# Soil biodiversity

Rémy Beugnon, Malte Jochum, Helen Phillips

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Rémy Beugnon, Malte Jochum, Helen Phillips. Soil biodiversity. Frontiers Media SA, 2022, Frontiers for Young Minds, 978-2-88976-674-1. 10.3389/978-2-88976-674-1. hal-04329887

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# SOIL BIODIVERSITY

EDITED BY: Rémy Beugnon, Malte Jochum and Helen Phillips

PUBLISHED IN: Frontiers for Young Minds

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FOR YOUNG MINDS

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ISSN 2296-6846

ISBN 978-2-88976-674-1

DOI 10.3389/978-2-88976-674-1

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# SOIL BIODIVERSITY

**Topic Editors:**

**Rémy Beugnon**, German Centre for Integrative Biodiversity Research (iDiv)
Halle-Jena-Leipzig, Germany; Leipzig Institute for Meteorology, Leipzig University,
Germany; Centre d'Ecologie Fonctionnelle et Evolutive (CEFE), CNRS, France

**Malte Jochum**, German Centre for Integrative Biodiversity Research (iDiv)
Halle-Jena-Leipzig, Germany; Leipzig University, Institute of Biology, Germany

**Helen Phillips**, German Centre for Integrative Biodiversity Research (iDiv)
Halle-Jena-Leipzig, Germany; Leipzig University, Institute of Biology,
Germany; Saint Mary's University, Halifax, Canada.

This Collection is the work of more than 50 scientists and Young Reviewers from all around the globe. Our role as editors, together with the authors, was to share our love of soil biodiversity with you. In this Collection, you will discover that soils are full of life. We will introduce some of the methods and techniques used by scientists to observe the life below our feet. We will show you that belowground life is essential to have healthy soils and, therefore, for us. However, you will soon realize that belowground life is changing and under multiple threats. The authors will give ideas on how we can protect soil biodiversity and invite you to actively help us in studying and protecting this valuable ecosystem. We have divided this article Collection into four sections, each of which is introduced below. To make our articles accessible to as many of you as possible, we have created a website hosting translations to languages other than English.

**Soils are alive**

Soils are not just rock and dust but are astonishing living systems that are full of life! In this first section, you will read about little creatures that you might already know, like earthworms. You will also discover many new creatures, like springtails and mites, that live close to you in your garden, in the parks, or in nearby fields. Our authors will even show you an entire world of tiny creatures not visible by the naked eye: tiny bacteria, fungi, and protists. Soil biodiversity is about the diversity of these organisms.

But how many different organisms are there? How different are they from each other? To answer these questions, scientists need tools and methods to observe and understand the biodiversity under our feet.

**How can we observe this beautiful world under our feet?**

In the articles in this section, the authors describe the tools and methods they use to observe and understand soil biodiversity. It is not easy to see the creatures in the soil and what they are doing under our feet; therefore, soils are often called the "black box". Some scientists are using the body fat of soil creatures to identify them and monitor what they feed on; others use DNA to identify soil organisms, like forensic investigators in the movies. In addition, our authors will explain how soil organisms are "talking" to each other and how we study these interactions.

What are scientists learning from studying these soil creatures? Is soil biodiversity important to us?

**Why is soil biodiversity so essential to us?**

In this section, the authors illustrate that soil biodiversity maintains processes essential for our well-being. For example, you will learn that soil bacteria can keep your food safe by protecting it from diseases. We will highlight that soil biodiversity is essential
for nature to work. For example, the authors will demonstrate that soil organisms
are vital for recycling dead matter and releasing the nutrients in it. In addition, you
will see how soil organisms are directly affecting greenhouse gas emissions such
as carbon dioxide and methane by controlling soil processes. Controlling these
emissions is critical for keeping our climate stable.

Soil organisms are alive, moving, and interacting, but are all these organisms and their important functions changing with time? Are these communities of soil organisms set in stone?

Soil communities are changing

You probably know that a lot of trees, flowers, and animals can change over the year with the seasons; flowers and fruits appear in spring and summer, leaves drop from the trees in fall. Soil animals are also changing with the seasons. And, like us, soil organisms can move to new places or disappear from others, either permanently or temporarily. These changes can be natural but can also be the result of human activities. Our authors will show you that agricultural practices and the effects of climate change (such as reduced rainfall) are affecting soil organisms, their functions, and the services they provide to us.

As we saw previously, soil biodiversity is essential for us, so any changes could be
disastrous. So can we protect the organisms in the soil in the same way we protect
other organisms such as tigers and pandas?

Protecting soil biodiversity

In the final section of this Collection, our authors will show you how to protect soil biodiversity. We can reduce our impacts and conserve this wonderful belowground life. But we can even go a step further and restore lost soil functions using our knowledge of soil biodiversity; for example by using fungi to restore soils. However, this is only possible if we understand soil biodiversity and its function. This is where you can help, for example by participating in a citizen science project and going outside to help researchers.

Conclusion

This Collection is about illuminating the "black box" of soil and showing you some of the fantastic creatures living under our feet. You will learn how scientists are studying soil biodiversity and how this soil biodiversity is essential for us. However, you will also see that soil biodiversity is under threat and needs to be protected. Many people across the globe will be needed to effectively protect these vital systems below our feet. That's why it is important to spread the word about the beauty and fragility of belowground life. We hope that this Collection will make you a champion of soil biodiversity and that you will pass on this message so that everyone will become more aware of, and be better able to protect soil biodiversity.

Now it is your turn to explore and engage with the content of this Collection. We
hope there will be something for all of you!

Citation: Beugnon, R., Jochum, M., Phillips, H., eds. (2022). Soil biodiversity.

Lausanne: Frontiers Media SA. doi: 10.3389/978-2-88976-674-1
# Table of Contents

## CHAPTER I

### SOILS ARE ALIVE

**07** *Earthworms of the World*
Helen R. P. Phillips, Erin K. Cameron and Nico Eisenhauer

**13** *Belowground Mountaineers: Critters Living In Mountain Soils*
Michael Steinwandter and Julia Seeber

**20** *Springtails—Worldwide Jumpers*
Anton Potapov

**27** *Armored Mites, Beetle Mites, or Moss Mites: The Fantastic World of Oribatida*
Carlos Barreto and Zoë Lindo

**35** *Super-Small Predators in Soils: Who Are They and What Do They Do?*
Stefan Geisen

**42** *The Soil Fungi: A Web of Life That Protects Trees and Fight Climate Change*
Olivia Azevedo and Frank Ashwood

**49** *From the Soil to the Club in the Roots: Clubroot*
Edel Pérez-López

**54** *Having Babies in Soil: Is Sex Really Necessary?*
Hüsna Öztoprak, Alexander Brandt, Marcel D. Solbach, Jens Bast and Ina Schaefer

## CHAPTER II

### HOW CAN WE OBSERVE THIS BEAUTIFUL WORLD UNDER OUR FEET?

**63** *Studying the Activity of Leaf-Litter Fauna: A Small World to Discover*
Dolores Ruiz-Lupión, María Pilar Gavin-Centol and Jordi Moya-Laraño

**73** *Soil Ecologists as Detectives Discovering Who Eats Whom or What in the Soil*
Amandine Erktan, Melanie M. Pollierer and Stefan Scheu

**81** *Looking for Earthworms in Deadwood*
Frank Ashwood, Elena I. Vanguelova, Sue Benham and Kevin R. Butt

**88** *The Way Soil Organisms Look Can Help Us Understand Their Importance*
Pierre Ganault Léa, Beaumelle and Apolline Auclerc

**96** *The Fascinating World of Belowground Communication*
Cristiana Ariotti, Elena Giuliano, Paolina Garbeva and Gianpiero Vigani

**103** *How Do Plants Defend Themselves From Root-Eating Creatures?*
Axel J. Touw and Nicole M. van Dam
# CHAPTER III

## WHY IS SOIL BIODIVERSITY ESSENTIAL TO US?

**110 Bacteria in Soil Keep Your Hamburger "Healthy"**
Stephanie D. Jurburg

**117 Dung Beetles Help Keep Ecosystems Healthy**
Dung Beetles Help Keep Ecosystems Healthy

**125 Plant-Eating Nematodes and the Key to Fighting Them**
Elisabeth Darling, Marisol Quintanilla-Tornel and Henry Chung

**131 The Bizarre Role of Soil Animals in the Decomposition of Dead Leaves**
François-Xavier Joly and Jens-Arne Subke

**138 Decomposition in Peatlands: Who Are the Players and What Affects Them?**
Carlos Barreto and Zoë Lindo

**147 Does Plant Biodiversity Influence Nutrient Cycles?**
Eva Koller-France, Wolfgang Wilcke and Yvonne Oelmann

**154 Can Methane-Eating Bacteria in Drylands Help Us Reduce Greenhouse Gases?**
Angela Lafuente and Concha Cano-Díaz

**161 Earthworms and Their Role in Greenhouse Gas Emissions**
Pierre Ganault, Sacha Delmotte, Agnès Duhamet, Gaëlle Lextrait and Yvan Capowiez

# CHAPTER IV

## SOIL COMMUNITIES ARE CHANGING

**169 Soil Ecosystems Change With Time**
Enrique Doblas-Miranda

**176 Microbial Mats: Primitive Structures That Could Help us Find Life on Other Worlds**
Santiago Cadena, Paula Maza-Márquez, Sandra I. Ramírez Jiménez, Sharon L. Grim, José Q. García-Maldonado, Leslie Prufert-Bebout and Brad M. Bebout

**185 How Soil Invertebrates Deal With Microplastic Contamination**
Carlos Barreto, Matthias C. Rillig, Walter R. Waldman and Stefanie Maaß

**194 Dirt Is Not Dead: How Land Use Affects the Living Soil**
Jes Hines and Franciska T. de Vries

**202 Double Whammy for Life in Soil? The Effects of Drought and Fertilizer Use**
Marie Sünnemann, Julia Siebert and Nico Eisenhauer

**211 Soil Organisms Have Favorite Forage Plants**
Felicity V. Crotty

**218 How Introduced Earthworms Alter Ecosystems**
Malte Jochum and Nico Eisenhauer
PROTECTING SOIL DIVERSITY

**227** Can We Save the Beast by Conserving the Beauty?
Felix Gottschall, Erin K. Cameron, Inês S. Martins, Julia Siebert
and Nico Eisenhauer

**235** Protecting Soil Biodiversity: A Dirty Job, but Somebody's Gotta Do It!
Alberto Orgiazzi

**24Z** Tiny Fungi in the Soil Are Like Medicine for Nature
Lena Neuenkamp and Nadia I. Maaroufi

**251** Learning More About Earthworms With Citizen Science
Victoria J. Burton and Erin K. Cameron
<figure><img src="image_4.png" /></figure>

# EARTHWORMS OF THE WORLD

Helen R. P. Phillips<fnref n="1" />,<fnref n="2" />,<fnref n="3" />*, Erin K. Cameron<fnref n="3" /> and Nico Eisenhauer<fnref n="1" />,<fnref n="2" />

## YOUNG REVIEWERS:

<figure><img src="image_5.png" /></figure>

ANNA-MARIE

AGE: 16

<figure><img src="image_6.png" /></figure>

KAYTLIN

AGE: 14

For decades, scientists have known where the highest numbers of species that live aboveground are found. So, they made maps of the world showing these patterns. For most of the aboveground groups, the highest numbers of species occur in the tropics and numbers decrease toward the poles. However, until recently, we did not understand such global patterns for many organisms living in the soil. We decided to create global maps of earthworm species richness. Earthworms provide humans with many useful services, such as moving the soils and improving their quality, which can increase the amount of food that is grown. If we want to protect earthworms and the services they provide, these global maps of earthworms are important because we need to understand where they are and why they live there.

<footnote n="1">¹Experimental Interaction Ecology, German Centre for Integrative Biodiversity Research (iDiv), Halle-Jena-Leipzig, Leipzig, Germany</footnote>

<footnote n="2">²Institute of Biology, Leipzig University, Leipzig, Germany</footnote>

<footnote n="3">³Department of Environmental Science, Saint Mary's University, Halifax, NS, Canada</footnote>
## MAPPING THE WORLD'S ANIMALS

There is around 150 million km² of land on earth. That is an area so huge that it is hard to imagine. With so much land, how do we know where the animals are, and how many there are? Why would we even want to know about the numbers of animals and their patterns across the world? Well, for example, we may want to know where to create **nature reserves** to protect the most species. Or maybe we are simply interested in knowing what the general pattern of animal and plant populations are, and whether that pattern is consistent across lots of different species. For example, tropical forests are known for having many different species of birds, but is that true for other animals?

To learn about the numbers of animals, people (both scientists and non-scientists) usually do surveys. A **survey** is simply counting the number of species (or number of individuals present) using a suitable technique for that species. For example, if we want to survey butterflies, we use a hand-held net and try to capture as many butterflies as possible using consistent methods, surveying a certain area of land for a given amount of time. However, doing surveys takes time, and it can also cost a lot of money. Additionally, we will never be able to do a survey at every location in the entire world. So, how do we know how many animals there are across the world?

We can use math! Specifically, we can use something scientists call **statistical models**, or just models for simplicity. For many decades, scientists have been creating models to estimate how many species of birds, plants, and other aboveground species there are across the globe. Unfortunately, this method has never been used for many of the organisms beneath our feet. So, we decided to create a model for earthworms. Earthworms are particularly cool (Figure 1). These soil organisms provide humans with many **ecosystem services** [1]. They help break down the fallen leaves so that the nutrients go back into the soil, they help make our crops grow better, and they help keep our climate the way we need it. Also, for a soil organism, earthworms are quite easy to survey because we can see them! Besides, there is quite a lot of information available about earthworms.

## WHAT DID WE DO TO UNDERSTAND GLOBAL PATTERNS OF EARTHWORMS?

To create a model to estimate the number of earthworms across the world [2], we needed data specifically about earthworms. Earthworm data consists of the numbers of earthworm species, collected using surveys. One person cannot survey everywhere, but we wanted to get as many surveys from across the globe as possible. So, we asked lots of other scientists to send us data from their surveys. These people were earthworm scientists that we knew, or who had already published the results of their surveys in scientific journals. We were confident that

### NATURE RESERVES

Areas where the animals, plants, and the environment are protected.

### SURVEY

Counting the number of species (or number of individuals present) using a suitable technique for that species.

### STATISTICAL MODELS

The process of trying to use known factors (such as temperature) to predict a factor that we may not be able to measure (such as the number of earthworm species).

### ECOSYSTEM SERVICES

Benefits to humans provided by the natural environment and the organisms in it. Ecosystem services can include increasing food production, breaking down fallen leaves, and helping to keep our climate the way we need it.
<caption>**Figure 1**</caption>

There are 7,000 described species of earthworms across the globe [1], and they vary considerably in their appearance. (a) *Scherotheca gigas* is an earthworm often found in France and Spain (photograph taken by Iñigo Virto). (b) *Aporrectodea smaragdina* is found in the Alps and eastern Europe (photograph taken by Michael Steinwandter).

<figure><img src="image_7.png" /><figcaption>Figure 1</figcaption></figure>

## PH

The scale used to specify how acidic (lemon juice is acidic) or how alkali (baking soda is alkali) something is.

the data were trustworthy, especially the data that had already been analyzed and published. When scientists publish papers, their data are always checked and critiqued by other scientists. The surveys were often done using slightly different methods, but many scientists simply dug a square hole in the ground, searched the soil for earthworms, and counted the numbers of earthworm species they removed. In total, we gathered data from 180 researchers across the globe, containing just over 9,000 surveys of earthworms.

The number of earthworm species scientists counted in their surveys ranged from no species in several surveys to 12 species found in another. We also needed information about the climate (for example, the temperature and rainfall) and the soil (such as the pH) at the location of each survey. We got this type of information from freely available databases.

Models ultimately use a certain factor (such as climate, soil pH) to estimate the number of earthworm species in an area. To understand how models work, imagine this: we survey lots of beaches and ask ice cream sellers how many ice cream cones they have sold. We then get information on the average temperature at each beach. We could then create a model showing how temperature affects the number of ice cream cones sold at each beach. As you might expect, the hotter the temperature, the more ice cream cones are sold. Using this model, we could then estimate how many ice creams will be sold at any temperature, which gives us an idea about ice cream cone sales on beaches where we cannot survey. We can do something similar for earthworms to see how the numbers of species found in a survey changes with an environmental factor like temperature.

Our earthworm model contains many details about the environment—12 different aspects in total—but the basic principle remains the same. The 12 environmental details included information about the soil, the type of vegetation covering the ground, and the climate. Using our model, we then estimated how many species of earthworms there are for all points in the world, and we made a map of that (Figure 2).
## Figure 2

The number of earthworm species across the world, created using our model. In total, 180 researchers provided data from just over 9,000 surveys. This survey data was combined with data about the environment, such as pH, so that we could predict the number of earthworm species across all areas of the world—even where no surveys have been done. Typically, the number of earthworm species in any one place varied between 1 (areas in dark purple) and 4 (areas in bright yellow), but areas in the temperate region, such as Europe, had the highest number of earthworm species (shown in the yellow shades).

<figure><img src="image_8.png" /><figcaption>Figure 2</figcaption></figure>

## TEMPERATE REGION

The earth's middle latitudes, which span between the tropics and the polar regions. The temperate region typically has more distinct seasons (spring, summer, autumn, and winter) compared to tropical climates.

## WHAT WE FOUND OUT ABOUT EARTHWORMS

As we mentioned at the beginning of this article, we usually expect the tropics to have the highest numbers of species. This is because, typically, we find more species in places that have higher temperatures. What our maps show is that this is not the case for earthworms. Our model indicates that, if you were to do a survey in a tropical region and one in a **temperate region**, you would find more earthworm species in the temperate region.

Why might this be? There are many aspects of the environment that shape the number of earthworm species found in a survey. And although the soil is important, we found that climate (for example, temperature and amount of rain) was the most important factor determining the number of species. As earthworms prefer to live in moist, warm conditions, the temperate region is much more suitable for them. There are more earthworm species where the environmental conditions are ideal. As long as the environment is not too extreme—too dry, too wet, too hot, too cold—it is very likely that there will be earthworms. Some species of earthworms may like conditions that are slightly different from most other earthworms. Alternatively, some species of earthworms may tolerate living in regions that are less than ideal, because there are fewer species to compete with for food, for instance, but this is an area scientist are still studying.

## EARTHWORM MODELS CAN BROADEN CONSERVATION EFFORTS

Earthworms are really important for many ecosystem services that humans need, such as increasing food production. With the new knowledge gained from our model, we hope that earthworms will
now be considered when scientists and conservationists think about
creating nature reserves. Typically, nature reserves are established
based on the number of species of plants or other aboveground
organisms. But, since high numbers of earthworm species do not exist
in the tropics (unlike many aboveground plants and animals), we need
to think about earthworms and other soil organisms separately, and
potentially establish nature reserves just for them.

Also, as we found that climate is the main aspect of the environment
correlated with the numbers of earthworms, the fact that our climate
is changing is concerning. Our future research will establish how
the numbers of earthworms change as the climate changes, since
some species may respond positively to changes in climate, whereas
others may not. We need to understand how climate change will
affect earthworms and other soil organisms, so that we can prepare
to protect these valuable organisms for the future.

ORIGINAL SOURCE ARTICLE

Phillips, H. R. P., Guerra, C. A., Bartz, M. L. C., Briones, M. J. I., Brown, G., Crowther, T. W., et al. 2019. Global distribution of earthworm diversity. *Science* 366:480–5. doi: 10.1101/587394

REFERENCES

1. Orgiazzi, A., Bardgett, R. D., Barrios, E., Behan-Pelletier, V., Briones, M. J. I., Chotte, J. L., et al. 2016. *Global Soil Biodiversity Atlas*. Luxembourg: Publications Office of the European Union. doi: 10.2788/2613

2. Phillips, H. R. P., Guerra, C. A., Bartz, M. L. C., Briones, M. J. I., Brown, G., Crowther, T. W., et al. 2019. Global distribution of earthworm diversity. *Science* 366:480–5. doi: 10.1101/587394

SUBMITTED: 31 March 2020; ACCEPTED: 08 April 2021;

PUBLISHED ONLINE: 10 May 2021.

EDITED BY: Vishal Shah, West Chester University, United States

**CITATION:** Phillips HRP, Cameron EK and Eisenhauer N (2021) Earthworms of the World. Front. Young Minds 9:547660. doi: 10.3389/frym.2021.547660

**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Phillips, Cameron and Eisenhauer. This is an open-access
article distributed under the terms of the Creative Commons Attribution License
(CC BY). The use, distribution or reproduction in other forums is permitted, provided
the original author(s) and the copyright owner(s) are credited and that the original
publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### ANNA-MARIE, AGE: 16

My favorite subject is biology, I like bionics. In future I would like to work on new materials, new substances. I am curious about the mysteries of the universe, there is so much to discover.

<figure><img src="image_9.png" /></figure>

### KAYTLIN, AGE: 14

I am a first-year high school student that likes to travel and explore different cultures and things. In my free time I read, do martial arts, teach myself Japanese or let my creativity run loose by writing stories, drawing, or taking pictures.

<figure><img src="image_10.png" /></figure>

## AUTHORS

### HELEN R. P. PHILLIPS

Helen has always loved animals, but never really enjoyed doing field work. She continued learning about ecology, and eventually realized that using large datasets and doing computer-based work, such as programming, was what she found interesting. Since then, Helen has focused on global datasets of biodiversity, using them to investigate where biodiversity is in the world, and how human activities might be affecting global patterns. Recently, her work has involved earthworms and other soil biodiversity. When not working, Helen likes to play computer and board games, sew, make music, and play with her pet rabbit. *helen.phillips@smu.ca

<figure><img src="image_11.png" /></figure>

### ERIN K. CAMERON

Erin enjoyed playing outside as a child and loved science but did not consider becoming a biologist. Eventually, she started helping out with research examining how human activities affect songbirds and found it fascinating. Once she started to work on soil organisms and saw how much is still unknown about them, she was convinced that she wanted to study ecology. Now she investigates how human activities affect soil biodiversity and functioning of ecosystems and enjoys cross-country skiing, bicycling, and kayaking in her free time.

<figure><img src="image_12.png" /></figure>

### NICO EISENHAUER

Nico has been interested in nature since his early childhood. He dug for earthworms, caught frogs and fish, and helped lizards survive the winter months. He has always been fascinated by the beauty of nature and driven by the question of why a specific plant or animal species occurs in one place, but not in another. During his study of biology, he discovered his interest in earthworms and their important activities, which are crucial for the functioning of ecosystems. When not at work, Nico likes playing soccer and badminton, running, and spending time with his family and friends.

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /></figure>

# BELOWGROUND MOUNTAINEERS: CRITTERS LIVING IN MOUNTAIN SOILS

Michael Steinwandter<fnref n="1" />* and Julia Seeber<fnref n="1" />,<fnref n="2" />

## YOUNG REVIEWERS:

<figure><img src="image_15.png" /></figure>

MERCY
SCHOOL
INSTITUTE

AGES: 15

The soil beneath our feet still requires more exploration, although we know that it is home to a vast number of organisms. It is basically a black box we cannot see into to observe its inhabitants and the processes they contribute to. In this article, we will tell you about soils that are even less explored. We are talking about mountain soils at high elevation: they are not easily accessible but harbor many exciting species, some of which are specialized to live only in mountain soils. We are a team of soil ecologists who dedicate our research to revealing the secrets of animals living in mountain soils. We will tell you which and how many soil critters can be found in high-elevation soils of the European Alps, in both natural mountain ecosystems and those used for farming. Further, we explain some clever ways these species have adapted to the harsh alpine environment.

<footnote n="1">¹AlpSoil Lab, Institute for Alpine Environment, Eurac Research, Bozen, Italy</footnote>

<footnote n="2">²Department of Ecology, University of Innsbruck, Innsbruck, Austria</footnote>
## ALPINE ZONE

A climatic zone that can be found in higher mountains or arctic areas. It is the zone where tree cannot grow because of the low temperatures.

## INVERTEBRATES

A large group of animals that have no vertebrae (backbones). Common soil invertebrates include insects (beetles, fly larvae), earthworms, millipedes, centipedes, woodlice, and spiders.

## KEMPSON APPARATUS

A device in soil laboratories for extracting animals from soil blocks. Being heated from above, soil animals seek to escape the dry and hot conditions and fall into a collection bucket.

## PITFALL TRAP

A simple method to catch ground-dwelling insects and invertebrates. These are glass jars that are dug into the soil and are left active for one or several days.

## SUBALPINE ZONE

A climatic zone that includes the mountain forests up to the natural treeline. It includes also a transition zone between these forests and the alpine grassland where only small shrubs and single trees can grow.

## WHAT ARE MOUNTAIN SOILS?

When we talk about mountain soils, we usually refer to soils at high elevation, particularly soils above the treeline where there are only scattered trees, or no trees at all. In the European Alps, this zone can be found above 2,000 m; in the Central European Alps, above 2,300 m. Of course, some mountain soils are found on lower mountains or even hills, but we will focus on the beautiful world of soil animals that live in natural grasslands, grazed pastures, and even bare soils on higher mountains like the European Alps, from 1,500 to 3,000 m.

Some mountain areas above the treeline, in what is called the **alpine zone**, may seem to be untouched by humans. But this is not true—farmers have gently used many such grasslands for centuries and even millennia, as pastures for small stocks of cows, sheep, and goats in the summer months (Figure 1A). Why did farmers of the past (and some even today) hike up so far with cows and sheep? Why not just use the meadows in the valley bottom, which are easily accessible? Well, they do use the valley meadows, but the alpine pastures are home to many colorful and nutritious herbs and grasses that cows and sheep love to eat and which are very healthy for them. Also, during hot summers, the temperatures at higher elevations are much more bearable for the animals.

## WHICH AND HOW MANY ANIMALS LIVE IN MOUNTAIN SOILS?

The beauty of mountains lies not only aboveground in the colorful flowers and shrubs, but also beneath our feet. To study the soil critters, we remove square soil blocks of 20 x 20 and 15 cm deep (Figure 1C) and extract the **invertebrates** using a **Kempson apparatus**. The Kempson apparatus uses heat and light from lightbulbs to force the animals to escape from the soil as the blocks dry out. The animals are then collected from water-cooled buckets containing a collection fluid. Further, we also install **pitfall traps** in our study locations (Figure 1B). Pitfall traps are open glass jars, dug into the ground so that organisms like spiders and beetles will fall into them and be trapped. This is a very useful method to see what is crawling on the soil surface. Using these techniques, we found a diverse community of earthworms, millipedes, beetles, and insect larvae in mountain soils.

Soil biodiversity is especially high in the **subalpine zone**, from 1,500 to 2,000 m. This border area contains forests and manmade pastures. In this zone, soil animals typically found in mountain forests (such as woodlice and centipedes) co-exist with species from natural grasslands (earthworms and millipedes). In subalpine pastures in the Central European Alps, we found up to 115 earthworms per square metre in the upper 15 cm of the soil layer, as well as 60 millipedes,
Figure 1

In the European Alps, centuries of traditional farming created species-rich grassland soils. (A) The alpine zone above the treeline is often grazed by cows and sheep. Below in the subalpine zone, forests were also cut down to create pastures and hay meadows. Rocky areas increase at higher elevations and are dominant in the high alpine, which is the area above most vegetation (Gsies/Valle di Casies, South Tyrol, Italy). (B) We assess soil invertebrates by installing pitfall traps (yellow arrow) (Dolomites, South Tyrol, Italy). (C) Soil blocks are removed and taken to the laboratory to be studied (Matsch/Mazia, South Tyrol, Italy) (Photograph credits: Michael Steinwandter).

<figure><img src="image_16.png" /><figcaption>Figure 1</figcaption></figure>

NIVAL ZONE

A rocky and often
snow-covered area of
high mountains and
arctic regions that
follows the alpine zone.
Here, almost no plants
grow, but mostly
lichens and mosses.

55 beetles, and 50 larvae of flies and midges [1]. All these soil invertebrates profit from the sporadic presence of animals that roam the fields, including cows, sheep, and wild mammals such as deer, chamois, and Alpine ibex. The grazing animals keep the vegetation short, remove troublesome shrubs, and therefore keep the grasslands open. Additionally, they leave behind lots of dung, which is a welcome food source for many soil animals such as earthworms, millipedes, and dung beetles (Figure 2) [2]. However, if farmers bring too many cows and sheep up to these areas, we can observe negative effects of trampling and too much dung, which results in reduced numbers of some types of soil animals. For example, we found only about 5 specimens of millipedes and 45 beetles per square metre in an area that was overused by farmers.

The higher we go, the fewer individuals and types of soil animals we find (Figure 3). In the alpine zone, from 2,000 to 2,800 m, trees cannot grow because temperatures are too low and the summer growing season is short. In this alpine zone, some types of soil animals reach the upper limit of their comfort zone. Earthworms and millipedes decrease in numbers and can rarely be found at elevations above 2,500 m, even if roaming sheep add extra food in the form of dung. We found only 20 earthworms and 10 millipedes per square metre in the alpine zones we studied. On the other hand, fly and midge larvae increase massively in numbers (more than 750 per square metre) and partly take over the important ecosystem functions of earthworms, such as the breakdown of dead plant material [3].

In the European Alps, areas above 2,500 m are often covered with snow for much of the time, making life very challenging for soil animals. These zones, called the high alpine zone and the **nival zone** (above 3,000 m), are usually not used by farmers. These areas are inhabited mainly by small soil animals such as springtails and mites. These cold-weather specialists survive beneath the snow cover, which acts like a blanket to keep temperatures slightly above freezing, even when the air temperature falls well-below freezing.
## Figure 2

Mountain soil invertebrates from our research sites in the Stubai Alps and Oetztal Alps in Tyrol, Austria. (A) A lifted cowpat unveils the feeding burrows of earthworms (Lumbricus rubellus), dung beetles, and insect larvae. (B) The pill millipede (Glomeris transalpina) is commonly found in alpine shrubland in the Central European Alps. (C) Larvae of fungus gnats (Mycetophilidae) feed on sheep dung in high alpine pastures (Photograph credits: Michael Steinwandter).

<figure><img src="image_17.png" /><figcaption>Figure 2</figcaption></figure>

## ADAPTATION

The ability to adapt to new environmental conditions by optimising body characteristics and/or behaviour. For example, alpine invertebrates adapted to the harsh mountain environment.

## HOW CAN THESE ANIMALS SURVIVE?

Adaptation is key to survival at high elevations. Adaptation is the ability to adapt to new environmental conditions by optimising body characteristics and/or behaviour. Soil animals in these ecosystems experience low temperatures and often encounter snow even in the summer months. They also face stronger sun rays and less and less living space the higher the elevation. These animals cannot be picky about what they eat—they must feed on any available food sources. For example, while in lowlands some beetles feed on plant-based food only, in alpine pastures they also feed on other animals and their shed-off skins or carcasses, as well as on dung, if it is present [4]. The adaptation to this broader menu increases the chances that these beetles will successfully gather enough energy to survive and thrive.

Another survival strategy of mountain soil animals is that they can prolong their life stages if the summers are too short to allow them to reach the next stage. For example, during a snowy, cold summer when a millipede cannot get enough energy to produce its eggs, it can wait another season and produce eggs the following year when the conditions are more favourable. While this is a useful adaptation, it also means that these millipedes must survive longer in the harsh environment to successfully finish their egg production.

Soil animals at high elevations have also adapted their bodies to the harsh conditions by decreasing their body size (smaller bodies can heat up more easily), by losing their wings (without wings the animals can stay close to the soil surface and avoid strong winds), by changing their body colours (darker bodies can heat up more easily), or/and
<caption>Figure 3</caption>

Distribution of typical soil invertebrates at the various elevation zones of the Central European Alps. The zones start and end at different elevations on the sunny side and the shady side. The table shows how many animals per square metre can be expected at each elevation zone, based on currently available data. Generally, the number of soil animals is decreasing with increasing elevation, with different animal groups reaching their limit at different elevations (such as millipedes at 2,500 m, beetles at 3,000 m). Asterisks (*) indicate that very limited, highly variable data are available (Image credits: modified after Wikimedia Commons).

<figure><img src="image_18.png" /><figcaption>Figure 3</figcaption></figure>

by producing anti-freeze in their bodies, to prevent their bodies from freezing at low temperatures.

## DO MOUNTAIN SOILS NEED HELP?

Now you know that mountain soils are interesting places that are home to many soil invertebrates, some of which can only be found in mountain soils. Because these soils and their inhabitants are still understudied, the possibility of finding new species there is quite high. However, like many ecosystems nowadays, mountain soils are threatened and need to be protected. One big problem that species-rich subalpine pastures face is that farmers are abandoning these lands, because these traditional methods of farming do not generate enough money. When cows and sheep are no longer grazing the alpine grasslands at the treeline, these areas will be taken over by persistent shrubs that form dense and unpassable shrubland. Also, the increasing temperatures resulting from climate change will lead to an upwards migration of soil animals, as they try to escape temperatures that are too warm for them. But because there is less and less space as the elevation increases, these animals will have problems
finding enough room to live, and they might experience a higher risk of extinction.

But the good news is that we can all help! For instance, by supporting mountain farmers and buying their products (like milk and cheese) we can increase the chances that they can continue to maintain beautiful alpine pastures. Further, we can personally take care of mountain soils by not damaging them when we go hiking, mountaineering, or skiing. We should stay on the trails and take our trash back home instead of leaving it in the mountains. Lastly, we can take political action, by raising our voices against the building of new entertainment facilities such as ski areas, mountain huts, and mountain bike trails, which can harm these natural and sensitive areas.

We all need to be extremely careful with this precious ecosystem called soil. Let us keep in mind that mountain soils needed centuries—and in high mountains, even millennia—to form, but these ecosystems and the fascinating soil organisms that live there can be destroyed in minutes, without our protection!

## REFERENCES

1. Steinwandter, M., Schlick-Steiner, B. C., Seeber, G. U. H., Steiner, F. M., and Seeber, J. 2017. Effects of Alpine land-use changes: soil macrofauna community revisited. *Ecol. Evolut.* 7:5389–99. doi: 10.1002/ece3.3043

2. Curry, J. P., and Schmidt, O. 2007. The feeding ecology of earthworms—a review. *Pedobiologia*. 50:463–77. doi: 10.1016/j.pedobi.2006.09.001

3. Kitz, F., Steinwandter, M., Traugott, M., and Seeber, J. 2015. Increased decomposer diversity accelerates and potentially stabilises litter decomposition. *Soil Biol. Biochem.* 83:138–41. doi: 10.1016/j.soilbio.2015.01.026

4. Steinwandter, M., Rief, A., Scheu, S., Traugott, M., and Seeber, J. 2018. Structural and functional characteristics of high alpine soil macro-invertebrate communities. *Euro. J. Soil Biol.* 86:72–80. doi: 10.1016/j.ejsobi.2018.03.006

**SUBMITTED:** 28 January 2021; **ACCEPTED:** 07 January 2022;

**PUBLISHED ONLINE:** 03 February 2022.

**EDITOR:** Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**SCIENCE MENTOR:** Fares Z. Najar

**CITATION:** Steinwandter M and Seeber J (2022) Belowground Mountaineers: Critters Living in Mountain Soils. Front. Young Minds 10:660110. doi: 10.3389/frym. 2022.660110

**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.

**COPYRIGHT** © 2022 Steinwandter and Seeber. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### MERCY SCHOOL INSTITUTE, AGE: 15

We are fun and dynamic leaders, and we love to hang out with our friends.

<figure><img src="image_19.png" /></figure>

## AUTHORS

### MICHAEL STEINWANDTER

I am a soil ecologist and soil zoologist in the AlpSoil Lab, mainly working with critters such as earthworms, spiders, and preferably millipedes. I conduct my research in soils at all elevations, including the lowlands, but I really love to discover the soil life in mountain forests and pastures above the treeline. This comes from my passion for hiking and mountaineering, something you are born with when you grow up in the Dolomites (South Tyrol, Northern Italy). Beside doing science, I am fascinated by all of nature. Therefore, I am also a professional hiking guide and environmentalist. *michael.steinwandter@eurac.edu

<figure><img src="image_20.png" /></figure>

### JULIA SEEBER

I am a soil ecologist in the AlpSoil Lab, interested in understanding the relationships between soil animals and their habitats, and which soil processes the animals contribute to. I like to go on field trips to see and investigate the habitats, but I also like to do experiments with the animals in the laboratory, to watch them do marvellous things such as breaking down dead plant material. My favourite soil animals are earthworms because without them, the soil ecosystem would be much less efficient. My love of mountain sports, such as skiing and hiking, is easy to combine with my love of doing science in mountains.

<figure><img src="image_21.png" /></figure>
<figure><img src="image_22.png" /></figure>

# SPRINGTAILS—WORLDWIDE JUMPERS

Anton Potapov<fnref n="1" />,<fnref n="2" />*

## YOUNG REVIEWERS:

<figure><img src="image_23.png" /><figcaption>ANSHUL</figcaption></figure>

AGE: 9

<figure><img src="image_24.png" /><figcaption>LUVENA</figcaption></figure>

AGE: 11

<figure><img src="image_25.png" /><figcaption>MILES</figcaption></figure>

AGE: 8

<figure><img src="image_26.png" /><figcaption>PRANATEE</figcaption></figure>

AGE: 12

Springtails are tiny, six-legged animals that you meet every day, but hardly notice. They can survive in big cities, on ice in Antarctica, in the deepest caves, and in rainforest canopies. Some scientists call them the earliest known and the most numerous insects on Earth. Springtails are famous jumpers—if they were as large as humans, they would easily be jumping over 10-story buildings. This ability allows them to escape from danger. Every day, springtails are very busy, improving soil health and supporting numerous species of spiders, beetles, ants, and other small predators on our planet. They are a key part of soil biodiversity, but we still need to learn a lot about them and many of these beautiful animals are yet to be discovered.

## WONDERFUL DIVERSITY OF SPRINGTAILS

If you walk out of your house, you will probably meet a springtail, but you might not notice it because most of these animals are only one millimeter long. Springtails, also called Collembola, are insect relatives that can be found in soils all over the world. The best

<footnote n="1">¹Animal Ecology, J.F. Blumenbach Institute of Zoology and Anthropology, University of Göttingen, Göttingen, Germany</footnote>

<footnote n="2">²Soil Zoology and General Entomology, A.N. Severtsov Institute of Ecology and Evolution, Russian Academy of Sciences, Moscow, Russia</footnote>
<caption>**Figure 1**</caption>

Springtails in their natural environments all over the world. (A,B) Most springtails live in dead leaves or wood. (C-E) Some springtails are associated with living plants. (F) A number of species can be found in extreme environments, like the snow surface shown here (Photo credits: A—Dunmei Lin from China; C-E—Marie Huskens from Belgium; F—Ferenc Erdélyi from Hungary; B—Andy Murray from the UK).

<figure><img src="image_27.png" /><figcaption>Figure 1</figcaption></figure>

place for a springtail to live is the forest floor, where you can find thousands of them in a handful of fallen leaves. But they can also live in other environments, often in moist places where fungi are growing (Figure 1). In fact, springtails live almost everywhere: they are numerous in Antarctica on snow and rocks, they are diverse in tree canopies of tropical forests, they are found on the highest mountains and down in the deepest caves. Several years ago, scientists found the springtail *Plutomurus*, which lives two kilometers below the land surface in a cave in the Caucasus mountains [1]. They lured it out with the help of a smelly cheese. In winter, some springtails jump and wander on the snow surface, which earned them the name "snow fleas." Snow fleas like *Hypogastrura* (Figure 1F) can exist in herds of millions, making the snow gray with their bodies! As masters of survival, springtails also live with us—in gardens, backyards, parks, and sometimes flowerpots.

Springtails were surviving and thriving on the planet long before dinosaurs, and they are among the first animals that walked on land. We know this because scientists found a fossil springtail in prehistoric rocks dated about 410 million years old. This springtail was named *Rhyniella praecursor*, "the earliest known insect." Interestingly, some
## Figure 2

Beautiful springtails from Australia, Tasmania, and New Zealand. (A) Masterpiece *Katianna*, with spotted coloration. (B) Shiny *Lepidocyrtus*, covered with scales. (C) Dragon *Womersleymeria*, large and horned. (D) Baby *Neelides*, only about half a millimeter in size (Photo credits: A,B,D—Andy Murray from the UK https://www.chaosofdelight.org; C—Cyrille D'Haese from France).

<figure><img src="image_28.png" /><figcaption>Figure 2</figcaption></figure>

## MASS EXTINCTION

Widespread and rapid decrease of biodiversity on Earth during evolutionary history. Five main mass extinctions are recognized.

## FURCA

A forked tail-like appendage attached to the abdomen of many springtail species.

modern springtails look very similar to *Rhyniella*, which means that springtails survived on Earth through four out of five **mass extinctions** without changing much in appearance. We know of about 9,000 species of springtails living now, which are all listed in a web catalog,<fnref n="1" /> but scientists think that there are at least four times as many unknown springtail species on our planet [2]. Some remote areas in Tasmania and New Zealand are inhabited by beautiful and unusual species (Figure 2) and many more are yet to be discovered.

Springtail species can look quite different from each other. Some species have a round shape, while other species are elongated. Some do not have coloration, others are blue, or black, or have colorful spots and stripes, such as the ball-like *Katianna* (Figure 2A). *Lepidocyrtus* (Figure 2B) has shiny scales, just like fish do. Most springtails are about 1 mm long, but there are tiny and giant species. For example, *Neelides* (Figure 2D) is only about half a millimeter long, while *Womersleymeria* (Figure 2C) can be longer than a centimeter! Most of these giants live on the dead wood of wild tropical forests and are called "dragon springtails."

## WHAT DOES A SPRINGTAIL LOOK LIKE?

The name "springtail" comes from the organism's **furca**, which looks like a forked tail and allows many springtails to spring away from danger, just like tiny grasshoppers (Figure 3). The furca can be found under the body, on the abdomen, but not all springtails have one. If a springtail is walking or eating, the furca is attached to the body under high tension, like a compressed spring. As soon as springtail wants to jump, the furca is released and the animal catapults itself

<footnote n="1">¹ http://www.collembo-la.org</footnote>
Figure 3

What are the parts of a springtail? This sketch of a springtail was made by taxonomist Mikhail Potapov, who has described more than 200 springtail species new to science.

<figure><img src="image_29.png" /><figcaption>Figure 3</figcaption></figure>

² Check out "trash can springtails" doing 22,440 rotations per minute in slow motion on this video https://www.youtube.com/watch?v=Qu01E UeE5PM.

COLLAPHORE

A tube-like structure that is used by springtails to attach to surfaces.

MANDIBLES

Lower jaws, used to bite and chew the food. Unlike humans, arthropods chew horizontally, not vertically.

away from predators, or curious scientists. While jumping, springtails flip over many times, so it looks like they are doing a fantastic acrobatic trick before they crash into something.² Springtails are champions at jumping—if they were the size of humans, they could jump over 10-story buildings! As you can imagine, after such a jump it is not easy to land without a parachute. Instead, springtails use a special tube called **collaphore**, which allows them to stick to the surface (for example to a leaf or a stone) when landing.

Like insects, springtails are hexapods, meaning that they have six walking legs. Unlike insects, they never have wings. Springtails can have from two to sixteen eyes (Figure 1D), but species living in soil are often blind. To orient themselves in the environment and communicate with others, many springtails use antennae, which are long organs on their heads. They move the antennae to touch and check the surfaces in front of them. If they find some food, they grasp it and chew it with their **mandibles**.

HOW DO YOU CATCH A SPRINGTAIL?

If you are interested enough to look for a real springtail, you need to know where to look and how to do it. Springtails like wet places, like moist fallen leaves or mosses. Some large species can be found hiding under the bark of decaying fallen trees. Others are found on stream banks, rocks, mosses, or flowers. If you are lucky, you can also find springtails in flowerpots—if so, chances are it will be the white *Folsomia candida*—one of the most commonly used soil animals in laboratory experiments. While searching for a springtail, be very patient—they are everywhere, but they are masters of hiding and often are colored like things in the environment around them (Figure 1E).
## ENTOMOLOGICAL ASPIRATOR

Aspirator used to collect small organisms visible to the naked eye [https://en.wikipedia.org/wiki/Aspirator_(entomology)].

## TULLGREN FUNNEL

Device used to extract living organisms, particularly arthropods, from soil, detritus, moss, and other substrates (https://en.wikipedia.org/wiki/Tullgren funnel).

## DETRITUS

Dead organic material, for example dead leaves or wood, bodies of dead animals, and excrements. Detritus is inseparable from the microorganisms that decompose it, such as bacteria and fungi.

If you find a springtail, you can simply observe it crawling around or jumping, and maybe you can take a photo. Web platforms like iNaturalist<fnref n="3" /> or Flickr<fnref n="4" /> have thousands of springtail photos from around the world. Such observations can help scientists to understand where species live, and may even help them to discover new species.

To catch a springtail, you can also use what is called an **entomological aspirator**. But if you want to keep the springtail, remember that springtails do not like dry conditions. After several minutes in an aspirator, some species may die. Many scientists collect springtails from leaf litter, soil, rotten wood, and mosses using a **Tullgren funnel**, which can be built with relatively little effort at home. Springtails can be closely inspected under a microscope. Scientists also keep springtails as laboratory pets—they need a permanently moist surface (for example, a jar with fallen leaves, soil, or clay), some food (baker's yeast would be a good choice), and air (make holes in the lid). Unfortunately, only a few dozen species like to live at home or in the laboratory—and we do really not know why.

## BUSY SPRINGTAILS RUN THE WORLD

What are all these springtails doing in nature and why should we care about them? Springtails have an important role in ecosystems: as ecosystem “cleaners” they recycle dead material called **detritus**, and they feed on microbes, such as bacteria and fungi [3]. By doing this, they improve soil structure and make nutrients available to plants. Springtails can also pollinate mosses, just like bees pollinate flowers [4]. Being delicious food for many predators is also important—numerous species of spiders, beetles, ants, and other invertebrates survive by hunting springtails. Sometimes springtails are also directly useful to humans. In agricultural fields, they may help plants by feeding on the microbes that cause plant diseases, or they may support other predators that can kill plant pests. However, scientists have only recently started to explore these functions of springtails, and there is a lot to learn yet.

In the modern world many ecosystems are changing. Cities are growing, tropical forests are being cut down to grow food, and increasing temperatures are making frozen places like Antarctica and the northern tundra melt. These changes affect springtails as well as other soil organisms. The most remarkable species are often also the most vulnerable and can become extinct if their natural environments are destroyed. The number of springtails on our planet is likely to decline in the future, since they are numerous in the cold, Polar regions that will be strongly affected by climate change. One hectare of tundra can be inhabited by as many springtails as there are humans on the entire planet. As masters of survival, springtails will adapt to the changing world and live in the new ecosystems. However, many species are likely to become extinct even before being

<footnote n="3">³ https://www.inaturalist.org</footnote>

<footnote n="4">⁴ https://www.flickr.com</footnote>
discovered. Studying springtails and sharing the knowledge about them as a hidden but very important part of biodiversity can help us to understand how the nature is organized and how we change it with our actions. Sharing your new knowledge with your friends and family can help with this—the more people who will know about the importance of hidden biodiversity, the better we will be able to understand and protect the nature and our future as a part of it.

## AUTHOR CONTRIBUTIONS

AP developed the idea and wrote this manuscript.

## ACKNOWLEDGMENTS

I am grateful to my father, Mikhail Potapov, who tells me springtail stories and draws great pictures. I am also grateful to Andy Murray, Cyrille D'Haese, Marie Huskens, Dunmei Li, Ferenc Erdélyi, and Frans Janssens, who helped to find or provide photographs for this article.

## REFERENCES

1. Sendra, A., and Reboleira, A. 2012. The world's deepest subterranean community—Krubera-Voronja Cave (Western Caucasus). *IJS* 41:221–30. doi: 10.5038/1827-806X.41.2.9

2. Hopkin, S. P. 1997. *Biology of Springtails: (Insecta: Collembola)*. Oxford: Oxford Science Publications.

3. Rusek, J. 1998. Biodiversity of Collembola and their functional role in the ecosystem. *Biodiv. Conserv.* 7:1207–19. doi: 10.1023/A:1008887817883

4. Cronberg, N., Natcheva, R., and Hedlund, K. 2006. Microarthropods mediate sperm transfer in mosses. *Science* 313:1255. doi: 10.1126/science.1128707

**SUBMITTED**: 24 March 2020; **ACCEPTED**: 30 October 2020;

**PUBLISHED ONLINE**: 24 November 2020.

**EDITED BY**: Helen Phillips, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Potapov A (2020) Springtails—Worldwide Jumpers. Front. Young Minds 8:545370. doi: 10.3389/frym.2020.545370

**CONFLICT OF INTEREST**: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2020 Potapov. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution
or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### ANSHUL, AGE: 9

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<figure><img src="image_30.png" /></figure>

### LUVENA, AGE: 11

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### MILES, AGE: 8

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### PRANATEE, AGE: 12

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<figure><img src="image_33.png" /></figure>

## AUTHOR

### ANTON POTAPOV

I am a soil ecologist working at the University of Göttingen, Germany. I particularly enjoy studying springtails, which I am doing in different environments, from Russian taiga to tropical rainforests. I want to understand how springtails and other small animals form complex food webs and drive biodiversity and functioning of ecosystems. *potapov.msu@gmail.com

<figure><img src="image_34.png" /></figure>
<figure><img src="image_35.png" /></figure>

# ARMORED MITES, BEETLE MITES, OR MOSS MITES: THE FANTASTIC WORLD OF ORIBATIDA

Carlos Barreto *†* and Zoë Lindo †

Soil Biodiversity and Ecosystem Function Laboratory, Department of Biology, Biotron Experimental Climate Change Research Centre, Western University, London, ON, Canada

## YOUNG REVIEWERS:

<figure><img src="image_36.png" /></figure>

ISABEL

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<figure><img src="image_37.png" /></figure>

MARGARIDA

AGE: 12

Oribatid mites are a group of animals related to spiders, scorpions, and ticks. However, they are typically much smaller (most are < 1 mm) and are full of defensive mechanisms to protect them from predators. Generally, oribatid mites live in soils and feed on fungi, bacteria, and soil particles, making them very important for decomposition processes. Oribatid mites also help cycle soil nutrients and contribute to soil formation. Oribatid mites can also be found in aquatic environments and even treetops. They are present all over the world, from forests to deserts, and along the edges of lakes and oceans. They are often the most abundant mesofauna found in soils, reaching impressive populations of up to 500,000 individuals per m² in forests. The diversity of oribatid mites varies across vegetation types, climate, and soil properties, such as moisture, pH, nutrient concentrations, and heavy metals.
## Figure 1

Examples of oribatid mites, with their scientific names.

(A) *Suctobelbella* sp.;

(B) *Hoplophorella* sp.

(also called box mites);

(C) defensive hairs on *Palaeacarus* sp.;

(D) hard body of *Diapterobates notatus* with an arrow pointing to tooth-like structures to help during feeding;

(E) juvenile *Lepidozetes* sp.; (F) adult

*Lepidozetes* sp. (notice that young mites and parents are not alike);

(G) juvenile *Tyrphonothrus* sp.; and

(H) adult *Tyrphonothrus* sp. (notice that young mites and parents are alike).

<figure><img src="image_38.png" /><figcaption>Figure 1</figcaption></figure>

## EXOSKELETON

The external skeleton that supports and protects an animal's body, in other words, the animals' outer body covering. In can be very hard in oribatid mites.

## WHO ARE THE ORIBATID MITES?

Oribatid mites (formal name: Suborder Oribatida) are animals related to spiders, scorpions, and ticks, as they also have eight legs (Figures 1, 2). While many people are fearful of spiders, scorpions, and ticks, oribatid mites are not a concern for the health of humans and are actually beneficial, because they help with soil formation and with returning nutrients to the environment. Oribatid mites are also called beetle mites or armored mites because they typically have hard bodies (like beetles) to protect them from being eaten (Figures 1B,D). They are also sometimes called moss mites because they are abundant within mosses, a type of plant that grows close to the soil surface.

## WHAT DO ORIBATID MITES LOOK LIKE?

Most adult oribatid mites are brown, but species range in color from nearly white, to yellow, to reddish-brown (Figures 1A, 2D), to almost black. Males and females look very similar in most species, but the young mites rarely look like the adult mites (Figures 1E–H). The females lay eggs and when the eggs hatch, the young mites (larvae) grow through three more stages before becoming adults. At each stage, they shed their outer body covering, called the **exoskeleton**, as their bodies get bigger. Some oribatid mites carry this old exoskeleton around on their backs as a form of camouflage to protect them from predator mites. Predator mites can eat some oribatid mites. Oribatid mites are mostly oval shaped, but some are very round like a ball when they curl their legs up into their bodies (Figure 1B). All oribatid mites have heads that are attached to the rest of their bodies and eight legs that attach in the middle region of their bodies. Most oribatid mites have tiny holes in their armpits that allow them to breathe; they do not have noses like we do. However, their legs have hairs to help them feel, taste, and even smell (Figures 1C,D, 2C). Their legs also have claws on the tips, similar to human nails (Figure 1D). Most oribatid mites do
## Figure 2

Diversity of oribatid mites, with their scientific names.
**(A) Melanozetes crossleyi; (B)**
*Hydrozetes* sp. with an arrow pointing to the lenticulus, the structure that allows this oribatid mite to perceive light;
**(C) Collohmannia johnstoni; (D) Cersella** sp.;
**(E)**
*Nehypochthonius porosus* and **(F)** juvenile *Eupterotegeaus*.

<figure><img src="image_39.png" /><figcaption>Figure 2</figcaption></figure>

## LENTICULUS

The structure that allows oribatid mites to perceive light since they do not have real eyes.

## CHELICERAE

Mouthparts present in arachnids like spiders, scorpions, ticks and also oribatid mites.

not have real eyes, but they have structures that can perceive light, for example, a structure called the **lenticulus**, seen in Figure 2B. They also do not have real teeth, but instead have tooth-like structures to help them feed (Figure 1D), called **chelicerae**.

Oribatid mites are very small organisms. The majority are <1 mm long (most 0.3–0.7 mm), which is the diameter of two human hairs side-by-side, but some can be as large as a pencil tip. Because these organisms are so small, we need to use a hand lens or a microscope to observe them. Even though oribatid mites are tiny and might look fragile at first glance, they have existed for many million of years, even before dinosaurs [1]!

Tiny oribatid mites move around in soil on their eight legs, but because they are so small, they usually move only a few meters during their entire lifetime. However, other strategies help oribatid mites to get to distant places, too. Oribatid mites can hitchhike on other animals like birds, frogs, and mammals for example, and then move long distances. Another strategy is, because they are so light, they can be picked up in the wind and travel in the air! How cool!

## DEFENSIVE MECHANISMS

Oribatid mites may live for one or more years (some species up to 5 years) in their natural habitats. Because it takes them a long time to become mature adults, they do not have many babies, and they must protect themselves from predators. To protect themselves, oribatid mites have developed many different defensive mechanisms. These protective mechanisms include: hairs that raise (like a frightened cat) (Figure 1C), armored structures that cover their bodies like a tank (Figures 1D, 2A), bodies that roll into a protective ball (Figure 1B), or camouflage obtained by covering themselves in soil, their exoskeletons, or other debris. Many oribatid mites also possess
Figure 3

Oribatid mites are found in many different environments. (A) Forest (Schwangau, Germany); (B) wetland (Massachusetts, U.S.); (C) desert (Arizona, U.S.); (D) treetop (Vancouver island, Canada); (E) edge of the ocean (Espírito Santo, Brazil); and (F) edge of a lake (Ásólfsskáli, Iceland).

<figure><img src="image_40.png" /><figcaption>Figure 3</figcaption></figure>

NUTRIENT CYCLING

The movement of nutrients between living things and the earth, including the atmosphere, rivers, and soils. Oribatid mites help recycle nutrients in soils.

DECOMPOSITION

The break-down of dead plants and animals by fungi and bacteria, facilitated by oribatid mites and other soil animals.

MESOFAUNA

This term refers to intermediate-sized animals in soils. Their size ranges from 0.1 to 2 mm.

specialized defensive glands that can release unpleasant chemicals when they are attacked by their predators.

WHY ARE ORIBATID MITES IMPORTANT?

A typical diet for oribatid mites consists of different types of fungi and bacteria. Because of what they eat, oribatid mites are important for ecosystem services like **nutrient cycling**. The mites chew dead leaves, which breaks the leaves down into smaller pieces that fungi and bacteria can further decompose. When the oribatids then eat the fungi and bacteria, the nutrients from the leaves are returned to the soil after they are discharged in oribatid feces. Oribatid mites are considered part of the soil food web, helping with processes like **decomposition** that are necessary for soil formation.

WHERE DO ORIBATID MITES LIVE?

Oribatid mites have been found all over the world, from forests to deserts, and along the shores of lakes and oceans in all countries (Figure 3). They live mostly in soils, where they usually are the most abundant and diverse group of small soil animals called **mesofauna** [2]. But they can also live in other habitats, like on the bark and trunks of trees, in mosses, and on other plant leaf surfaces. Oribatid mites are also found at the top of the tallest trees, as well as in some aquatic environments. They are almost everywhere!

While there are over 10,000 species of oribatid mites known, we have good reasons to believe that there are many, many more species of oribatids that have not been discovered yet. The number of species of oribatid mites found in any location varies, depending on a number
## EXTRACTOR

Tool used to help separate oribatid mites and other soil animals from the surrounding soil. The lamp on top of it slightly heats the soil up and have the animals to leave the soil.

of factors including the type of plants in the location, the climate, the amount of moisture in the soil, the pH of the soil, the concentration of soil nutrients, and the levels of soil pollution. Different species of oribatids prefer different conditions; for example, some prefer soil with lots of nutrients while others prefer low-nutrient soil. Depending on the environment, oribatid mites can reach impressive numbers—up to 500,000 individuals per m² in forest soils [3]! This is equivalent to 4000 mites in a handful of soil!

## HOW CAN I FIND ORIBATID MITES?

If you want to observe oribatid mites and other soil organisms, you will need an **extractor** to help separate the oribatid mites from the surrounding soil, and a microscope or hand lens. In Box 1, we describe the materials you will need to build your own soil oribatid mite extractor. You will need adult supervision for this, but it is easy. Once you have your soil organisms separated from the soil, use a hand lens to see how different they all are. If you have access to a microscope, then even better!

Go to your backyard, or to a public park. Oribatid mites can be found in cities too. You will need to collect two to three handfuls of soil, leaf litter, or mosses (dig up to 10 cm deep) for your extractor. You will also need to be a little patient; it will likely take a minimum of 3 days to extract the oribatids from your soil sample. Okay, time to build the extractor!

Last, if you are interested in soils and who lives in them, there are many other activities that can be found in Chapter VII of the Global Biodiversity Atlas [4]. Links for books, games, videos, and more are all available! Have fun!

### Box 1 | Building an oribatid mite extractor

Materials:

* Two sheets of cardboard (about 100 x 60 cm)

* 2 m of adhesive laminate sheet

* Kitchen strainer (about 24 cm diameter)

* Toilet paper roll—10 cm

* Plastic vial

* Table lamp with an incandescent bulb/halogen light bulb (it needs to heat up)

* Sealing/duct tape

* Scissors

* Pencil

* Stapler

* Ruler

<figure><img src="image_41.png" /></figure>

(Continued)
Box 1 | Continued

Methods:

* Draw a semi-circle on one of the cardboard sheets. The semi-circle should have a radius of 25 cm. Cut it out using scissors (A). Cover both sides with the adhesive laminate sheet and cut a semi-circle in the middle.

* Fold it to form a funnel and staple it using the stapler (B). The funnel must have an opening at the bottom—this is where the oribatid mites and other soil animals will pass through.

* Attach the toilet paper roll to the bottom of the funnel using the tape (C).

* Cover the other piece of cardboard with the laminate sheet (D). Use it to create a 25 cm-long cylinder. Close it with staples. This will be attached later onto the big opening (top) of the funnel.

* Place the kitchen strainer on top of the funnel/bottom end of the cylinder (E).

* Use the tape to secure the cylinder to the funnel (with the strainer in between) (F).

* Put the soil sample on the strainer from the top of the cylinder.

* Put the plastic vial at the bottom end of the extractor.

* Put the table lamp near the top opening of the cylinder and turn the light on. This will encourage the oribatids to leave the soil (G).

<figure><img src="image_42.png" /></figure>

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AUTHORS CONTRIBUTIONS

CB and ZL wrote the manuscript. CB created the figures and made the manual for creating the fauna extractor.

ACKNOWLEDGMENTS

We were very grateful to Dr. Roy Norton and Dr. Valerie Behan-Pelletier, our mentors in the field of oribatid mites, for the great suggestions. Special thanks to Caitlyn Lyons, who helped us with making the language appropriate for children, and to our great young reviewers
and their Science mentors. We thank Marilia Paulon for the help building the extractor. We also thank Dr. Malte Jochum for the invite to be part of this great initiative.

## REFERENCES

1. Labandeira, C. C., Phillips, T. L., and Norton, R. A. 1997. Oribatid mites and the decomposition of plant tissues in paleozoic coal-swamp forests. *Palaios* 12:319–53.

2. Behan-Pelletier, V. M., and Bissett, B. 1992. Biodiversity of Nearctic soil arthropods. *Can. Biodivers. 2:5–14.

3. Coleman, D., Crossley, D., and Hendrix, P. F. 2004. *Fundamentals of Soil Ecology, 2nd Edn*. Amsterdam: Elsevier Academic Press, 1–386.

4. Orgiazzi, A., Bardgett, R. D., Barrios, E., Behan-Pelletier, V., Briones, M. J. I., Chotte, J. L., et al. (Eds.). 2016. *Global Soil Biodiversity Atlas*. Luxembourg: European Commission, Publications Office of the European Union.

**SUBMITTED**: 24 March 2020; **ACCEPTED**: 03 September 2020;

**PUBLISHED ONLINE**: 03 November 2020.

**EDITED BY**: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Barreto C and Lindo Z (2020) Armored Mites, Beetle Mites, or Moss Mites: The Fantastic World of Oribatida. Front. Young Minds 8:545263. doi: 10. 3389/frym.2020.545263

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2020 Barreto and Lindo. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

**ISABEL, AGE: 10**

Hi, I am Isabel and I am from Portugal. I am 10 and I like reading, writing, and music. I have three cats and I like to learn about history. I have no idea of what I want to be when I grow up. I really like vegetables (and fruit).

<figure><img src="image_49.png" /></figure>
<figure><img src="image_50.png" /></figure>

MARGARIDA, AGE: 12

My name is Margarida, I am 12 years old and I like reading, climbing, and writing.
I love science, especially anything about black holes and I have absolutely no idea
what I want to do when I grow up. I also really like biology.

AUTHORS

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CARLOS BARRETO

At a very young age Carlos realized that he liked animals, maybe too much. In school, science was always his favorite discipline, all the way through to high school. That is when he decided that he wanted to do something that involved science and animals. He tried to be a vet; it did not work out. No regrets. So, he became an Ecologist a few years later, and since then, he has been working with little animals (mostly insects and mites) in tropical forests, iron ore and limestone caves, boreal forests, urban fields, and peatlands on three continents: South America, North America, and Europe. *cbarreto@uwo.ca; orcid.org/0000-0003-2859-021X

<figure><img src="image_52.png" /></figure>

ZOË LINDO

Dr. Zoë Lindo is an expert on soil biodiversity and ecosystem function. She has worked extensively in Canadian forests, including the mixed-wood boreal of Alberta, the subarctic taiga of Quebec, the coastal temperate rainforest of British Columbia, and the black spruce/peatlands of Ontario. "The overall focus of my research aims to mitigate biodiversity loss in association with human-induced environmental change and maintain ecosystem functioning in Canadian forest and soil ecosystems. I describe myself as a biodiversity scientist to encompass the breadth of my research in the areas of community ecology, soil ecology, and taxonomy." orcid.org/0000-0001-9942-7204
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# SUPER-SMALL PREDATORS IN SOILS: WHO ARE THEY AND WHAT DO THEY DO?

Stefan Geisen<fnref n="1" />,<fnref n="2" />*

YOUNG REVIEWER:

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AYA

AGE: 10

There are millions of species living in soils. Most of this biodiversity is made up of bacteria and fungi, tiny organisms that make up what is called the soil microbiome. The size and composition of the soil microbiome is mainly controlled by two groups of predators: protists and nematodes. Protists are tiny single-celled organisms, while nematodes are tiny worms and the most numerous animals on Earth. Protists and nematodes together weight more than all the other animals on Earth! Protists and nematodes keep the soil microbiome in balance, which helps plants to grow and keeps soils functioning properly. Without protist and nematode soil predators, the functions and services provided by soils would change so much that it could even affect the Earth's climate. So, let us not forget the importance of these tiny soil organisms!

<footnote n="1">¹Laboratory of Nematology, Wageningen University, Wageningen, Netherlands</footnote>

<footnote n="2">²Department of Terrestrial Ecology, Netherlands Institute of Ecology NIOO-KNAW, Wageningen, Netherlands</footnote>
BIODIVERSITY

The variety of species
in an ecosystem.

SOIL MICROBIOME

All the microorganisms
that live in the soil.

MICROBIOME
PREDATORS

Organisms that feed on
bacteria and fungi.

MICROORGANISMS

Tiny organisms,
including bacteria,
fungi, and protists.

NEMATODES

Tiny worms that live in
all environments and
inside hosts. Most
abundant animals
on Earth.

1 For an overview of nematode diversity, see https://www.wur.nl/en/Research-Results/Chair-groups/Plant-Sciences/Laboratory-of-Nematology/Nematode-in-the-picture/Nematode-Pictures.htm.

PROTISTS

All eukaryotes except
for fungi, animals and
plants, most of which
are single celled.

EUKARYOTES

Organisms that have
nuclei in their cells, in
which DNA is located.
Eukaryotes include
fungi, protists, plants,
and animals.

SOIL BIODIVERSITY

Maybe you already know that soils are not just dark-colored dirt consisting of sand, clay, and loam; soils have high **biodiversity**, meaning that millions of organisms of all types and sizes live there. Not only do soils have more biodiversity than any other ecosystem on Earth, but the weight of all soil organisms (including plants whose roots make them about half soil organisms) is far greater than the weight of all other organisms combined [1]. Soil organisms range in size from microscopic viruses to huge fungi that can spread over hundreds of meters—and everything in between! The microscopic life in soils is called the **soil microbiome**, and it is mostly composed of bacteria and fungi. Without its biodiverse microbiome, soils would not be able to recycle nutrients and promote plant growth. This article focuses on two groups of soil organisms that few scientists study: protist and nematodes. By the time you are done reading, I hope you will understand why more research should focus on those tiny organisms!

PROTISTS AND NEMATODES: TINY PREDATORS

**Microbiome predators** are organisms that feed on **microorganisms**, such as bacteria and fungi [2]. Protists and nematodes are among the most important microbiome predators. **Nematodes** are super small—at least 100 times smaller than the width of a human hair—so they are not visible by the naked eye. The nematode most well-known to scientists is a roundworm that scientist call *Caenorhabditis elegans*, which is frequently used in research. Many scientists, doctors, farmers, and gardeners know nematodes as pests, because some species can infect humans, while others cause plant diseases. In soils, nematodes prey on bacteria, fungi, or other nematodes, and can be parasites of animals and plants. While most nematodes look basically the same, many hundreds of thousands of diverse species exist (Figure 1A)¹. Eight of 10 animals on Earth are nematodes. In other words, 60 billion nematodes exist for every single human being [3]!

Now try to imagine that there are 1,000 times more **protists** than nematodes in soils! Protists are microorganisms that cannot be classified as animals or plants. Protists are **eukaryotes**, which means they contain a nucleus, just like the cells of animals, fungi, and plants. While the term “protist” might be new to you, you have probably heard of some well-known examples of protists: all eukaryotic algae that perform photosynthesis in lakes and oceans are protists. Some diseases are caused by protists, such as the malaria-causing *Plasmodium falciparum*. Amoebae and *Paramecium* are protists, and there are many more! It is likely that most of the eukaryotic diversity on Earth is composed of protists, with many millions of existing species. But we know fewer than 1 in 100 of the protist species that exist. In soils, protists perform a large range of functions. Most protists feed
**Figure 1**

An impression of how nematodes (A) and protists (B) look like under the microscope. Similar to lions (C), which are well-known predators in savanna ecosystems, microbiome predators (D) have essential roles in underground ecosystems. These protists and nematodes feed on bacteria and fungi, changing the composition and activity these soil organisms, which helps to keep the soil healthy [image credits: A: nematodes (Anaplectus porosus and Aphelenchoides) in from Hanny van Megen]; (B) protists (Hyalosphenia papilio, Heliamoeba sp., and Mayorella viridis) from www.penard.de with permission from Eckhard Völcker and Steffen Clauß; (C) lion and zebra pictures from https://purepng.com/ (under CCO license). (D) Bacterial cartoons under the CCO license from Wikipedia.

² For an overview of protist shapes, see http://www.penard.de/.

³ Here is an online guide: https://www.ars.usda.gov/northeast-area/beltsville-md-barc/beltsville-agricultural-research-center/mycology-and-nematology-genetic-diversity-and-biology-laboratory/people/zafar-handoo/extracting-nematodes-from-soil-samples/!

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on bacteria, but many also feed on fungi or even on bigger animals. Some protists can be parasites of plants and animals, and some live in relationship with fungi, as lichens [4]. Protists have super diverse shapes and can be beautiful (Figure 1B)².

Similar to lions or other large predators, microbiome predators prey on many things they can catch (Figures 1C,D). By doing so, these small predators control microbial growth, change the kinds of organisms present in the soil ecosystem, and help the soil to perform its important functions. Microbiome predators commonly feed on the less active microbes, thereby keeping the entire microbiome more active—a feeding strategy again similar to lions that induce prey fitness by feeding on older, slower, and weaker individuals. Microbiome predation also leads to a release of nutrients into soils, which can be taken up by other microbes and plants, helping these organisms to grow.

**HOW CAN WE STUDY MICROBIOME PREDATORS?**

Since nematodes and protists are tiny and not visible in soils, scientists face the big problem of not being able to directly study them in their natural habitat. One way to study these organisms is to isolate them from the soil. This is a bit easier for nematodes, and you can even try it yourself at home, using coffee filters or tissue paper, water, and a bowl (Figure 2)³.
Figure 2

Nematode extraction at home. Place a handful of soil in two coffee filters on top of each other, close them with clothespins as shown, and place the filters in a bowl filled with water (the water should cover the soil in the filter). The next day, remove the coffee filters, mix the solution and transfer it to a thin glass. Wait and hour, then carefully pour off most of the liquid, leaving only a little in the bottom of the glass. You can now observe nematodes in the remaining solution with a magnifying glass or microscope.

DNA SEQUENCING

A method to determine the DNA sequence of organisms. It can be used to identify organisms.

<figure><img src="image_56.png" /><figcaption>Figure 2</figcaption></figure>

Studying protists is more complicated. Since most soil protists stick strongly to soil particles, the best way to study them in the past was to place tiny amounts of soil together with bacteria, to extract the bacteria-eating protists. However, only few types of protists can be grown this way, and therefore many protist species still are unknown. Today, most researchers use molecular tools to identify soil organisms. Like criminal investigators, we can use DNA extracted from soil to identify these small organisms. Since every organism has a unique DNA sequence, we can use a **DNA sequencing** technology to differentiate individual species based on their unique DNA. This can tell us which soil organisms are present, even though we cannot see them with our eyes or grow them in the lab (Figure 3).

These extraction, cultivation, and sequencing approaches show us which species are *present* in a soil sample, but not what these species *do*. There are ways to understand the functions of protists and nematodes in soils, including combining predators with various prey microorganisms to observe how they interact. Other scientific approaches can reveal what organisms feed on in soils, by tracing certain soil compounds using special methods as described in [5].

WHY SHOULD WE CARE ABOUT MICROBIOME PREDATORS?

Without microbiome predators, our soils would have a big problem... and a problem with the soil would cause problems for us! Almost all nutrients that enter soils are taken up by bacteria and fungi. If bacteria in soils were not controlled, the most numerous ones would take up all the nutrients and retain them for long periods. Microbes can remain alive in soils for months without predators, and can even enter a long-term survival stage, in which they can survive for decades.
<caption>**Figure 3**</caption>

In addition to being well hidden in the soil, microbiome predators are too small to be seen by the naked eye. Scientists often study these organisms by extracting DNA from soils and sequencing that DNA to identify all the species of protists and/or nematodes that are present [image credit: https://www.bioanalysis-zone.com/ and copyright free(CC0)].

<figure><img src="image_57.png" /><figcaption>Figure 3</figcaption></figure>

As a consequence, the food chain would stop and bigger organisms could not survive. Plants would also take up much less nutrients from soils and would grow slowly, if at all. Microbiome predators make sure that this situation does not happen. By preying on microbes and releasing some of the nutrients that those microbes take up, microbiome predators ensure that plants can access nutrients. The microbiome predators themselves can also serve as prey for larger organisms. So, microbiome predators ultimately save our food supply, by helping the soil to perform its critical functions.

## CONCLUSIONS

Protists and nematodes are among the most biodiverse and numerous organisms on Earth. Both protists and nematodes are important microbiome predators and ensure that soils function the way they should. Simply said, without microbiome predators, life in and on soils would not be possible! Indeed, microbiome predators are of key importance for plant growth and they also play critical roles in other microbial processes that occur in healthy soils. By increasing our knowledge about microbiome protists and nematodes, we will also increase our understanding of how soils work. If we can learn how to adjust the types and numbers of microbiome predators in soils, we could help to keep the soil microbiome healthy and protect soil functions. The crucial role of microbiome predators in soils clearly shows that we should not forget any group of life on the planet, because even the smallest organisms can have a unique functional importance that helps to keep many other organisms, including humans, alive!

## REFERENCES

1. Bar-On, Y. M., Phillips, R., and Milo, R. 2018. The biomass distribution on Earth. *Proc. Natl. Acad. U.S.A.* 115:6506–11. doi: 10.1073/pnas.1711842115
2. Thakur, M. P., and Geisen, S. 2019. Trophic regulations of the soil microbiome. *Trends Microbiol.* 27:771–80. doi: 10.1016/j.tim.2019.04.008

3. van den Hoogen, J., Geisen, S., Routh, D., Ferris, H., Traunspurger, W., Wardle, D. A., et al. 2019. Soil nematode abundance and functional group composition at a global scale. *Nature* 572:194–98. doi: 10.1038/s41586-019-1418-6

4. Geisen, S., Mitchell, E. A. D., Adl, S., Bonkowski, M., Dunthorn, M., Ekelund, F., et al. 2018. Soil protists: a fertile frontier in soil biology research. *FEMS Microbiol. Rev.* 42:293–323. doi: 10.1093/femsre/fuy006

5. Erktan, A., Pollierer, M., and Scheu, S. 2020. Soil ecologists as detectives discovering who eats whom or what in the soil. *Front. Young Minds* 8:544803. doi: 10.3389/frym.2020.544803

**SUBMITTED**: 21 August 2020; **ACCEPTED**: 07 October 2021;

**PUBLISHED ONLINE**: 23 November 2021.

**EDITED BY**: Rémy Beugnon, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Geisen S (2021) Super-Small Predators in Soils: Who Are They and What Do They Do? *Front. Young Minds* 9:597620. doi: 10.3389/frym.2021.597620

**CONFLICT OF INTEREST**: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Geisen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWER

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**AYA, AGE: 10**

Aya wants to study marine biology. She wants to specialize in sharks and rays. Her favorite subjects in school are reading, writing, math, and music. In her free time she likes to read books, try out challenging puzzles, train for track and cross country, and play the violin.

## AUTHOR

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**STEFAN GEISEN**

Stefan is an Assistant Professor at the Laboratory of Nematology at Wageningen University, the Netherlands. He tries to make sense of all the living organisms in soil.
Meaning, he wants to explore what lives in soil and what these organisms do for the ecosystem and for us by steering plant growth. A major focus of his work is on protists and nematodes, small organisms that are the most important predators of microbes in soil. He is father of three boys and in the remaining time he likes to meet friends and do sports. *stefan.geisen@wur.nl
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# THE SOIL FUNGI: A WEB OF LIFE THAT PROTECTS TREES AND FIGHT CLIMATE CHANGE

Olivia Azevedo<fnref n="1" />* and Frank Ashwood<fnref n="2" />

## YOUNG REVIEWERS:

<figure><img src="image_61.png" /><figcaption>ANNA</figcaption></figure>

AGE: 16

<figure><img src="image_62.png" /><figcaption>CATHERINE</figcaption></figure>

AGE: 15

Ectomycorrhizal fungi are a type of fungi that develops a mutually beneficial relationship with plant roots. These fungi form ancient and extremely successful partnerships with forest trees worldwide. The trees and their associated fungi have developed a trading partnership: the fungi help the plants reach hard-to-get nutrients, and, in return, the fungi get constant and uninterrupted access to carbohydrates (such as sugars) from the plant. This largely invisible interaction affects the storage and cycling of carbon in soil and benefits plant health and nutrition. Ectomycorrhizal fungi are also important for breaking down dead plants and animals. These fungi contribute to soil biodiversity and can help us to protect our forests in the face of environmental stresses, such as climate change and excessive land use.

<footnote n="1">¹Biological and Environmental Sciences, University of Stirling, Stirling, United Kingdom</footnote>

<footnote n="2">²Forest Research, Northern Research Station, Roslin, United Kingdom</footnote>
### DECOMPOSER

They are bacteria, fungi or invertebrates that break down dead plants and animals, releasing nutrients back into the soil. Without them, dead organisms and waste would just pile up and plants would not be able to get essential nutrients.

### ORGANIC MATTER

Any material produced originally by living organism such as plants, animals and microorganisms that is returned to the soil and can be further broken down (decomposed). Organic matter is carbon rich.

### SYMBIOSIS

Any type of close, long-term biological interaction between two different organisms.

### MUTUALISM

A relationship between two or more species, in which each species benefits.

### MYCORRHIZAL FUNGI

Fungi that form a mutualistic relationship with the roots of plants. The plants receive nutrients and protection from the fungi and the fungi receive sugars from the plants.

## FUNGI AND THEIR ROLE IN FOREST ECOSYSTEMS

We often hear that variety is the spice of life. This statement can easily be applied to the many interactions in nature, such as those taking place in forest ecosystems. To lead long and healthy lives, nearly all plants in the wild rely on a complex and varied network of soil organisms feeding on one another. This mostly invisible underground network is made up of tiny bacteria, archaea, fungi, and many other microscopic organisms.

In forest soils, the role of fungi is one crucial piece of the wider ecological network. Fungi have many ecological roles, but two of these are especially important. First, fungi play an important role as **decomposers**. Fungi excel at decomposing dead plant material (called **organic matter**) because they are better than other organisms at completely breaking down the particularly tough materials found in the cells of woody plants [1]. Equipped with a wide range of enzymes, which are special proteins that help produce and speed up chemical reactions, fungi can degrade organic matter and release hard-to-get nutrients, making the nutrients available to plants and other soil dwellers. However, during decomposition, fungi release CO2 gas as a waste product, which results in movement of carbon from the soil into the atmosphere. Fungi are such excellent decomposers that fungal decomposition is one of the largest global sources of carbon emissions, releasing 85 gigatonnes (one gigatonne equals 1 billion tons) of carbon into the atmosphere every year. For comparison, in 2018, the combustion of fossil fuels produced around 10 gigatonnes [2].

In this article, we will focus on a different important role also played by fungi: their **symbiotic** relationship with trees and other plants. A symbiotic relationship in which both species benefit is known as **mutualism**. Fungi that form mutualistic relationships with plants are known as **mycorrhizal fungi**, from "myco," which means "relating to fungi" and "rhizal," which means "roots." Mycorrhizal fungi form ancient, mutualistic relationships with the roots of around 80% of all terrestrial plant species [1]. Even seemingly barren Antarctica has a fossil record of mycorrhizal communities. Studies show that this teamwork has been ongoing for 400 million years, from the time that plants started to colonize land [3]. Like all fungi, mycorrhizae cannot make their own food, so they receive sugars from their plant hosts, and in exchange provide the plants with water and nutrients, such as nitrogen and phosphorus, from the soil.

Unlike the decomposing fungi that tend to predominantly inhabit the upper portion of the soil and release lots of carbon from their activities, symbiotic mycorrhizal fungi delve deeper into the soil. By doing this, the fungal network becomes a significant carbon sink in the soil, meaning that these fungi keep the carbon locked away from the atmosphere, stored as hard-to-decompose organic matter in the
Figure 1

The ectomycorrhizal fungus *Lactarius camphoratus*, forming a white sheath around the roots of an oak tree (Photograph credit: Laura Martinez-Suz).

<figure><img src="image_63.png" /><figcaption>Figure 1</figcaption></figure>

soil. It has been estimated that plants with mycorrhizal association can transfer up to 35% more carbon to soil than non-mycorrhizal plants, and a considerable amount of the carbon in mycorrhizal tissues may stay in the soil for many years [4]. This is important because we need to keep carbon stored in the soil for long periods of time so that there is less in the atmosphere causing rising global temperatures.

Despite the critical roles that fungi play in forest ecosystems, fungal diversity is often overlooked during forest management decisions. Human interference, such as logging or indiscriminate use of fertilizers, can alter the underground network and upset the balance of the entire ecosystem. Like any other web, if just one of the connecting segments is missing or weakened, the whole structure may suffer.

ECTOMYCORRHIZA

The relationship between a fungus and the roots of certain plants (plural, ectomycorrhizas, or ectomycorrhizae).

FRUITING BODY

Structures made by fungi so they can reproduce. A mushroom is a common type of fruiting body.

HYPHAE

Long, branching structures of a fungus that spread through soil to absorb and transport nutrients (singular, hypha).

THE FUNGAL SPHERE

There are two main types of mycorrhizal fungi. One type, called endomycorrhiza, live inside plant cells. While we will not focus on these in this article, they are interesting because they are incredible experts in adaptation to many different environments. Endomycorrhizal fungi include arbuscular, ericoid and orchid mycorrhizas. However, the main focus of our article is **ectomycorrhiza** (pl. ectomycorrhizae), which live on the outside of plant cell walls. While multiple mycorrhizal types can coexist in an ecosystem, ectomycorrhiza are dominant in temperate and boreal forests, with ~6,000 fungal species establishing symbiotic associations with many trees and woody plants.

Ectomycorrhizae consist of two types of important structures: **fruitsing bodies** and **hyphae**. Fruiting bodies are structures containing spores used by ectomycorrhizae in order to reproduce. Around 4,500 species of ectomycorrhizae have above-ground fruiting bodies (mushrooms),
<caption>Figure 2</caption>

The well-known poisonous fungus called fly agaric (*Amanita muscaria*) can form a mycorrhizal association with several types of trees, including pine trees, as shown here (Image credit: Angela Mele).

<figure><img src="image_64.png" /><figcaption>Figure 2</figcaption></figure>

while up to a quarter have underground fruiting bodies (for example, truffles). Ectomycorrhizae also have hyphae (from Greek meaning "web"), which are long filaments or tube-like structures used by the fungi to absorb and transport nutrients. Hyphae form a mat of fungal tissue around plant roots, creating a casing that envelops the roots like a cast on a broken bone (Figure 1). Fungal hyphae also grow outwards like veins, pushing their way between soil particles, roots, and rocks to capture nutrients that are beyond the normal reach of plant roots. The ectomycorrhizal association also produces antibiotics, hormones, and vitamins useful to the plant, and protects plant roots from harmful conditions in the soil such as low nutrients, disease-causing organisms, and toxic substances. In return, the fungi gain constant and direct access to carbohydrates (such as sugars) produced by their plant hosts during photosynthesis.

ECTOMYCORRHIZAE AND FORESTS: TEAMWORK AT ITS
BEST

Ectomycorrhizae prefer woody plant species such as trees and shrubs as their partners. Occasionally they can form exclusive relationships, in which only one fungal species pairs up with a specific tree species. However, ectomycorrhizae typically associate with a wide range of tree species. It is common to find several different mycorrhizal fungi on the root system of a single tree, or one fungal species that is associated with several different tree species. For instance, the Norway spruce can form symbiotic associations with over 100 different fungal species. The well-known poisonous fungus called the fly agaric can colonize the roots of several types of trees, including pine, birch, spruce, and eucalyptus (Figure 2).

The range of plant species colonized by ectomycorrhizae is relatively
small—only around 2% of the world's plants. However, the plants
that ectomycorrhizae partner with cover large land areas and have
a high economic value, for example as a source of timber. In northern temperate regions, pine, poplar, spruce, fir, willow, beech, birch, and oak trees all show ectomycorrhizal associations, while eucalyptus and southern beeches are more commonly associated with ectomycorrhizae in the southern hemisphere.

Ectomycorrhizae give trees and forests the ability to adapt to seasonal and landscape changes, for example by providing adequate levels of water throughout the year and helping the plants to establish themselves in new soils. The fungi also protect the plants from soil degradation, pollution, and shifting climatic conditions. Scientists have observed a direct relationship between the decline of ectomycorrhizal fungi and declining tree health. Because each type of ectomycorrhizal fungus has its own unique set of characteristics, every species is necessary and irreplaceable. For example, certain species prefer cool or moist conditions; others operate better during warm or dry seasons; some are experts at obtaining phosphorus and nitrogen from the soil; and still others are more effective at getting these nutrients from decaying organic matter [5].

Ectomycorrhizal fungi are also an extremely important link between plants and the soil food web, which is the complex community of organisms found in soil. These fungi provide important nutrients to organisms living in the soil around plant roots, such as other tiny fungi, bacteria, protozoa, and invertebrates. The fungi also produce fruiting bodies, which are essential food for wildlife in forest ecosystems. For example, many rodents, such as the northern flying squirrel and western red-backed vole, depend on truffles as their primary food source. Many other mammals eat fungi, including bears, deer, and mice. Humans marvel at the complex and often pretty structures of mushrooms, taking great joy from learning to identify them and studying their ecology, as well as enjoying them as culinary delicacies. Wild fungi are also used for medicines, and the pharmaceutical industry commonly studies the antibacterial properties of ectomycorrhizal fungi.

## PROTECTING OUR FORESTS AND FUNGI

Despite the importance of mycorrhizal symbiosis in forests worldwide, conservation of ectomycorrhizal fungi and monitoring them to assess forest health are rarely considered in forest-management decisions. The functions and services that forests provide are dependent on soil biodiversity. Fungi are a major component of this biodiversity, which makes them an important partner for overcoming the global challenges that we currently face. Fungi could contribute to the long-term removal of carbon from the atmosphere, which could help us to combat the effects of climate change. By cycling essential nutrients, fungi could also help to prevent the degradation of soils, so that the land can continue to produce food and sustain
life. Scientists and land managers must continue to study and protect ectomycorrhizal fungi, so that these important organisms can maintain their critical role in the web of life on our planet.

## ACKNOWLEDGMENTS

We would like to thank the financial and/or logistical support of University of Stirling, Forest Research, Natural England, Woodland Trust, Forestry Commission Scotland, and Tarmac.

## REFERENCES

1. van der Heijden, M. G. A., Martin, F. M., Selosse, M. A., and Sanders, I. R. 2015. Mycorrhizal ecology and evolution: the past, the present, and the future. *New Phytol.* 205:1406–23. doi: 10.1111/nph.13288

2. Friedlingstein, P., Jones, M. W., O'sullivan, M., Andrew, R. M., Hauck, J., Peters, G. P., et al. 2019. Global carbon budget 2019. *Earth Syst. Sci. Data.* 11:1783–838. doi: 10.5194/essd-11-1783-2019

3. Brundrett, M. C. 2002. Coevolution of roots and mycorrhizas of land plants. *New Phytol.* 154:275–304. doi: 10.1046/j.1469-8137.2002.00397.x

4. Frey, S. D. 2019. Mycorrhizal fungi as mediators of soil organic matter dynamics. *Annu. Rev. Ecol. Evol. Syst.* 50:237–59. doi: 10.1146/annurev-ecolsys-110617-062331

5. Amaranthus, M. P. 1998. *The Importance and Conservation of Ectomycorrhizal Fungal Diversity in Forest Ecosystems: Lessons From Europe and the Pacific Northwest*. Portland, OR. doi: 10.2737/PNW-GTR-431

**SUBMITTED**: 12 January 2021; **ACCEPTED**: 14 February 2022;

**PUBLISHED ONLINE**: 16 March 2022.

**EDITOR**: Rémy Beugnon, German Centre for Integrative Biodiversity Research (iDiv), Germany

**SCIENCE MENTOR**: Ryan Thomas Weir

**CITATION**: Azevedo O and Ashwood F (2022) The Soil Fungi: A Web of Life That Protects Trees and Fight Climate Change. Front. Young Minds 10:652660. doi: 10.3389/frym.2022.652660

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2022 Azevedo and Ashwood. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice.
No use, distribution or reproduction is permitted which does not comply with these
terms.

YOUNG REVIEWERS

ANNA, AGE: 16

I wanted to do Frontiers for Young Minds because I thought it would be a great opportunity to learn more about the world around me! I love the sciences, particularly biology and physics. After school I would love to do something with those subjects.

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CATHERINE, AGE: 15

I love music and singing, I play the violin and guitar and I also enjoy writing! I am part of a highland dance troup and volunteer with children at local kids clubs and guides. I enjoy attending youth events at my church and doing fitness. I hoped that by reviewing these articles I could learn about new and interesting stuff!

<figure><img src="image_66.png" /></figure>

AUTHORS

OLIVIA AZEVEDO

I am a former journalist who decided to become a soil scientist. I moved from Portugal to Scotland where I am currently a Ph.D., student at the University of Stirling. My research involves a lot of digging in forest soils in the hope of finding answers to one important question: what happens to soils and the things living in soils when you plant trees. My research interests include, but are not limited to, the following areas: Soil biodiversity; nutrient cycling; mutualistic relationships linking the above and belowground components of terrestrial ecosystems; soil degradation and management; historical soil landscapes. *olivia.azevedo@stir.ac.uk

<figure><img src="image_67.png" /></figure>

FRANK ASHWOOD

A passion for nature encouraged me to study Biology at university, where I volunteered for research projects on invertebrate ecology in Scotland and Mexico. After working as an environmental consultant for a few years, I went back to Uni and did a Ph.D., studying earthworms on reclaimed landfill sites. I now have a great job as a soil ecologist for Forest Research, where I study soil biodiversity in UK woodlands. In my spare time I am a soil biology tutor and do macrophotography (taking photos of the tiny animals living in soil).

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# FROM THE SOIL TO THE CLUB IN THE ROOTS: CLUBROOT

Edel Pérez-López*

Département de Phytologie, Faculté des Sciences de l'Agriculture et de l'Alimentation, Centre de Recherche et d'Innovation des Végétaux, Université Laval, Québec City, QC, Canada

## YOUNG REVIEWER:

<figure><img src="image_70.png" /></figure>

JUNIPER

AGE: 11

Among the millions of microorganisms inhabiting the soils, some can be plant pathogens, meaning they can become a disease to plants. Some diseases are more well-known than others. This is the case of clubroot, a very atypical microorganism that infects cruciferous plants, such as cabbage, kale, canola, and the common research plant thale cress. In this article, I will tell you more about clubroot and clubroot disease because there is still a lot to discover about the pathogen and the disease. Maybe you will be part of our lab in the future and investigate a fascinating soil-borne pathogen.

## A TALE OF PROTISTS AND PLANTS

Soils are alive. Insects, worms, and microorganisms live in the soil and many of them live in close interaction with plants. Those interactions are not always beneficial for the plants. The soil also harbors creatures that represent a risk for ecologically and economically important plants. Some fungi and worms are
## Figure 1

Two important plant hosts susceptible to clubroot: thale cress and rapeseed. Figure created with BioRender.

<figure><img src="image_71.png" /><figcaption>Figure 1</figcaption></figure>

## SOIL-BORNE PATHOGEN

Microorganisms that survive and move about in the soil and can make plants ill.

## PROTISTS

Tiny, one-celled organisms that are not animals, plants, or fungi.

## CRUCIFEROUS VEGETABLES

Vegetables of the family Brassicaceae, including many green leafy vegetables like cauliflower, cabbage, kale, garden cress, bok choy, broccoli, and Brussels sprouts.

well-known **soil-borne pathogens** for plants, but they are not the only ones. Some **protists** are also devastating soil-borne plant pathogens, although they are less studied and known. Protists are one-celled organisms that are not animals, plants, or fungi.

The clubroot pathogen, or *Plasmodiophora brassicae* like scientist call it, is a protist that affects the roots of **cruciferous vegetables**, such as broccoli, cabbage, cauliflower, the oilseed plant known as canola, and thale cress that is commonly used in plant research [1]. Thale cress and canola are one important host for clubroot, but for different reasons (Figure 1). Thale cress is a very useful plant for scientists that study various aspects of plants, such as their genomes, ecology, and relationships with beneficial microorganisms or pathogens [2]. Thale cress plants infected by clubroot are the perfect model to study how the pathogen affects the plant. Another plant that is extremely important from an economic point of view is canola (Figure 1). Canola is the source of canola oil, a product used in many kitchens around the world. In Canada alone, canola represents around $26 billion dollars (CAD) for the Canadian economy, so a pathogen that puts canola production at risk could be devastating for the economy [1].
Figure 2

Lifecycle of clubroot and establishment of clubroot disease. (A) Clubroot can exist for years as a resting spore in the soil. When the spore contacts a susceptible plant, turns into zoospore and, infects the roots and forms galls. The infected roots eventually release more spores into the soil, which can go on to infect new plants. (B) Healthy plant vs. a clubroot infected plant. (A) Based on Auer and Ludwig-Müller [3] and created with BioRender.

<figure><img src="image_72.png" /></figure>

B

Expectation

Reality

<figure><img src="image_73.png" /></figure>

<figure><img src="image_74.png" /></figure>

<figure><img src="image_75.png" /></figure>

<figure><img src="image_76.png" /><figcaption>Figure 2</figcaption></figure>

SPORE

Structure created by clubroot that has a thick cell wall to help it survive for many years in the soil when it is not able to infect a plant.

GALLS

Abnormal swelling or outgrowth of plant tissues caused by microbial plant pathogens or insects.

THE LIFE CYCLE OF CLUBROOT

The clubroot pathogen can live in the soil for up to 20 years as an inactive **spore**. Spores are structures created by some plant pathogens that are released to the environment and have a thick cell wall to help it survive for many years in the soil when are not able to infect a plant. If a susceptible plant grows in soil containing clubroot spores, then the resting spores "wake up" turning now in zoospores and attach to the roots of the plant and start an infection. The infection process is divided into two phases: primary infection and secondary infection (Figure 2A). During primary infection, the clubroot pathogen encounter the roots and penetrate, starting the infection. Although primary infection is important, it is during secondary infection that clubroot induces **galls**, also called clubs, in the roots of susceptible plants. Galls are abnormal swelling of plant tissue induced by plant pathogens or insects. The galls prevent the plant from absorbing water and nutrients from the soil and eventually cause the plants to die. The clubroot life cycle concludes with the production of new resting spores, which are released back to the soil, ready to infect a new plant.

CLUBROOT OR A CLUB IN THE ROOTS?

The clubs are the reason why this disease is known as clubroot. When I heard of clubroot for the first time, I thought it meant "a club in the roots." However, as fun as it sounds, that expectation is quite different from reality (Figure 2B). If anyone is having fun in the infected root, it is definitely not the plant! The reality is that the clubs induced by
<caption>Figure 3</caption>

Canola (A), broccoli (B), arugula (C), and kale (D,E) roots affected by clubroot, and healthy canola roots (F).

<figure><img src="image_77.png" /></figure>

clubroot are devastating. In the laboratory, we found that 21 days after adding clubroot to the soil, susceptible plants start to die. The xylem and phloem, which are the channels used by plants to transport water and nutrients, are totally blocked. It is also interesting to note that the galls caused by clubroot are quite similar in different plant hosts (Figure 3).

## HOW CAN WE MANAGE CLUBROOT AND FIGHT CLUBROOT DISEASE?

Although clubroot and the disease it causes were discovered back in the 1600s, there are a lot of things that scientists do not know [1]. To manage clubroot, farmers have been using chemicals, such as fungicides, with little success. Growing plants that are resistant to clubroot disease is the best strategy. However, after only a few years, clubroot develops new strategies to infect those resistant plants, making them susceptible to clubroot disease.

We are trying to fight an enemy that we do not understand very well, so the first step in the war against clubroot disease is to uncover all the tricks that this pathogen is hiding up its sleeves. In the University Laval (Quebec City, Canada), scientists are taking important steps toward understanding clubroot and the disease it causes. They want to know how clubroot escapes the plant immune system and how it can make the cells grow to form the galls. Their ultimate goal is to develop plants that are resistant to clubroot disease that can survive in a field with or without clubroot spores in it. Maybe in a few years you, the young curious mind reading this, will be part of the team tackling this intriguing pathogen.
## REFERENCES

1. Botero, A., García, C., Gossen, B. D., Strelkov, S. E., Toddd, C. D., Bonham-Smith, P. C., et al. 2019. Clubroot disease in Latin America: distribution and management strategies. *Plant Pathol.* 68:827-33. doi: 10.1111/ppa.13013

2. Krämer, U. 2015. Planting molecular functions in an ecological context with *Arabidopsis thaliana*. *eLife* 4:e06100. doi: 10.7554/eLife.06100

3. Auer, S., and Ludwig-Müller, J. 2015. Biological control of clubroot (*Plasmodiophora brassicae*) by the endophytic fungus *Acremonium alternatum*. *J. Endocytobiosis Cell Res.* 26:46-9. Available online at: https://zs.thulb.uni-jena.de/receive/jportal_jparticle_00346200

**SUBMITTED**: 16 May 2020; **ACCEPTED**: 20 May 2021;

**PUBLISHED ONLINE**: 15 June 2021.

**EDITED BY**: Helen Phillips, Saint Mary's University, Canada

**CITATION**: Pérez-López E (2021) From the Soil to the Club in the Roots: Clubroot. Front. Young Minds 9:562915. doi: 10.3389/frym.2021.562915

**CONFLICT OF INTEREST**: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Pérez-López. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWER

**JUNIPER, AGE: 11**

I like reading and coding. I also like animals. My favorite subjects in school are mathematics and art.

<figure><img src="image_78.png" /></figure>

## AUTHOR

**EDEL PÉREZ-LÓPEZ**

I am an Assistant Professor at the department of Plant Sciences at the University Laval, Quebec City, Canada. I am from a small agricultural town in Cuba and agriculture was always part of my life. That is why I decided to study biochemistry and to investigate the pathogens affecting plants and agriculture. In 2018, when I first learned about clubroot and clubroot disease, I felt in love with that intriguing pathogen, and it is now a central part of my research. I hope that someday one of you, a curious mind reading this, can be part of Edelab (https://edelabcriv.com/) to discover the mysteries behind pathogen-plant interactions. *edel.perez-lopez.1@ulaval.ca

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# HAVING BABIES IN SOIL: IS SEX REALLY NECESSARY?

Hüsna Öztoprak<fnref n="1" />*, Alexander Brandt<fnref n="2" />,<fnref n="3" />, Marcel D. Solbach<fnref n="4" />, Jens Bast<fnref n="1" /> and Ina Schaefer<fnref n="3" />

## YOUNG REVIEWERS:

<figure><img src="image_81.png" /></figure>

DARIO

AGE: 14

<figure><img src="image_82.png" /></figure>

LUVENA

AGE: 11

<figure><img src="image_83.png" /></figure>

PRANATEE

AGE: 12

<figure><img src="image_84.png" /></figure>

VALERIE

AGE: 13

Finding a partner and having sex to produce babies is a common way to reproduce. Yet, upon closer look, we see that nature provides many ways for reproduction. What about a world without males? What first sounds impossible is the reality for many organisms that reproduce asexually, meaning without having sex. Females produce daughters that are clones of themselves, so no partner is required and males are dispensable. An Example of such all-female societies are several species of oribatid mites, which live in soils. These mites were already on earth long before the dinosaurs. Have oribatid mites always been asexual? Why do they reproduce without males? Does asexual reproduction have any advantages? Keep reading to learn about asexual reproduction and why oribatid mites are a key organism to investigate the question, "Why sex?".

<footnote n="1">¹Bast Lab, Institute of Zoology, University of Cologne, Cologne, Germany</footnote>

<footnote n="2">²Schwander Lab, Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland</footnote>

<footnote n="3">³Animal Ecology, Johann-Friedrich-Blumenbach Institute of Zoology and Anthropology, University of Göttingen, Göttingen, Germany</footnote>

<footnote n="4">⁴Terrestrial Ecology, Institute of Zoology, University of Cologne, Cologne, Germany</footnote>
## Figure 1

Overview of different reproductive modes described in the text. (A) In hermaphroditism, each individual possesses both male and female reproductive organs; earthworms fertilize each other and each individual lays eggs. (B) In binary fission, a cell divides into two cells of equal size after duplicating its genetic material. Two individuals are produced from a single parent cell. This method is used by many Protists, for example amoebae. (C) Parthenogenesis is a form of asexual reproduction in which an offspring develops from an unfertilized egg cell. Some oribatid mites reproduce via parthenogenesis.

<figure><img src="image_85.png" /><figcaption>Figure 1</figcaption></figure>

## SEXUAL REPRODUCTION

Reproductive mode that requires an egg cell of a female and sperm cell of a male individual. The offspring is a unique individual that carries half of each parent's DNA.

### ZYGOTE

A cell resulting from the fusion of a male's sperm cell with a female's egg cell i.e., a zygote is a fertilized egg.

### HERMAPHRODITE

A single individual that can produce both egg cells and sperm cells. It is basically both a female and a male.

## REPRODUCTION IN SOIL ORGANISMS

All living organisms reproduce to generate new offspring. Almost all organisms, including humans, use some form of sex to reproduce. In **sexual reproduction**, an egg cell produced by a female and a sperm cell produced by a male fuse. The result is a **zygote** that develops into a unique offspring. Each offspring is a mixture of its parents, since it inherits half of its DNA from its mother and half from its father. The new offspring grows, becomes an adult, finds a partner, and finally produces offspring itself. This is the circle of life.

Many kinds of organisms live in soil, and they have a variety of different ways to reproduce. For example, earthworms are **hermaphrodites**, which means that one worm has both male and female reproductive organs. Earthworms reproduce sexually—if two earthworms meet, they exchange sperm cells that fuse with the egg cells of the other earthworm (Figure 1A). Since they have both male and female sex organs, earthworms do not have to worry about finding a partner of the opposite sex because there is always a match. Earthworms belong to a group of soil organisms called the macrofauna, which includes all animals larger than 2 mm. Macrofauna are soil giants compared to most soil organisms.
## PROTIST

Single celled organism that are eukaryotes. Many feed on bacteria and release excess nitrogen, providing essential nutrients for plants and other organisms.

## ASEXUAL REPRODUCTION

Any type of reproductive mode that produces offspring without the fusion of egg and sperm cells, also called gametes. The offspring are identical to the parent and/or to each other.

## BINARY FISSION

A cell divides into two cells of equal size after duplicating its genetic material. Two individuals are produced from a single parent cell i.e., one daughter and one parent cell.

## PARTHENOGENESIS

A form of asexual reproduction in which an offspring develops from an unfertilized egg cell.

## EUKARYOTES

Can be animals, plants, single-celled organisms like protists and fungi. These are complex cells, where the genetic information is organized in a nucleus.

The greatest number of soil organisms belongs to the microfauna, which consists of organisms smaller than 0.1 mm. Most are single-celled organisms called **protists**. They are so small that one handful of soil contains more protists than there are humans in the entire world. For reproduction, protists do not need a partner at all. Some protists reproduce via a type of **asexual reproduction**, by making exact copies of themselves through a process called **binary fission**. First, they duplicate all their genetic material, and then one cell divides in two (Figure 1B). If repeated several times, many identical copies of one individual independently populate the soil.

Another essential group of soil organisms is known as the mesofauna, which includes all soil animals between 0.1 and 2 millimeters in size. The soil mesofauna includes springtails and oribatid mites. These organisms are very common and play a key role in the soil food web. They shred dead organic matter from plants, making the nutrients available to other organisms, including bacteria and fungi, which are then eaten by other belowground organisms. Many oribatid mites live in all-female populations and have done so for millions of years. They do not need sex for reproduction, because they can lay eggs that develop without being fertilized by a male, in a process called **parthenogenesis** (Figure 1C). Each egg contains the DNA only of the mother, which means that the offspring are clones of the mother.

## ADVANTAGES OF ASEXUAL REPRODUCTION

Copying or cloning oneself seems to be much easier than finding a partner, but there are even more advantages of asexual reproduction. If you follow a sexual and an asexual population over time, two major differences appear. An asexual female only produces daughters, and these daughters produce only daughters again when they reproduce. The sexual females, however, must produce sons to fertilize the eggs—but only daughters can produce offspring. So, even if sexual and asexual females produce the same number of offspring, the asexual female has more daughters, which means more offspring that can reproduce.

Over time, the asexual population grows much faster and may outcompete the sexual population just by numbers (Figure 2). Scientists call this the "cost of males." In addition to providing greater population growth, asexual reproduction seems to have other advantages: no sexually transmitted diseases, no energy loss, and no chance of being eaten by predators while trying to find a mate.

## WHAT IS THE USE OF SEX IF REPRODUCTION CAN HAPPEN WITHOUT IT?

If there are successful ways of asexual reproduction, why do **eukaryotes** bother with complex, risky, and costly sexual reproduction?
<caption>**Figure 2**</caption>

Predicted population growth for sexual and asexual populations. In this example, each female produces two offspring. The size of the sexually reproducing population remains constant over time, because males are needed to fertilize the eggs of the females, but they cannot have offspring themselves. In asexual reproduction, the female produces twice as many childbearing offspring (females), leading to exponential population growth. (F0: parental generation, F1: first set of offspring from parents, F2: next generation of offspring from F1).

<figure><img src="image_86.png" /><figcaption>Figure 2</figcaption></figure>

More than 98% of all animals use sex to reproduce. This means that sexual reproduction must have clear advantages over asexual reproduction. Therefore, scientists try to explain the benefits of sex mainly by looking for potential problems if sex is not used. One disadvantage that scientists proposed for asexual reproduction has to do with mutations. Mutations are changes in the DNA that are an important cause of diversity among organisms. Occasionally, mutations are beneficial—sometimes they are very harmful—and most of the time they are slightly harmful. When an organism copies itself all the time, slightly harmful mutations keep adding up over generations, causing more and more harm. Once they build up enough, these mutations could lead to the extinction of the species. This does not happen in sexual organisms, because the harmful mutations of one parent can be compensated for by the unmutated DNA from the other parent. Think of two bikes, one with a flat tire and one with a broken pedal. By combining parts from the two, you could still have a functional bike. It would be preferable to have one fully functional instead of two semi-functional ones. Some scientists suggest that this repair mechanism is an essential advantage of sexual over asexual reproduction.

Furthermore, just making copies or clones means that an organism will stay the same for many generations. This leads to problems when the environment changes. For example, the availability of resources such as food could change over time, e.g., due to climate change or the presence of other organisms competing for food. Food may develop a defense strategy, like running faster or becoming poisonous. Parasites can also be a problem. Since all individuals in an asexual population are very similar, they will not have the tiny differences that could help them adapt quickly enough to the ever-changing environment, so they would eventually die out. This means that offspring produced through
sexual reproduction have higher chances of survival simply because they are different from those that came before them. This seems to tell us that all asexual organisms should go extinct in the long term. But, knowing that so many asexual organisms thrive in soil, we asked whether asexual organisms truly are doomed to die.

## GENES CAN TELL US ABOUT ASEXUAL REPRODUCTION

Our research groups work with soil-living oribatid mites because many asexual species have survived without males for millions of years. We analyze the genes of sexual and asexual oribatid mite species because genes can tell us what happened to organisms in the past. Imagine genes as a captain's logbook, in which important things that happened to an organism in the past were recorded and passed on to future generations. We are still able to see the disadvantages of asexual reproduction in these logbooks. If we compare logbooks of oribatid mites that reproduce sexually to those that reproduce asexually, we can figure out which of the above-mentioned problems occurred and how they were solved. Since we are comparing two very similar species of mites, most of the genes are very similar. However, for some genes we can identify certain differences that must be caused by the consequences of the different reproductive modes.

In contrast to what scientists theorized, we found that asexual species do not accumulate more harmful mutations than sexual species (Figure 3) [1]. They do not need to combine two broken bikes to get a functional bike, because they keep the bikes functional. We also found that the asexual mites do maintain variability in their large populations [2]. The genes in separate mother-daughter lines are as different from each other as are individuals that mix genes by reproducing sexually. Last, we found that the genes of two (or more) populations of asexual oribatid mites can be as varied as the genes of sexual species. This means that asexual oribatid mites do not stay the same for many generations, so they can adapt to new environments and are even able to diverge into new species [3].

From our research, it appears that asexual oribatid mites have no disadvantages compared with the ones that reproduce sexually. They keep mutations in check, maintain genetic variability, and adapt over time, all without sex!

## WHY IS THIS IMPORTANT?

There are many ways to produce offspring, and they do not always include sex. In theory, asexual species are expected to go extinct. In nature, however, there are various asexual organisms that have persisted over time. There must be ways to overcome the disadvantages of asexuality, as we saw in the asexual oribatid mites.
<caption>**Figure 3**</caption>

Asexual oribatid mites get rid of slightly harmful mutations even more effectively than sexual oribatid mites [1]. Asexuals are even better at maintaining healthy genes (indicated by the fancy blue bike) than the sexual species (which still have decent red bikes). A boxplot shows the middle 50% of the data, each with a line in the middle representing the median (center of data). The two lines outside the box indicate the highest (maximum) and lowest (minimum) observations. The three stars on top indicate that this result is statistically significant, meaning it is very unlikely to observe this just by chance.

<figure><img src="image_87.png" /><figcaption>Figure 3</figcaption></figure>

But how do they do it? Do they have special mechanisms that fix mutations? Do they have to maintain a large population size to keep genetic variability? Is it easier to be asexual if you feed on a dead food source, which cannot develop defense strategies that you must adapt to? One future challenge will be to find the mechanisms that most asexual organisms have in common. Scientists continue to work on pairs of related sexual and asexual organisms to shed light on the variety of reproduction modes and to attempt to answer the question, "Why is sex so common?"

## ACKNOWLEDGMENTS

We thank Mascha Gavrik for valuable comments on the manuscript.

## REFERENCES

1. Brandt, A., Schaefer, I., Glanz, J., Schwander, T., Maraun, M., Scheu, S., et al. 2017. Effective purifying selection in ancient asexual oribatid mites. *Nat. Commun.* 8:873. doi: 10.1038/s41467-017-01002-8
2. Palmer, S. C., and Norton, R. A. 1992. Genetic diversity in thelytokous oribatid mites (Acari; Acariformes: Desmonomata). *Biochem. Syst. Ecol.* 20:219-31. doi: 10.1016/0305-1978(92)90056-J

3. Heethoff, M., Domes, K., Laumann, M., Maraun, M., Norton, R. A., and Scheu, S. 2007. High genetic divergences indicate ancient separation of parthenogenetic lineages of the oribatid mite *Platynothrus peltifer* (Acari, Oribatida). *J. Evol. Biol.* 20:392-402. doi: 10.1111/j.1420-9101.2006.01183.x

**SUBMITTED**: 29 September 2020; **ACCEPTED**: 09 September 2021;

**PUBLISHED ONLINE**: 22 October 2021.

**EDITED BY**: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Öztoprak H, Brandt A, Solbach MD, Bast J and Schaefer I (2021) Having Babies in Soil: Is Sex Really Necessary? Front. Young Minds 9:611659. doi: 10.3389/frym.2021.611659

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Öztoprak, Brandt, Solbach, Bast and Schaefer. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

### DARIO, AGE: 14

My name is Dario. I live in a small village in Austria. It is full of nature so in my freetime I like to go out with my dogs or climb trees. My parents are both biologist so I got into biology pretty early.

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### LUVENA, AGE: 11

Hi, my name is Luvena! I love music, sports, and food. My favorite subjects in school are math and language arts. In my spare time, I enjoy playing piano and reading books with my sister. When I grow up, I would like to be a neurosurgeon.

<figure><img src="image_89.png" /></figure>

### PRANATEE, AGE: 12

Hello! I love to bake, especially tarts and pies. In school, my favorite subjects are science, lunch, and recess. I like spending time outdoors and going hiking. I also love going to the beach and have an interest in photography. Watching my favorite TV shows, painting, listening to music, singing, and hanging out with friends are my

<figure><img src="image_90.png" /></figure>
favorite things to do in my free time. In the future, I would like to either like to be a scientist, or a singer/songwriter and actress.

### VALERIE, AGE: 13

I am in 8th grade of a middle school in Austria. My hobbies are horsebackriding, skating, and dancing. I have got a very old cat and we are getting a dog soon. I also like meeting my friends and listening to some music.

<figure><img src="image_91.png" /></figure>

## AUTHORS

### HÜSNA ÖZTOPRAK

At first, I studied biology to become a scientific journalist. During my studies I quickly realized I want to find answers to my own question, so I became a researcher. During my master's research I found and described new species of testate amoebae. I became interested in how asexual organisms diverge into new species. So, I started my Ph.D. at the University of Cologne, Germany. I like to travel and try new foods. At the moment, staying at home is the responsible thing to do. So, when I am not in the lab, I stay in and binge-watch anime. *h.oeztoprak@uni-koeln.de

<figure><img src="image_92.png" /></figure>

### ALEXANDER BRANDT

I am an evolutionary biologist at the University of Lausanne in Switzerland. In my research I mostly analyze genes using the computer. However, I always enjoy watching (soil) animals (under the microscope) when I have some free time in the lab. I spent a lot of time comparing how many harmful mutations have accumulated in asexual and sexual oribatid mites during their evolution. For this work, I just recently obtained my Ph.D. from the University of Göttingen (about which I am really proud)! In my free time I like watching documentaries about dinosaurs, meeting friends, and I play drums in a rock band.

<figure><img src="image_93.png" /></figure>

### MARCEL D. SOLBACH

I am a biology Ph.D. student from Cologne. I conducted work on various microorganisms including microalgae, intracellular bacteria in amoebae, and currently on protists as potential plant pathogens. When I am not sitting in the laboratory, I try to build a side-career as a fantasy artist and illustrator—hence I painted the pictures in this article! In my remaining time, I like to play basketball and go swimming, to prevent my aging and aching body from falling apart.

<figure><img src="image_94.png" /></figure>

### JENS BAST

As a kid (and still as an adult), I was fascinated by animals, mostly cats. And genes. I genetically manipulated my first bacteria at 17. This is how I got into biology and when I started thinking about why the things in nature are the way they are. I use my passion for genes and animals to try to understand what happens in evolution when sex is lost. I enjoy working with other scientists so that we can solve scientific questions together. In my free time I like to cook and eat, to travel, to make music, and to play games.

<figure><img src="image_95.png" /></figure>
INA SCHAEFER

The diversity and distinctness of the natural world made me think about what we think is normal and what is unusual. This is why I became a scientist, because science is supposed to be objective. I am fascinated by the soil environment because life in soil is so different from our everyday above-ground experience in many ways. However, because I am a large and clumsy human that cannot see nor move in soil, I need to investigate the DNA of soil organisms to understand what they do and who they are.

<figure><img src="image_96.png" /></figure>
<figure><img src="image_97.png" /></figure>

# STUDYING THE ACTIVITY OF LEAF-LITTER FAUNA: A SMALL WORLD TO DISCOVER

Dolores Ruiz-Lupión*†, María Pilar Gavín-Centol and Jordi Moya-Laraño

Department of Functional and Evolutionary Ecology, Estación Experimental de Zonas Áridas, Consejo Superior de Investigaciones Científicas (EEZA-CSIC), Almería, Spain

## YOUNG REVIEWER:

<figure><img src="image_98.png" /></figure>

SASYAK

AGE: 12

Hundreds of thousands of little creatures live in soils. Some eat live plants, live animals, or both. Others, called decomposers, consume dead plants, and the waste of other living beings (their feces and their dead bodies), and transform them into food for plants. The health of soils depends largely on the presence of decomposers, and thus it is necessary to study how these creatures may be affected by climate change. To this end, we built a new type of traps to catch live soil animals, which we called cul-de-sac and basket traps. Here, we show how these traps are better for studying animal activity (how much they move in the soil) compared to the most used devices to date, pitfall traps. Comparatively, our traps capture more active animals and prevent predators from killing prey inside, which will improve the accuracy of future studies all over the world.
## WHY ARE LIVING BEINGS SO IMPORTANT FOR SOILS?

Soils are largely unknown universes, complex systems formed by a mixture of air, minerals, organic compounds, and living organisms that are related to each other and with the environment. These relationships between living beings are called interactions, which take place when organisms communicate, feed on each other or pollinate flowers, among others. Currently, we do not know how many species of animals, fungi, and bacteria live in the first four meters of soil (Figure 1, soil profile); but we do know that soils contain the greatest biodiversity on Earth, with about 1.5 million described species out of a total of an estimated 2 billion. Among this overwhelming biodiversity, **soil fauna** perform many important functions necessary for both soil health and human well-being [1]. For example, one of these functions is the decomposition of dead animals and plants, a process in which dead matter is transformed into food from which plants feed on (Figure 1, orange arrows). Without decomposer animals healthy soils would disappear, and both wildlife and humanity would be affected. Moreover, some of these animals act as ecosystem engineers, creating, modifying, and maintaining the soil structure (as ants or earthworms when digging holes). Other soil-living animals are natural enemies of pests, helping farmers to protect their crops. Therefore, a soil with greater abundance and diversity of fauna will provide more benefits and this is why these organisms are good indicators of soil health [2]. Hence, sampling and analyzing these creatures is essential if we are to understand and preserve the soils and the functions they supply.

### SOIL FAUNA

The set of animals that live within or on top of the soil (springtails, mites, spiders, centipedes, earthworms, etc.). This contrasts to soil microbiota (bacteria and fungi) also important for soil functioning.

## THE IMPORTANCE OF LEAF-LITTER FOOD WEBS

About 97% of the species of soil fauna are invertebrates, animals without an internal skeleton such as nematodes, potworms, earthworms, slugs, or snails. We mostly like to study a type of invertebrates that often live in the **leaf-litter** layer and that have external skeletons, segmented bodies, and pairs of appendages with joints: the arthropods. These animals can vary widely in size from very tiny to larger than a hand. Soil arthropods are grouped into two size categories: mesofauna (0.2–2.0 mm) such as mites and springtails, and macrofauna (>2.0 mm) such as spiders, beetles, centipedes, and millipedes (Figure 1, grey boxes). All these animals organize around what we call **food webs**, in which predator-prey relationship take place governed by the rule that "big fish eats little fish;" that is, larger species prey (kill and feed) on several smaller ones, while small species prey on even smaller species or on the offspring of larger ones (Figure 1, black arrows). This interdependence among species means that food webs are fragile systems, in which the extinction of one species can cause the extinction of others, making the entire food web to crumble as a "house of cards" (Video 1). Therefore, studying the functioning of food webs is crucial for soil health monitoring.

### LEAF-LITTER

The top soil layer of dead leaves (1 cm–1 m depth) in terrestrial ecosystems, such as forests and scrublands, which provides habitat and food to a large diversity of organisms.

### FOOD WEB

A natural network of connections (interactions) among organisms that feed on each other.

### VIDEO 1

An illustrative video of a crumbling food web as a "house of cards."
<caption>**Figure 1**</caption>

The leaf-litter food web. The abundance and diversity of soil fauna is critical for soil health. Black arrows show "who eats whom," and organisms can be classified into: (1) primary decomposers that directly feed on the leaf-litter; (2) secondary decomposers that feed on primary decomposers; (3) small predators that feed on primary and secondary decomposers; and (4) large predators that feed on small predators and large decomposers. Orange arrows show the decomposition process, by which dead matter is transformed into food from which plants feed upon. To complete the decomposition process of the falling leaves (brown arrow), sunlight, rainfall and a good airflow are also necessary.

<figure><img src="image_99.png" /><figcaption>Figure 1</figcaption></figure>

## ACTIVITY

The amount of animal movement per unit of time (minutes, hours, etc.).

## USING TRAPS TO MEASURE THE ACTIVITY OF LEAF-LITTER ARTHROPODS

Not all soil animals search food at the same time, nor for the same amount of time, which determines which animals encounter and interact with each other in the soil. Therefore, predator-prey interactions in food webs could be influenced by the activity patterns (e.g., diurnal or nocturnal) of different animals. Our main goal is to study the **activity** of leaf-litter arthropods using different types of traps. The use of traps for measuring animal activity may be an appropriate approach to monitor the functioning and health of soils. Even so, it is still unclear if trap catches are an estimate of activity, abundance, or a mixture of both [3]. This is because traps in a location with more arthropods will catch a larger number of them if they are more abundant, but also if they move around for longer time (that is, because they are more active). Since the early 1900s, the standard method for collecting soil fauna and measuring the abundance and/or diversity of soil arthropods has been the use of pitfall traps. These traps consist of cups buried in the soil in which fauna crawling through the leaf-litter fall. The cups are usually partially filled with a liquid to kill and preserve the trapped organisms. Besides pitfall traps, there are other methods of sampling abundance and/or diversity of soil arthropods [4]. In our study, we were interested in assessing the activity—not just the abundance—of soil arthropods. Therefore, we used pitfall traps without preservative liquid, which allowed collecting live specimens that could be returned to the leaf-litter.
However, pitfall traps have many problems, including the fact that animals that are too small or too large do not fall into them, either because they are longer than the trap entrance or so small that they detect the edge of the trap and move away from it. Some animals can even walk on the trap walls without falling in. This means that pitfall traps might not give us good estimates of the entire leaf-litter community, which could lead to improper conclusions about predator-prey interactions when using pitfall traps [3]. Thus, we developed two new trapping devices to improve the monitoring of animal activity, small mesh fabric bags in the shape of a sock named "cul-de-sac" traps, and square baskets made of wireframe similar to a box with holes without a lid named "basket" traps [5], in which the transition between the edge of the trap and the surrounding litter is less noticeable to soil animals than in pitfall traps.

## DISTINGUISHING ACTIVITY FROM ABUNDANCE

Our main goal was to find which traps were better at monitoring animal activity, by means of distinguishing abundance (how many animals are there) from activity (how much do they move). We use traps to estimate activity; that is, animals that fall inside a trap while it is placed in the field should reflect how much they move. However, things are not this simple. For instance, the number of animals directly captured in the leaf-litter is an estimate of their abundance; that is, the quantity of animals present regardless of the moment. Hence, differently than activity, abundance is not affected, for instance, by the weather conditions of the day of the experiment in which we capture the animals. And this is when things get tricky: animal abundance affects how many animals are caught in the traps regardless of their activity. Therefore, in order to differentiate activity from abundance, we had to count and classify the arthropods inside the traps (activity) and within the leaf-litter outside the traps (abundance). This distinction is very important because the measure of soil arthropod activity cannot be done without knowing the abundance. Imagine one location in the soil inhabited by 2 individuals of a species (Sp1) of very active beetles that move around a lot, and by 20 beetles of a very abundant but sedentary species (Sp2); i.e., that do move very little. If our traps collected 2 beetles of each species in that location, we would conclude that the activity of both species was similar. However, in reality the 2 individuals of the former species (Sp1) would be caught by their high activity, while the second two (Sp2) would be caught because this species is much more abundant. Just because Sp2 is present in large numbers two are caught in the traps, even though these beetles move much less. An independent measurement of abundance serves the researchers to correct for these differences and have accurate measurements of activity.
## SETTING UP OUR MESOCOSM EXPERIMENT

In the spring of 2013, we conducted an experiment in 4 beech forests (*Fagus sylvatica* L.) from the Cantabrian Mountains, Spain (Figure 2A). Beech trees have relatively large leaves that fall in autumn, forming a leaf-litter layer often deeper than 10 cm, which is shelter to large numbers of arthropods [6]. Working in soils with leaf-litter offers a great advantage: animals live and are active in superficial layers of the soil during most of the time, while in the soils of other terrestrial ecosystems the fauna is mostly active in deeper layers, from which it is difficult to trap live animals.

All traps used in the study were handmade. Pitfall traps (Figure 2C, left) consisted of a plastic cup with the base cut and a fabric attached at the bottom to prevent small animals from escaping while allowing water to drain out. This fabric was very fine, with a mesh of about 200 µm. In addition, to minimize the entry of sunlight as to mimic the dark conditions within the litter, a wooden square cap was placed on top of each pitfall trap in the leaf-litter. Cul-de-sac traps (Figure 2C, center) were made firm by sewing an oval-shaped wire around the mouth trap. The fabric of these bags was the same as that used for the pitfall traps. Basket traps (Figure 2C, right) consisted of 20 × 20 × 7 cm wireframe baskets of 1 × 1 cm mesh. Once we built all traps, we placed several square shaped metal enclosures buried in the forest leaf-litter, named **mesocosms** (Figure 2B). We then collected leaf-litter in the surroundings of each mesocosm, removed all fauna from this litter in the laboratory, filled all traps with this litter free of fauna, and finally placed all traps in the field. Cul-de-sac and basket traps were embedded in the litter layer within the mesocosms, and pitfall traps were buried in the mesocosm soil (Figure 2D). In total, 4 mesocosms were placed in each forest, each with 4 pitfalls, 2 cul-de-sac, and 2 basket traps (Figure 2E). To begin our observations with similar moisture conditions in the traps and the surrounding litter, we placed roofs on the mesocosms to block all rainfall 15 days in advance. This procedure provided with uniform moisture conditions within the mesocosms, ensuring that animals were not moving to seek for, or to avoid humidity.

After collecting the samples from the traps and the leaf-litter outside the traps, five steps were then followed in the laboratory: (1) we weighed the leaf-litter of each sample (wet weights); (2) we removed and counted the arthropods; (3) using a dissecting microscope we classified the arthropods according to their size (macrofauna vs. mesofauna), diet (predators vs. prey) and main group (mites, springtails, spiders, centipedes, millipedes, or beetles); (4) we dried the leaf-litter of each trap and weighed it again (dry weights); and (5) we calculated the water content within each trap from the difference between wet and dry leaf-litter weights. We used statistical tools to reach conclusions from the numerical data that we obtained from the field. We needed to use these tools for including a correction

## MESOCOSM

A device, normally placed outdoors, that encloses part of an ecosystem and allows scientists to control parameters, such as rainfall, more realistically than in laboratory experiments.
Figure 2

Our experimental setup. (A) Our study was conducted in 4 beech forests in Asturias, Spain. (B) We set up mesocosms that were partially buried in the soil, each with a fiberglass screen on top to prevent the escape of arthropods and a plastic roof to block rainfall. (C) Each mesocosm contained “pitfall” traps and the new “cul-de-sac” and “basket” traps. (D) Pitfall traps were buried in the soil, whereas cul-de-sac and basket traps were embedded within the leaf-litter layer. (E) Top-down view of a mesocosm with 4 pitfall, 2 cul-de-sac and 2 basket traps (photographs and some drawings were reused from the Original Source Article).

<figure><img src="image_100.png" /><figcaption>Figure 2</figcaption></figure>

for abundance (see the above section), ensuring that we were testing for differences in activity and not in abundance [7]. In summary, we tested the activity of each group and if some trap types captured some groups more than others. For instance, we compared the catches of large (macrofauna) vs. small (mesofauna) animals, as well as those of predators vs. prey. With this set of comparisons, we could assess what kind of trap works better for studying arthropod activity.
WHAT TYPE OF TRAP BEST MEASURES ACTIVITY?

Through our experiments, we found that cul-de-sac and basket traps
performed better than pitfalls trap. First, pitfall traps retained almost
twice the amount of water than cul-de-sac and basket traps, which
could result in attraction for some animals or repulsion for others.
Additionally, the litter in these new traps had similar water content
than that in the surrounding litter outside the traps (Figure 3A). Second,
cul-de-sac and basket traps caught around 3–5 times more animals
per unit time, for example per hour, than pitfall traps. Thus, the latter
underestimated the activity of leaf-litter fauna (Figure 3B). Third, pitfall

<caption>Figure 3</caption>

What type of trap best
measures the activity of
leaf-litter fauna? To
estimate the natural
abundance of
arthropods, we
collected 5 samples of
leaf-litter from each
mesocosm. We also
collected pitfall,
cul-de-sac and basket
traps to estimate
arthropod activity.
Compared to the new
traps, pitfall traps: (A)
retained twice the
amount of water as the
other two traps; (B)
captured 20-33% the
amount of animals per
unit of time; for
example, per hour, than
the new traps; (C)
caught more
macrofauna than
mesofauna; and (D)
caught more predators
than prey. This told us
that the new traps are
better measuring the
activity of soil
arthropods
(photographs and
some drawings were
reused from the
Original Source Article).

<figure><img src="image_101.png" /></figure>
traps captured more macrofauna than mesofauna and more predators than prey. This means that the predators that fell into the pitfall traps could have eaten some of the smaller fauna before we collected the traps (Figures 3C,D). Therefore, we concluded that cul-de-sac and basket traps performed much better than pitfall traps.

## WHY DO THESE NEW TRAPS MATTER?

Our new cul-de-sac and basket traps are promising tools for soil ecologists, since they perform much better than pitfall traps, which have been widely used to assess abundances over the last century. These new traps will help scientists to more accurately estimate the activity of soil arthropods, which will improve our knowledge of terrestrial ecosystems with leaf-litter. Besides these are cheap devices that one can easily build him/herself (with fabric, wires, plastic, and/or metal meshes of different sizes, some glue, and leaf-litter), allowing one to learn more about soil animals and, therefore, about the health of ecosystems. These new traps capture animals more efficiently, minimize the predation on small ones and do not attract/repel animals because of differences in moisture between trap and the surrounding leaf-litter. In addition, these traps can not only be used in soils with a deep leaf-litter layer (like forests or jungles), but also in any ecosystem that has a well-defined leaf-litter layer, such as under shrubs in scrubs and savannas. This work is also very important because we need to understand how climate change may negatively affect food webs (feeding interactions among species) and the important functions and benefits that soil ecosystems provide. We are already working on it, through tools like these new traps and field experiments in which we modify rain or predators. Thus, we should not miss this opportunity and get to know the wonderful but hidden soil fauna. Because it is everyone's heritage and because it protects us all, now more than ever we have to study and preserve the soils and the life they host.

## ACKNOWLEDGMENTS

We thank J. Pascual, N. Melguizo-Ruiz, and O. Verdeny-Vilalta, co-authors of the original scientific article; E. de Mas, J. Pato, and G. Jiménez, who helped with field sampling; and E. de Mas for photographs. We also thank the Research Unit of Biodiversity (UMIB, UO/CSIC/PA) at Mieres (Asturias) for logistical support. This work was done under Asturias Government permit 2011/059163 and funded by grants CGL2010-18602 and CGL2015-66192-R from the Spanish Ministerio de Economía y Competitividad (European funds FEDER), 020/2008 from Spanish Organismo Autónomo de Parques Nacionales, and P12-RNM-1521 from Junta de Andalucía (European funds FEDER). DR-L enjoyed a FPU (FPU13/04933) scholarship from the Spanish Ministerio de Educación, Cultura y Deporte.
ORIGINAL SOURCE ARTICLE

Ruiz-Lupión, D., Pascual, J., Melguizo-Ruiz, N., Verdeny-Vilalta, O., and Moya-Laraño, J. 2019. New litter trap devices outperform pitfall traps for studying arthropod activity. *Insects*. 10:147. doi: 10.3390/insects100150147

REFERENCES

1. Briones, M. J. I. 2018. The serendipitous value of soil fauna in ecosystem functioning: the unexplained explained. *Front. Environ. Sci.* 6:149. doi: 10.3389/fenvs.2018.00149

2. Brackin, R., Schmidt, S., Walter, D., Bhuiyan, S., Buckley, S., and Anderson, J. 2017. Soil biological health - what is it and how can we improve it? *Proc. Aust. Soc. Sugar Cane Technol.* 39:141-54.

3. Lang, A. 2000. The pitfalls of pitfalls: a comparison of pitfall trap catches and absolute density estimates of epigeal invertebrate predators in Arable Land. *J. Pest Sci.* 73:99-106. doi: 10.1007/BF02956438

4. McCravy, K. W. 2018. A review of sampling and monitoring methods for beneficial arthropods in agroecosystems. *Insects* 9:170. doi: 10.3390/insects9040170

5. Ruiz-Lupión, D., Pascual, J., Melguizo-Ruiz, N., Verdeny-Vilalta, O., and Moya-Laraño, J. 2019. New litter trap devices outperform pitfall traps for studying arthropod activity. *Insects* 10:147. doi: 10.3390/insects10050147

6. Melguizo-Ruiz, N., Jiménez-Navarro, G., De Mas, E., Pato, J., Scheu, S., Austin, A. T., et al. 2020. Field exclusion of large soil predators impacts lower trophic levels and decreases leaf-litter decomposition in dry forests. *J. Anim. Ecol.* 89:334-46. doi: 10.1111/1365-2656.13101

7. Shultz, B. J., Lensing, J. R., and Wise, D. H. 2006. Effects of altered precipitation and wolf spiders on the density and activity of forest-floor Collembola. *Pedobiologia* 50:43-50. doi: 10.1016/j.pedobi.2005.10.001

SUBMITTED: 16 April 2020; ACCEPTED: 06 September 2021;

PUBLISHED ONLINE: 08 October 2021.

EDITED BY: Helen Phillips, Saint Mary's University, Canada

CITATION: Ruiz-Lupión D, Gavín-Centol MP and Moya-Laraño J (2021) Studying the Activity of Leaf-Litter Fauna: A Small World to Discover. *Front. Young Minds* 9:552700. doi: 10.3389/frym.2021.552700

**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2021 Ruiz-Lupión, Gavín-Centol and Moya-Laraño. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited
and that the original publication in this journal is cited, in accordance with accepted
academic practice. No use, distribution or reproduction is permitted which does not
comply with these terms.

YOUNG REVIEWER

SASYAK, AGE: 12

Sasyak is a 12-year old student from India. He is an avid reader of several genres of books. He is a keen participant in quiz contests and olympiads, and is a spell bee champion. He attends football classes and enjoys cycling.

<figure><img src="image_102.png" /></figure>

AUTHORS

DOLORES RUIZ-LUPIÓN

I started my academic career with two bachelor degrees one in Marine Sciences and another in Environmental Sciences, and later I received a Master degree in Evaluation of Global Change. Afterwards, I completed my PhD in Evolutionary Ecology at the Estación Experimental de Zonas Áridas, Consejo Superior de Investigaciones Científicas (EEZA-CSIC). My scientific interest is the study of terrestrial and aquatic food webs through field and laboratory experiments, as well as theoretical approaches using mathematical models and computer simulations. I am very enthusiastic about scientific illustration and I design figures for other researchers. *loli.ruiz@eeza.csic.es; loli.ruizlupion@gmail.com †Dolores Ruiz-Lupión, Laboratory of Arid Zones and Global Change, Department of Ecology, Instituto Multidisciplinar para el Estudio del Medio "Ramón Margalef" (IMEM), Universidad de Alicante (UA), Spain

<figure><img src="image_103.png" /></figure>

MARÍA PILAR GAVÍN-CENTOL

I am a pre-doctoral student at the Estación Experimental de Zonas Áridas, Consejo Superior de Investigaciones Científicas (EEZA-CSIC). Since I was a child, animals have always amazed me, but even more since I learned that nematodes (tiny worm-shaped animals) can "wake up" after spending about 40,000 years on ice! That is why, after my studies in Biology, a master's degree and two internships, I began to investigate the mechanisms by which these and other soil animals are affected by increased droughts and how their drought-driven inactivity affects ecosystem functioning, both in natural and human-modified ecosystems.

<figure><img src="image_104.png" /></figure>

JORDI MOYA-LARAÑO

I have been live-collecting and observing bugs in nature since I was a child. I am passionate about wilderness and I consider myself a naturalist. As a consequence I also adore my job as an evolutionary ecologist. In our group, we conduct both field and laboratory experiments to understand the role of water in soil food webs. We also perform computer simulations that recreate both ecology and evolution in food webs, and my dream is to achieve realistic simulation scenarios that match our field experiments.

<figure><img src="image_105.png" /></figure>
<figure><img src="image_106.png" /></figure>

# SOIL ECOLOGISTS AS DETECTIVES DISCOVERING WHO EATS WHOM OR WHAT IN THE SOIL

Amandine Erktan<fnref n="1" />*, Melanie M. Pollierer<fnref n="1" /> and Stefan Scheu<fnref n="1" />,<fnref n="2" />

## YOUNG REVIEWERS:

<figure><img src="image_107.png" /></figure>

CECÍLIA

AGE: 9

<figure><img src="image_108.png" /></figure>

NYNKE

AGE: 12

Did you ever notice that dead leaves never accumulate in forests? For that service, we can thank a cleaning team of tiny recyclers living in the soil. Dead organisms are their food source, and they recycle them by just eating them. Knowing who eats what or whom in soils is essential to understand this recycling machinery. But that is hard to know, because many soil animals are tiny, hidden in the soil, and unable to tell us what they ate! To bypass these difficulties, soil ecologists have developed a special method. They track specific markers of bacteria, fungi, and plants in the fat of animals and hence can identify what they fed on. Some animals consume a large variety of food sources, others are more specific. Remarkably, many organisms have developed amazing strategies to feed in soil, because finding food in such a dark maze is not that easy!

<footnote n="1">¹J.F. Blumenbach Institute of Zoology and Anthropology, University of Göttingen, Göttingen, Germany</footnote>

<footnote n="2">²Centre of Biodiversity and Sustainable Land Use, University of Göttingen, Göttingen, Germany</footnote>
## WHY IS IT IMPORTANT TO UNDERSTAND WHO EATS WHOM OR WHAT IN SOILS?

Have you ever noticed that we never see huge piles of dead leaves accumulating in the forest? It is also very rare to encounter a dead animal lying on the forest floor. That should encourage us to ask the question, "who is cleaning the forest floor?" In cities, city workers remove all the dead leaves. In the forest, the work is done by a team of tiny recyclers living in the soil. For these small soil animals, dead organisms are a food source and they recycle leaves and other dead things by just eating them. When the soil animals defecate (meaning poop) they release nutrients that can be used by plants to grow. Or, small soil animals are eaten by larger animals, and that allows the bigger animals to grow. This process allows the recycling of nutrients and is essential to help plants grow. It is also very important for humans, as plants provide us with plenty of goods, such as foods like vegetables, cereals, and fruits, and also wood to build furniture and houses. Understanding who consumes what in soil is essential to understand this precious recycling machinery.

## WHAT KIND OF FOOD IS IN SOIL ... AND WHAT IS A SOIL FOOD WEB?

What kind of food is in the soil? If you dig in soil in the forest, you would not find a plate with spaghetti Bolognese! We are not talking about this kind of food, of course! In soil, the basic food sources are dead tissues from plants and other organisms (dead leaves or dead soil organisms of any size), and plant roots (Figure 1A). These basic food sources are mainly consumed by bacteria and fungi, which are called **primary consumers**. Fungi and bacteria themselves are the main food source for larger organisms, such as protists, nematodes, springtails, and mites (about 0.1–2 mm; Figure 1). These organisms again are eaten by larger predators (a few millimeters in size), such as centipedes and spiders (Figure 1B). Earthworms also mainly eat bacteria and fungi, but in a special way, they eat them together with soil (Figure 1B). This is a bit like if you ate the food on your plate together with the plate!

Although some soil animals, such as springtails or nematodes, mainly eat tiny living creatures like bacteria and fungi, they can also eat plant tissue, notably the roots or the nutritious liquid roots release. Altogether, there are many food sources in soil, ranging from plant to animal tissues and from dead to living organisms, and most soil organisms consume several of these food sources. All the links of who consumes what or whom is called a **soil food web** (Figure 1A).

### PRIMARY CONSUMER

Organisms directly consuming the dead plant material.

### SOIL FOOD WEB

All the links indicating who consume whom or what in soil.
<caption>**Figure 1**</caption>

Organisms in a typical soil food web. (A) A food web contains basic food sources, like plant roots and dead organisms, as well as primary and secondary consumers and a selection of predators. Arrows indicate who consumes what or whom. Note that primary consumers are a food source for secondary consumers, which themselves are a food source for predators. (B) Here you can see examples of soil primary and secondary consumers as well as predators.

<figure><img src="image_109.png" /></figure>

HOW DO WE STUDY SOIL FOOD WEBS?

Despite all the research that has happened in the last decades, researchers still know little about who consumes what or whom in soil. This lack of knowledge is because soil animals are tiny, hidden
Figure 2

Determining what soil organisms eat by looking at fatty acid markers. (A) Soil animals have different types of fats in their bodies, depending on the food sources they eat, such as bacteria, fungi, or dead leaves. (B) The fats can be extracted (removed) from these animals in the form of fatty acids. (C) Those fatty acids can then be analyzed using a piece of equipment called a gas chromatograph. The data from the gas chromatograph allows researchers to identify which foods the soil organisms ate.

<figure><img src="image_110.png" /><figcaption>Figure 2</figcaption></figure>

FATTY ACID
MARKERS

Molecules of fat which are specific from a food source, namely bacteria or fungi.

FOOD GENERALIST

Soil organisms consuming many types of food resources.

FOOD SPECIALIST

Soil organisms consuming specifically one or a few types of food resources only.

in the soil, and cannot tell us what they ate! To know who consumes what in the soil, researchers must act as detectives. They developed a curious method: they study the fat of soil animals (Figure 2). When you eat something, the food serves as a source of energy, so you can grow and be active. However, you cannot use all the energy at once, so your body must store it for later use. How is energy stored? When we eat more than we need at that moment, the body builds up fat as an energy-storage tissue. The fat will later be "burned" for energy when we need it. For both humans and animals, it is even easier for the body to take up and store the fat that is already in the food, instead of making new fat. Now, the trick is that not all fat is the same! Bacteria, fungi, and plants have different types of fat, and researchers can track these so-called **fatty acid markers** in the animals that consumed these food sources [1]. So, in the end, we can identify if the fat stored in an animal has come from bacteria, fungi, or plants—and thus know what they ate.

FOOD GENERALISTS VS. FOOD SPECIALISTS

By studying the fat of soil animals, researchers discovered that many springtails preferably feed on fungi, but can also eat bacteria or plants. Because they can eat different food sources, they are considered **food generalists** [2]. This means that they are not difficult to satisfy if you invite them for dinner! Some other organisms are **food specialists** and tend to eat only one thing. For example, some nematodes prefer
<caption>**Figure 3**</caption>

Animals have developed ways to deal with the difficulty of feeding in the dark soil. (A) Small holes in the soil provide a place for tiny soil organisms (nematodes, protists, and bacteria) to hide from the animals that eat them. (B) To reach its prey, an amoeba (a type of protist) can extend an arm 20 microns long and 1 micron thin to catch bacteria hidden in small soil holes. (C)

Springtails are flexible in the food sources they consume, allowing them to have something to eat each day. (D) Earthworms ingest soil along with their food and digest the bacteria and fungi contained in the ingested soil. This also creates a path through the soil maze, making it easier for them to travel through it. Remember 1 micron is 1,000 times smaller than 1 mm.

<figure><img src="image_111.png" /></figure>

eating bacteria, while others prefer fungi. There are even predatory nematodes that eat other nematodes! Each class of nematode has a different mouth shape, specific for eating a certain type of food.

## WHY IS IT SO SPECIAL TO FEED IN THE SOIL?

Soil is dark. Have you ever tried to eat your dinner in the dark? It is not that easy to find where the food is. Soil animals have the same problem. Eyes do not work in the soil, so many soil animals do not even have them. By contrast, most soil animals have very fine "noses." For example, nematodes, springtails, and earthworms are very efficient at smelling their food. They can detect the location of food and move toward it. Nematodes can "smell" bacteria up to 50 cm away and reach them in 2 weeks [3]. This is a considerable distance for a nematode, as these little worms are usually only a few hundred microns long (1 micron = 0.001 mm; for comparison, the width of a human hair is 100 microns). This would be like humans being able to smell food from about 70 km away!

Soil is not only dark, it is also a maze, through which soil organisms cannot freely move. Soil is like a sponge with larger holes and smaller holes. The smaller organisms, such as the bacteria, typically measure 1–2 microns and can “hide” in small holes. For example, we know that protists (Figure 1) cannot reach bacterial prey if the bacteria are located in holes smaller than 2–3 microns [4] (Figure 3A). The same applies to nematodes, which cannot eat bacteria located in holes with openings smaller than 30 microns [4] (Figure 3A). The smaller the holes in soil, the more the bacteria can hide in them and avoid being caught and eaten by predators. But predators have developed strategies to feed despite these problems. For example, amoebae (the
plural of "amoeba") are protists with soft bodies that can adopt any
shape (Figure 3B). Amoebae can extend a very thin and long "arm" into
small soil holes to catch hidden bacteria [5]. Springtails have a different
strategy: they are simply not very picky about the food sources they
consume—they are food generalists. They can eat bacteria and fungi,
but also dead leaves and nematodes (Figure 3C). Depending on what
is available in the small holes of the soil they are crawling through,
they eat one food source or another. This flexibility helps them to
have something to eat every day. Earthworms are less affected by the
difficulties of accessing food in the soil. These animals directly ingest
soil, enabling them to search for and access food easily in the soil maze
(Figure 3D). Moreover, they can digest bacteria and fungi, as well as
dead plant material that is ingested with the soil. When earthworms
poop, the undigested leftovers typically form small bowls of soil.

A NEW LOOK INTO FEEDING IN A DARK MAZE

Feeding in soil is like finding food in a dark maze. To understand who eats whom in the soil darkness below our feet, soil ecologists have to act as real detectives and using all sorts of complicated techniques, either in the forest or in the laboratory. Now that you know what it is like to feed in soil, you will never look at soil animals in the same way!

ACKNOWLEDGMENTS

This research has been supported by the European Commission –
Horizon 2020 research and innovation program, Marie Skłodowska-Curie
actions (grant no. 750249). We are thankful to Audrey Marville for
drawing the soil animals.

REFERENCES

1. Ruess, L., and Chamberlain, P. M. 2010. The fat that matters: soil food web analysis using fatty acids and their carbon stable isotope signature. *Soil Biol. Biochem.* 42:1898–910. doi: 10.1016/j.soilbio.2010.07.020

2. Digel, C., Curtsdotter, A., Riede, J., Klarner, B., and Brose, U. 2014. Unravelling the complex structure of forest soil food webs: higher omnivory and more trophic levels. *Oikos* 123:1157–72. doi: 10.1111/oik.00865

3. Rasmann, S., Köllner, T. G., Degenhardt, J., Hiltpold, I., Toepfer, S., Kuhlmann, U., et al. 2005. Recruitment of entomopathogenic nematodes by insect-damaged maize roots. *Nature* 434:732. doi: 10.1038/nature03451

4. Rønn, R., Vestergård, M., and Ekelund, F. 2012. Interactions between bacteria, protozoa and nematodes in soil. *Acta Protozool.* 51:223–35. doi: 10.4467/16890027AP.12.018.0764

5. Foster, R. C., and Dormaar, J. F. 1991. Bacteria-grazing amoebae *in situ* in the rhizosphere. *Biol. Fertil. Soils* 11:83–7. doi: 10.1007/BF00336368
**SUBMITTED:** 22 March 2020; **ACCEPTED:** 26 October 2020;

**PUBLISHED ONLINE:** 19 November 2020.

**EDITED BY:** Rémy Beugnon, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION:** Erktan A, Pollierer MM and Scheu S (2020) Soil Ecologists as Detectives Discovering Who Eats Whom or What in the Soil. Front. Young Minds 8:544803. doi: 10.3389/frym.2020.544803

**CONFLICT OF INTEREST:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2020 Erktan, Pollierer and Scheu. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWERS

**CECÍLIA, AGE: 9**

Cecília is a bright young lady that loves to play chess and is very curious about all kind of random things. She just wants to know how everything works and likes to later show off all the facts that her brain collected.

<figure><img src="image_112.png" /></figure>

**NYNKE, AGE: 12**

Hi, my name is Nynke.

<figure><img src="image_113.png" /></figure>

## AUTHORS

**AMANDINE ERKTAN**

I am a post-doc at the University of Göttingen in Germany. I am interested in understanding how living organisms shape the soil structure, and vice-versa. I first worked on plant roots and studied how they structure the soil. I quickly realized that roots are not the only engineers of the soil. There are countless microbes and animals in the soil and their roles are crucial to determining the soil structure. I am now acquiring new skills studying soil animals and hope to shed light on how plant roots, microbes and soil animals interact in the soil matrix. *aerktan@gwdg.de

<figure><img src="image_114.png" /></figure>
<figure><img src="image_115.png" /></figure>

MELANIE M. POLLIERER

I am a post-doc at the University of Göttingen, Germany. My main interest concerns soil animal food webs. Since it is hard to observe what soil animals really eat, I use indirect methods to find out more. In my Ph.D. thesis, I analyzed fatty acids in soil animals and followed the fate of labeled carbon from plants to animals. Now, I use another new method: I analyze stable forms of carbon and nitrogen in amino acids, allowing even more detailed insights into the diet of consumers.

<figure><img src="image_116.png" /></figure>

STEFAN SCHEU

While studying biology in Tübingen and Göttingen between 1979 and 1986, I was fascinated by the enormous diversity and important role of soil invertebrates. Ever since, I have investigated the structure and functioning of soil animal communities, first during my Ph.D. at the University of Göttingen and later as a post-doc in Calgary and Göttingen. In 1997, I established my own research group as Professor of Zoology and Ecology at Darmstadt University of Technology, and in 2008 back at the University of Göttingen as Chair of Animal Ecology. Here, we investigate the structure, function, and evolution of soil animal species and communities.
<figure><img src="image_117.png" /></figure>

# LOOKING FOR EARTHWORMS IN DEADWOOD

Frank Ashwood<fnref n="1" />*, Elena I. Vanguelova<fnref n="1" />, Sue Benham<fnref n="1" /> and Kevin R. Butt<fnref n="2" />

## YOUNG REVIEWER:

<figure><img src="image_118.png" /></figure>

MARIA

AGE: 14

Fallen branches, logs, and tree stumps are a valuable habitat in forests, giving food and shelter to many organisms, including earthworms. Unfortunately, deadwood is often removed from forests because its value is not fully appreciated. We wanted to overcome this by developing a sampling method for earthworms living in deadwood in forests. By testing our new sampling method in oak forests, we found that including deadwood in earthworm surveys improves our knowledge of forest earthworm populations. We also found a greater number of young earthworms in deadwood, where conditions were warmer and moister than in the soil. By surveying deadwood for invertebrates, such as earthworms, we can better understand the important role that deadwood in forests plays in maintaining biodiversity.

## EARTHWORMS IN TREES?

Earthworms are important for maintaining healthy ecosystems: their burrows help air and water enter soil, and they break down dead

<footnote n="1">¹Forest Research, Alice Holt Lodge, Farnham, United Kingdom</footnote>

<footnote n="2">²Earthworm Research Group, University of Central Lancashire, Preston, United Kingdom</footnote>
Figure 1

An epigeic earthworm found in deadwood. The pale band (or "saddle") toward the head end (to the right) tells us this is an adult earthworm.

DEADWOOD

In forests, woody material that is no longer living, including fallen branches, logs, stumps, and standing dead trees.

EPIGEIC

Earthworms that live on the surface of the soil in organic matter-rich habitats, like leaf litter.

ENDOGEIC

Earthworms that make shallow, horizontal burrows, and feed on the soil.

ANECIC

Earthworms that build deep vertical burrows and feed on organic material on the soil surface.

ORGANIC MATTER

Matter composed of organic compounds that have come from the remains of organisms, such as plants and animals.

BIODIVERSITY

The variety of life in a habitat, a high level of which is usually desirable.

<figure><img src="image_119.png" /><figcaption>Figure 1</figcaption></figure>

plant material, recycling its nutrients back into the environment. But earthworms do not just live in the ground, they can be found in all sorts of unexpected places—even up in trees and inside logs! Logs and fallen trees are called **deadwood** and are an important habitat for many different types of earthworm, giving them shelter and food (Figure 1). There are three main groups of earthworms: **epigeic** (living above-ground), **endogeic** (shallow-burrowing in soil), and **anecic** (deep-burrowing in soil) [1]. Earthworm populations in a forest can affect how fast deadwood decays, with different groups and species more important at different stages [2]. This decaying wood is a source of nutrients and **organic matter** (organic compounds that are the remains of organisms, such as plants), and its decomposition is important for maintaining healthy forest soils.

DEADWOOD IS AN IMPORTANT BUT UNDER-STUDIED HABITAT

Despite being a key source of soil organic matter and an important habitat, deadwood is often removed from forests when trees are cut down for timber and firewood, putting many species of animals at risk of going extinct [3]. Protecting forest **biodiversity** is important. Biodiversity is the variety of life in a habitat, and we depend on the services that healthy and highly biodiverse forests provide, such as storing carbon and protecting soil. The more information we can learn about the amount of biodiversity present in deadwood, the better we can understand the importance of keeping it in forests. We do not have a full idea of which earthworms live inside deadwood, because we do not currently have a scientific way of looking for them there. Earthworm surveys normally only look in the soil and may miss earthworms living in other places. By developing a way to survey deadwood for earthworms, we can learn more about earthworm lifecycles and perhaps show how important it is to keep deadwood
<caption>**Figure 2**</caption>

Earthworm sampling in an oak forest plot. Dashed white lines within deadwood indicate sections divided into separate pieces. All deadwood within the plot and >10 cm in diameter (dark gray) was measured for total length and midpoint diameter, and five randomly selected pieces of mid- to late decay were sampled for earthworms. All deadwood <10 cm in diameter or outside the plot was excluded from the survey (light gray). Five 0.1 m² soil pits (indicated by crosses) were sampled for soil-dwelling earthworms.

<figure><img src="image_120.png" /><figcaption>Figure 2</figcaption></figure>

in forests. The main aim of our project was to develop and test a method for surveying deadwood for earthworms. To test whether our method worked, we compared our deadwood results with those from a standard soil-based earthworm survey.

## SURVEYING DEADWOOD FOR EARTHWORMS

To try out our method, we visited 12 forests of common oak trees (Quercus robur) in Surrey, UK, and in each forest, we marked out a 10 × 10 m square plot (Figure 2). In each plot, the total volume of deadwood was recorded and we tried to determine which tree species the deadwood belonged to. We also estimated how decayed the deadwood was, based on a 1-to-5 ranking system, in which 1 is least decayed (freshly fallen) and 5 is most decayed (deadwood completely broken down into the soil). Since small branches have very few earthworms in them, we selected five pieces of deadwood that were larger than 10 cm in diameter, and of mid- to late-decay stage (loose bark and wood beginning to soften). We analyzed the deadwood by placing it onto a sheet and measuring its length and diameter, so we could calculate its volume. Deadwood temperature was measured by inserting a digital kitchen thermometer beneath any loose bark. We then removed any moss and loose bark and inspected it for earthworms, and the remaining wood was taken apart and inspected. Once all earthworms were collected, the deadwood was then returned to its original location, with moss and loose bark replaced as best possible.

## SURVEYING SOIL FOR EARTHWORMS

We also sampled the soil for earthworms using a standard method. This involved digging soil pits (30 × 30 cm wide and 10 cm deep)
<caption>**Figure 3**</caption>

Summary of the results
from the earthworm
surveys of soil and
deadwood in
oak forests.

<figure><img src="image_121.png" /><figcaption>Figure 3</figcaption></figure>

at five locations in the plot (Figure 2). First, we placed the soil onto a sheet to hand-sort for earthworms, and then we poured 5 L of mustard water (25 g of table mustard powder mixed with 5 L of water) into each soil pit, to extract deep-burrowing earthworms, some of which can dig 2 m deep! This mustard powder irritates the earthworms' skin and encourages them to come up to the surface. Soil moisture and temperature measurements were also taken. Once all earthworms were collected, the soil was placed back into the holes. All earthworms from deadwood and soil were preserved in alcohol and were weighed, then each species was identified using a microscope and an earthworm identification guide [4].

## WHAT DID THE SURVEYS REVEAL?

Overall, we found a total of 1,012 earthworms and 13 different earthworm species. The number of earthworm species was different between soil and deadwood, with seven species found in deadwood, and twelve species found in soil (Figure 3). One species, *Eisenia fetida*, was found only within deadwood. This is an epigeic earthworm that is often found in compost heaps. Six species were found only in the soil: two epigeic, three endogeic, and one anecic species. All other earthworm species were found in both deadwood and soil. Many more juvenile (young) earthworms were found in deadwood than soil, and deadwood was much moister than the soil and warmer by around 1°C.
## ABUNDANCE

The number of a species or community in an ecosystem, such as the number of individuals per area.

## BIOMASS

The mass of an organism or community of organisms in an area or ecosystem.

Total earthworm **abundance** (number of individuals) and **biomass** (mass of all earthworms) was much greater in soil than in deadwood (Figure 3). On average, the deadwood surveys contributed an additional 81 earthworms and 209 g earthworm biomass per 10 m² forest plot.

## THE BENEFITS OF DEADWOOD SURVEYS

By adding deadwood to our forest earthworm surveys, we found both a greater abundance of earthworms and more earthworm species than we would have found by doing only soil surveys. There was a much greater proportion of juvenile earthworms inside deadwood, where temperature and moisture conditions were more favorable. Earthworms are very sensitive to temperature and moisture extremes, so being protected in decaying wood allows earthworms to stay active throughout the year, especially during summer drought and freezing winter conditions. Removing deadwood from forests may therefore have a negative effect on the many earthworm species that rely on it for shelter and food. Based on our results, we can say that forest-based earthworm research that does not include deadwood is likely to underestimate earthworm populations, and forests without deadwood will have fewer earthworms. With more development, our survey method could be used for studying other important invertebrates that live in deadwood, such as insects [3]. Hopefully we can use this information to improve forest management practices, to make sure that deadwood is left in place to protect forest biodiversity.

## ORIGINAL SOURCE ARTICLE

Ashwood, F., Vanguelova, E. I., Benham, S. and Butt, K. R. 2019. Developing a systematic sampling method for earthworms in and around deadwood. *For. Ecosyst.* 6:33. doi: 10.1186/s40663-019-0193-z

## REFERENCES

1. Hendrix, P. F. 1996. "Earthworms, biodiversity, and coarse woody debris in forest ecosystems of the southeastern U.S.A." in *Proceedings of the Workshop on Coarse Woody Debris in Southern Forests: Effects on Biodiversity* (Athens, GA). p. 43–8.

2. Bouché, M. B. 1977. "Strategies lombriciennes," in *Organisms as Components of Ecosystems*, eds U. Lohm and T. Person (Stockholm: Ecological Bulletin). p. 122–32.

3. Cálix, M., Alexander, K. N. A., Nieto, A., Dodelin, B., Soldati, F., Telnov, D., et al. 2018. *European Red List of Saproxylic Beetles*. Brussels: IUCN.

4. Sherlock, E. 2018. *Key to the Earthworms of the UK and Ireland. 2nd Edn.* Telford: Field Studies Council.
**SUBMITTED**: 31 March 2020; **ACCEPTED**: 20 October 2020;

**PUBLISHED ONLINE**: 16 November 2020.

**EDITED BY**: Malte Jochum, German Centre for Integrative Biodiversity Research (iDiv), Germany

**CITATION**: Ashwood F, Vanguelova EI, Benham S and Butt KR (2020) Looking for Earthworms in Deadwood. Front. Young Minds 8:547465. doi: 10.3389/frym.2020.547465

**CONFLICT OF INTEREST**: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

**COPYRIGHT** © 2020 Ashwood, Vanguelova, Benham and Butt. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

## YOUNG REVIEWER

### MARIA, AGE: 14

Hi, my name is Maria and I am from Poland. I absolutely love biology, especially solving tasks in genetics and cell metabolism. In my spare time I enjoy reading books and playing with my cat Roxi. I am keen on ballet. I train regularly at the Dance Conservatory. I really love it!

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## AUTHORS

### FRANK ASHWOOD

A passion for nature encouraged me to study Biology at university, where I volunteered for research projects on invertebrate ecology in Scotland and Mexico. After working as an environmental consultant for a few years, I went back to Uni and did a Ph.D. studying earthworms on reclaimed landfill sites. I now have a great job as a soil ecologist for Forest Research, where I study soil biodiversity in UK woodlands. In my spare time I am a soil biology tutor and do macrophotography (taking photos of the tiny animals living in soil). *francis.ashwood@forestresearch.gov.uk

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### ELENA I. VANGUELOVA

I have always been an outdoorsy person, spending lots of time in the mountains in Bulgaria as a child. At University I studied Forest Engineering but was not entirely happy until I did my Ph.D. in atmospheric pollution impacts on forest ecosystems. Then I realized that what I really like is the environmental side of forestry and soil. I work at Forest Research as a biogeochemist (a little bit of everything):

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