# Trait concepts, categories, and databases in soil invertebrates ecology – ordering the mess

Sophie Joimel, Anton Potapov, Benjamin Pey, Jonathan Bonfanti, Jérôme Cortet, Tania de Almeida, Sara Di Lonardo, Davorka K Hackenberger, Paul Henning Krogh, Ryszard Laskowski, et al.

► To cite this version:

Sophie Joimel, Anton Potapov, Benjamin Pey, Jonathan Bonfanti, Jérôme Cortet, et al. Trait concepts, categories, and databases in soil invertebrates ecology – ordering the mess. Soil Organisms, 2024, 96 (3), pp.151-156. 10.25674/431. hal-04920060

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# Trait concepts, categories, and databases in soil invertebrates
ecology – ordering the mess

Sophie Joimel<fnref n="1" />*, Anton Potapov<fnref n="2" />,<fnref n="3" />,<fnref n="4" />, Benjamin Pey<fnref n="5" />, Jonathan Bonfanti<fnref n="6" />, Jérôme Cortet<fnref n="7" />,
Tania De Almeida<fnref n="8" />, Sara Di Lonardo<fnref n="9" />,<fnref n="10" />, Davorka K. Hackenberger<fnref n="11" />, Paul Henning Krogh<fnref n="12" />,
Ryszard Laskowski<fnref n="13" />, Susana Loureiro<fnref n="14" /> and Mickael Hedde<fnref n="6" />

Received 23 Juli 2024 | Accepted 10 August 2024

Published online at www.soil-organisms.org 1 December 2024 | Printed version 15 December 2024

DOI https://doi.org/10.25674/431

## Abstract

The trait-based approach is increasingly used for soil invertebrates. Complementary to the taxonomy based approach, the trait-based approach can provide a more mechanistic understanding of the responses of organisms to environmental disturbances and of their effects on soil functioning. However, the application of the trait-based approach across studies is limited by the lack of conceptual consensus among researchers due to the historical development of the idea. There is a large ambiguity and variability in using the term ‘functional traits’ by zoologists and ecologists working on soil invertebrates. In this study, we used a questionnaire and literature scanning to review the practical use of functional traits concept in soil ecology over the last decade. We clarified and expanded the functional trait definition as ‘A functional trait is a measurable characteristic of an individual organism or its colony that has a link to the organism’s fitness and/or its effect on other organisms and/or the environment’. We also reviewed existing

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<footnote n="1">¹ Université Paris-Saclay, INRAE, AgroParisTech, UMR EcoSys, 22 place de l'agronomie, 91120 Palaiseau, France</footnote>

<footnote n="2">² Senckenberg Museum of Natural History Görlitz, Am Museum 1, 02826 Görlitz, Germany</footnote>

<footnote n="3">³ German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, Puschstrasse 4, 04103 Leipzig, Germany</footnote>

<footnote n="4">⁴ International Institute Zittau, TUD Dresden University of Technology, Markt 23, 02763 Zittau, Germany</footnote>

<footnote n="5">⁵ Centre de Recherche sur la Biodiversité et l'Environnement (CRBE), Université de Toulouse, CNRS, IRD, Toulouse INP, Université Toulouse 3 - Paul Sabatier (UT3), Toulouse, France</footnote>

<footnote n="6">⁶ Eco&Sols, Univ Montpellier, CIRAD, INRAE, Institut Agro, IRD, 34060 Montpellier, France</footnote>

<footnote n="7">⁷ CEFE, Univ Montpellier, CNRS, EPHE, IRD, Univ Paul-Valéry Montpellier 3, Montpellier, France</footnote>

<footnote n="8">⁸ Univ. Lille, IMT Nord-Europe, Univ. Artois, JUNIA, ULR 4515 – LGCgE, Laboratoire de Génie Civil et géo-Environnement, 59000 Lille, France</footnote>

<footnote n="9">⁹ Research Institute on Terrestrial Ecosystems-National Research Council (IRET-CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy</footnote>

<footnote n="10">¹⁰ National Biodiversity Future Center (NBFC), Piazza Marina 61, 90133 Palermo, Italy</footnote>

<footnote n="11">¹¹ University of Osijek, Department of Biology, Cara Hadrijana 8A, HR-31000 Osijek, Croatia</footnote>

<footnote n="12">¹² Department of Ecoscience, Aarhus University, C.F. Møllers Allé 4, Building 1120, 8000 Aarhus C, Denmark</footnote>

<footnote n="13">¹³ Institute of Environmental Sciences, Jagiellonian University, Gronostajowa 7, 30-387 Kraków, Poland</footnote>

<footnote n="14">¹⁴ CESAM-Centre for Environmental and Marine Studies & Department of Biology, University of Aveiro, Campus de Santiago, 3810-1930 Aveiro, Portugal</footnote>

<footnote>* Corresponding author, e-mail: sophie.boulanger-joimel@agroparistech.fr</footnote>
trait databases showing a high amount, but also high heterogeneity and low accessibility of data on the functional
traits of soil invertebrates. We suggest synthesising existing trait data and databases, using the functional trait-based
approach consistently and reproducibly, and disseminating it to facilitate research in soil ecology.

**Keywords** Functional traits | soil invertebrates | functional ecology | community ecology | terminology

# 1. Introduction

Soil organisms regulate nutrient cycling, dynamics of soil organic matter, greenhouse gas emissions, soil physical structure and water regimes, contributing thus to numerous ecosystem services (Kibblewhite et al. 2008, Wardle et al. 2004). To understand how soil communities assemble, protect them from threats, and manage their interactions to provide ecosystem services, we need better knowledge of soil biology and ecology, as well as a mechanistic understanding of the links between the diversity of soil communities and soil processes. Soil invertebrates present a vast diversity of sizes, shapes, life history strategies, and diets, each differently impacting soil functioning from the matrix scale (composition of mineral and organic matter) to the structural scale (arrangement of organic and mineral matter) and the soil profile scale (organisation of aggregates, pores and horizons) (Lavelle et al. 2006). While rapidly developing molecular tools fuel the progress in soil functional and community ecology, quantifying the role of invertebrates in soil functioning or understanding community assembly requires a description of their functional diversity.

Soil invertebrates have been grouped by ecologists according to several criteria to a number of ecological groups, such as guilds or functional groups, to embrace their tremendous biological diversity (Briones 2014, Hedde et al. 2022). Such an approach postulates that a group is represented by species sharing similar roles in a given ecological function and/or responding similarly to environmental gradients. While being generally satisfactory, the approach has several methodological and conceptual drawbacks. For example, such groups do not consider within-group variation and often consider adults and not juveniles (Hedde et al. 2022). Many misuses of concepts behind classifications are also noticed (Hedde et al. 2022).

The concept of functional traits was expanded from plants (Violle et al. 2007) to soil invertebrates to overcome these problems (Pey et al. 2014b). According to the conventional definition, functional traits are any morphological, physiological, phenological or behavioural features that can be measured in individuals and affect their fitness (Pey et al. 2014b). Over the last decade, a growing body of literature on traits appeared

in soil invertebrate ecology. Many studies focused on providing additional insights into understanding the responses of soil biodiversity to environmental pressures (the 'response' aspect of functional traits, Lavorel and Garnier, 2002). It was shown that traits may better explain soil biodiversity patterns in response to environmental changes including soil contamination (Hedde et al. 2013a, 2013b, Santorufo et al. 2014), land-use (Joimel et al. 2021, Marliac et al. 2016, Pelosi et al. 2016, 2014, Santorufo et al. 2014, Susanti et al. 2021), or climate change (Bonfanti et al. 2022). Other studies attempted to mechanistically link soil organisms with soil processes or functions (Hedde et al. 2022): the 'effect' aspect of functional traits. For example, the long-standing consensus on the relationship between functional groups and processes in termites (Jouquet et al. 2022) or earthworms (Capowiez et al. 2024) was revised. The trait approach could also be used in evolutionary ecology to predict biodiversity responses to environmental changes of various organisms (Kearney et al. 2021) or in the feeding ecology of springtails (Chen et al. 2017). However, eco-evolutionary approaches are still rarely used in soil invertebrate research (Dalos et al. 2022).

Due to the growing interest in functional traits, which is by far not limited to soil organisms, several papers discussing the concept of traits have appeared in recent years (e.g. Dawson et al. 2021, Kearney et al. 2021, Streit & Bellwood 2023, Weiss & Ray 2019). However, trait-based ecology is sometimes considered to be biased because research tends to focus on the same ecosystems, sometimes poor in species, on the same taxonomic groups, mainly plants (e.g. Díaz et al. 2013), with approaches that are rarely multi-group (Schleuning et al. 2023). As a result, trait-based ecology needs to integrate more animal taxa and a diversity of ecosystems in their approach. Reviews that focus on terrestrial arthropods (Brousseau et al. 2018, Wong et al. 2019) do not include other important groups of soil invertebrates, such as earthworms, springtails or potworms. Currently, the term 'functional traits' is used by zoologists and ecologists working on soil invertebrates ambiguously and with different meanings. The confusion in the understanding the trait concept seems particularly pronounced in arthropod studies (Wong et al. 2019) but is also present in other groups (Dawson et al. 2021). Adopting common definitions within the trait concept
is the basis for semantic integration in trait ecology. By
defining the terms and logical relationships between
them and linking them to data, we can avoid confusion in
science and improve the management of trait data to allow
studies and cross-taxa comparisons, as advocated by
Weiss et Ray (2019). Strong theoretical concepts, rarely
mentioned in trait data collection, could help interpret and
prioritise trait acquisition (Kearney et al. 2021). Thus, a
unification of functional trait semantics is urgently needed
to synthesise trait-based research on soil invertebrate taxa
and facilitate the methodology of this research field with,
for example, future AI-based semantic solutions.

Increasing trait data availability also requires semantic
homogenisation. Over the last years, the development
of trait-based approaches has resulted in the creation of
several databases of soil invertebrates traits. As for the
centralised TRY database on plant traits (Kattge et al.
2011), most of the trait data on soil invertebrates are
dispersed among taxon-specific databases (e.g. Formicidae
or Carabidae) (Homburg et al. 2014, Carabids, Parr et al.
2017, GlobalAnts). Recently, several database initiatives
emerged (e.g. BETSI, Ecotaxonomy), gathering traits of
soil invertebrates in general rather than for specific taxa
(Pey et al. 2014a, Potapov et al. 2019). However, until
today, there has been no single initiative that aims to link
the existing databases on soil invertebrate traits into a
single accessible and interoperable platform for cross-
taxa ecological and eco-evolutionary research. The large
amount of missing data is still a limit to their current use
(Auclerc et al. 2022).

As the first crucial step, there is an urgent need to review
the existing trait databases, traits used, trait data available,
and the understanding of the trait concept among soil
ecologists. Very few papers have been published on
soil invertebrate trait databases (but see Homburg et al.
2014, Parr et al. 2017, Potapov et al. 2019) and many
uncertainties about available data and animal groups they
cover remain. To fill these knowledge gaps, here we (i)
refine the existing functional trait concepts used by soil
ecologists to avoid scientific misunderstandings and
improve trait data interoperability and use and (ii) list
trait data sources, traits and their use in soil invertebrate
research to improve the accessibility of this information
and facilitate the development of integrative cross-taxa
and cross-database tools and projects based on trait-based
approaches.

## 2. Materials and methods

We used a questionnaire addressed to zoologists and
ecologists working on soil invertebrates, who were

familiar with the functional trait approach to identify
the existing concepts and the cognitive representations
about functional traits, the use of functional traits and
trait databases.

The questionnaire included 14 questions (online S1). The questions included information about (i) participants (career stage, experience in trait approaches and specific taxa), (ii) their definition and classification of the ‘functional traits’, (iii) examples of traits used, as well as information about available trait datasets and databases. The questionnaire was distributed electronically on the 10th January 2021 among participants of the EUdaphobase COST Action (https://www.eudaphobase.eu) and Global Soil Biodiversity Initiative (GSBI; https://www.globalsoilbiodiversity.org). We encouraged colleagues to distribute the questionnaire among their colleagues. Although these networks are biased towards the European and North American research communities, we believe they are informative and represent current trends in soil ecology.

Overall, answers from 78 respondents were collected
by the 30th of June 2021. Most respondents already used
a functional trait approach for soil invertebrates in their
research (79%), while others planned to do this approach
and were familiar with the concept. Mid- to late-
career researchers were better represented (73% were
professors or associate professors) compared to early-
career researchers (23% were PhD students or postdocs).
Other respondents were students or non-academic
people (4%). Respondents had experience mostly with
earthworms (55%) and springtails (52%). Other studied
taxa were mites (35%), beetles (27%), nematodes (27%)
and myriapods (23%).

The results of the questionnaire were complemented
with a literature search on the Web of Science and
Google Scholar platforms with the keywords: ‘traits’
AND ‘database*’ AND ‘soil invertebrates’ or ‘soil fauna’
and personal literature libraries of the paper authors.

## 3. Concepts and applications of functional traits in soil ecology

### 3.1 What is a functional trait?

Pey et al. (2014a) proposed the following definition of
functional traits for soil invertebrates: *'Functional traits
are any morphological, physiological, phenological or
behavioural feature that can be measured at individual
level and that affect its fitness'*. This definition seems
to be well accepted by the respondents. In the survey,
96% agreed with this definition, considering that it
fits a broader functional ecology set of concepts and is nicely balanced. This common definition is based on the widely accepted concept of functional traits developed for plants by Violle et al. (2007), who made many efforts to unify and standardise the terminology of ‘morphology, performance and fitness’ traits from Arnold (1983). This general definition is globally used with flexibility by scientists across taxa as, for example, for the notion of individual level, which can be extended to several individuals in the case of bacteria or corals (Dawson et al. 2021). Pey et al. (2014) added also the category of ‘behaviour’ to the definition of traits for soil invertebrates. As mentioned by Wong et al. (2019), if technical applications of trait-based approaches vary among fields, researchers specialising in plants and animals agree on the general properties of traits, defining them as phenotypic features that are strictly measurable in individual organisms. However, several suggestions for the modification of this definition were submitted by the respondents. Some respondents (38%) suggested modifying the definition by including: (i) reference to ecological functions or (ii) new trait categories. The next sections will detail the discussion around the functional trait definition.

## 3.2 ‘Trait’ or ‘functional trait’?

The main disagreement pointed out by respondents was that this definition does not take the trait ‘functionality’ into account. These researchers argue that the functional trait definition should clearly identify the link between the function of a trait and associated ecosystem functions or services for human well-being. Although many researchers agree that traits need to be ‘functional’ (Wong et al. 2019), there is no consensus in the current literature and the survey on understanding of what the ‘functional’ term in the trait definition means. Frequent inconsistencies in the literature led to recent suggestions to abandon the adjective ‘functional’ completely, as it became redundant to the ‘trait’ itself (Streit & Bellwood 2023). The semantic complexity of the term ‘functional trait’ could lead to misunderstandings in ecological research (Mlambo 2014). Indeed, a function presents multiple semantic facets in ecology (Glenk et al. 2012). The term ‘function’ is used as a synonym for processes, referring to object state changes in time (e.g. organic matter decomposition), as a term describing a sum of processes of the entire system (e.g. nutrient cycling), to describe the roles of objects (e.g. the function of earthworms), or as a synonym to ecosystem services. In ecotoxicology, functional traits are used to infer soil functions being affected by chemicals (Beaumelle et

al. 2014). In the definition of functional trait by Violle et al. (2007), the term ‘functional’ refers to the link of this trait to the growth, reproduction and survival of an individual (Arnold 1983, Violle et al. 2007) and not to the impact of this trait on a specific ecosystem function. This eco-evolutionary perspective of function refers to Darwinian fitness, i.e. the organismal reproductive success. Dispersal could also be listed here (Bonte & Dahirel 2017) but is not directly or systematically related to the fitness.

To take these different definitions into account, several overlapping or complementary concepts have emerged around the concept of functional traits. For example, Pey et al. (2014) specifically distinguished performance traits (i.e., growth, reproduction and survival, following Violle et al. 2007) and ecological preferences (e.g. microhabitat or moisture preference) for soil invertebrates. Lavorel & Garnier (2002) separated two types of traits: ‘response traits’, which reflect the influence of the environment on organisms (similar to the eco-evolutionary understanding of trait functions), and ‘effect traits’, which give information about the role of organisms in ecosystem functioning (similar to the ‘ecosystem effect’ understanding of trait functions). 1% of the respondents suggested integrating response traits as ‘traits’, and effect traits as the ‘functional traits’. However, for most respondents (85%), the definition of functional trait does not exclude the response trait aspect, as the majority accept the vision of the definition by Violle et al. (2007) around organism fitness.

The widespread ecosystem-focused point of view contrasts the original ‘fitness-centred’ functional trait definition and may be explained by the general focus on the ecosystem functioning research in soil ecology. Notably, the same trait is often linked to both environmental response and ecosystem affect aspects of organism’s ecology (e.g. body mass, mandible shape, reproduction rate). Wong et al. (2019) highlighted the ability of response traits to influence the resilience of an ecosystem (Lavorel & Garnier 2002, Violle et al. 2007, Wright et al. 2016), and there are numerous other examples of linkages between response and effect traits in plants and animals, making it difficult to strictly assign traits to these categories. That is why, although the distinction between traits and functional traits, or effect and response traits, seems to be clearly defined, it is seldom used in the literature. Parr et al. (2017) mentioned this difficulty and did not include trait categories (e.g. effect traits or response traits) in their database on ants because often this distinction depends on the scientific question asked (Petchey & Gaston 2006). There is also little empirical evidence on the links between ecosystem functioning and effect traits, and there are traits that are
considered functional by definition (e.g. 'trophic' traits, see 5.4) due to their links with ecosystem functioning. The traits used in research are almost always directly or indirectly related to fitness, and there are hardly any traits that do not affect fitness at all. Therefore, virtually any trait has the potential to influence at least one ecosystem function and related service, as well as to react to some environmental changes.

To avoid divergence with general trait-based research, we call for consistent use of the eco-evolutionary view on 'functional traits' as proposed by Violle et al. (2007) for plants or by Wong et al. (2018) and Pey et al. (2013) for animals. It means that traits are functional because they influence the fitness of organisms but not because they influence soil processes, functions or services. It is important to admit that functional traits remain a fuzzy concept, as all traits could be functional to some degree (Dawson et al. 2021). Nevertheless, we recommend further use of the term functional traits (contrary to Dawson et al. 2021) due to the very generic meaning of the word 'trait' in English.

The main task for soil ecologists working on trait approaches is to propose and validate links among specific environmental factors, functional traits and soil processes. These processes can further be connected to ecosystem services and soil functions using measurable indicators (Morgado et al. 2018). This task seems more complex for soil invertebrates than plants because of the many indirect links between soil invertebrates and ecosystem functioning (Weiss & Ray 2019). However, the operability of such a scheme should not be underestimated. Plant ecology has established core lists of traits that are routinely measured (e.g. leaf area, leaf dry matter content, height, seed mass) and used to explain biodiversity-ecosystem functioning relationships. Studies proposing mechanistic environment-trait-process links, and experiments validating them, are the key to developing a more functional trait-based approach for soil invertebrates. Until now, very few studies or databases link traits and associated ecosystems functions, but first attempts have been made (Brousseau et al. 2018, CRITTER; Parr et al. 2017, GlobalAnt). Trait-based trophic ecology (fluxes of carbon and energy) is a way to connect community ecology (responses) to functional ecology (Gravel et al. 2016, Schleuning et al. 2023, 2015).

## 3.3 Morphological, physiological, phenological and behavioural traits

As described above, the definition of a functional trait is evolving. From the definition of Violle et al. (2007), several adjustments were made to adapt this definition to

different taxonomic groups. We can thus note the addition of categories, such as behaviour, proposed by Pey et al. (2014b). Also, some respondents propose to add new categories, such as chemical, physical or by emphasing life history. These proposed additions are also found in Dawson et al. (2021) with behavioural and cultural traits. In this line, Moretti et al. (2016) used five categories of traits for terrestrial invertebrates: morphology (e.g. body size, number of eyes), feeding (e.g. ingestion rate, biting force), life history (e.g. ontogeny, clutch size), physiology (e.g. resting metabolic rate, relative growth rate), and behaviour (e.g. activity time, sociality). According to Pey et al. (2014b), life history is rather a population parameter that can serve as responses/indicators of the underlying trait-based processes and reflect the success of an organism in a specific environment. Finding a universally accepted categorisation is probably impossible for all biota (Dawson et al. 2021) and it seems complex even for soil invertebrates only. Categories proposed by Pey et al. (2014), i.e. morphological, physiological, phenological or behavioural, are often used in publications and seem generic enough to fit most needs. However, different categorisations are not exclusive and depend on the context of how traits are applied. The response-effect categorisation can be used to link specific environmental factors and ecosystem functions to traits. According to their associated function, traits can also be categorised into, e.g., feeding, mobility or engineering traits. While emphasising the flexibility of such categorisations, we also call for using standard categories whenever possible and clearly justifying the use of alternative categorisations and stating the differences.

## 3.4 Level of organisation of traits

Another frequent debate is the measurement at the 'individual' level 'without reference to any other level of organisation'. This disagreement predominantly stems from the superorganism or colony-level traits in social insects, for which several individuals act as a single unit (e.g. ant colony, Hölldobler & Wilson 2009) or for microorganisms, where individuals are difficult to define (fungi, Dawson et al. 2021). Parr et al. (2017) argue that natural selection can operate at the individual and colony levels for ants (Keller 1995), so ant functional traits may be quantified at both levels: an individual worker and the colony. Examples of such traits are the nesting or foraging type of a colony (Kreider et al. 2021). Such traits can help predict the occurrence of a species in the environment (e.g. due to the presence of suitable microhabitats) and the effect of this species on the ecosystem functions (e.g. resource acquisition). Thus, a broad trait definition
should include colony-level traits if a common approach is extended to social insects (e.g. ants) or other soil organisms beyond invertebrates such as microorganisms (De almeida et al. 2024, Elizalde et al. 2020).

## 3.5 Suggestion for a clear definition of functional traits

Due to these misunderstandings in the trait concept, some authors do not restrict themselves to any specific definition of traits and integrate all possible species characteristics, letting users select traits according to their study needs (Jeliazkov et al. 2020). Since the trait concept is unclear, many researchers do not use it. Several authors have, thus, called for simplifying and clarifying the definition of traits to facilitate their use and comparability among different groups of organisms (Dawson et al. 2021). We can only agree and propose to clarify the definition by Pey et al. (2014b) as follows: 'A functional trait is a measurable characteristic of an individual organism or its colony that has a link to the organism's fitness and/or its effect on other organisms and/or the environment'. This definition sensu lato is sufficiently broad to include all groups of soil invertebrates (Figure 1).

Measurable in this definition should be considered as the expression of traits on different mathematical

measurement scales (e.g. ordinal or nominal). In the case of a colour, for example, colour measurement is the quantitative expression of colour and there are a number of methods to quantify color (i.e. colorimetry and spectrophotometry). Also, nominal (or categorial) variables such as body shape or coloration are simplifications of more complex measurements involving ratios (body shape) or characterization tools that are not yet used in soil ecology (color).

## 4. List of traits

A total of 202 'traits' were listed by respondents in our questionnaire, illustrating the large current application of the functional trait concept and diversity in its interpretations. Some terms were quoted several times while some have only appeared once. After screening through the list, we merged records with a similar meaning (e.g. body size and body length) and classified all traits into ecological niches traits, morphological, physiological, and phenological traits. An additional category of elemental and molecular composition traits was suggested to categorise e.g. isotopic and biochemical composition of organisms (Table 1). The full unedited list of responses is given in the online S2.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Clarification of functional trait definition; The figure shows the different terminologies used to define a 'functional trait'. Part a) shows the primary terminology used, emphasising what is included in our sensu lato definition and what terms are excluded from it (in red). Part b) specifies trait categories that may be used in this definition.</figcaption></figure>
## 4.1 Morphological, physiological, phenological and behavioural traits

In line with the data available in databases, the vast majority of the traits cited belong to the category of morphological traits. Among the most frequent, the ‘body size’ trait were mentioned 59 times, mostly expressing the ‘body size’ or ‘size’ (n=30) and ‘body length’ or ‘length’ (n=21) (Table 1). In the databases, it is found almost every time and there is even a database focused solely on this trait (Egrowth). In the same vein, the size of the different parts of the body (e.g. legs, antennae), body shape, colouration, body mass and mouthpart morphology are also frequent. We also noted the citation of many dichotomous traits linked to the presence or absence of organs (e.g. furca, wings, glands). Among traits related to life history, reproduction mode (i.e. sexual versus parthenogenetic) was frequently named. However, other traits, such as reproductive output, were rarely mentioned despite being fundamental intrinsic components of organism fitness. Physiological traits were mentioned even less frequently despite their potential importance for digestion and environmental responses. Among behavioural traits, mobility and feeding behaviour were the most common.

## 4.2 Ecological preferences and tolerances: ecological niche traits

Many respondents listed traits beginning with ‘preference for’ humus, carbon content, habitat type (n=14) or with ‘tolerance to’, for example, temperature (n=14). Here, tolerance refers to the niche width (limits), whereas preference refers to the niche optimum. As highlighted by some authors (Middleton-Welling et al. 2020, Pey et al. 2014b, Wong et al. 2019), these characteristics, present in several publications, are not always considered as ‘traits’ and might be termed as ‘ecological preferences’ (Pey et al. 2014b). To fit better with the niche theory (Chase & Leibold 2003), we propose to rename this group of traits as ‘ecological niche’ that includes the parameters of the distribution of individuals on an ecological gradient: the position (max, min, mode, etc) and the dispersion (range, variance, etc). Position and dispersion parameters reflect eco-evolutionary adaptations through different, often undetermined, trait syndromes. Among those, respondents often mentioned temperature, (micro)habitat preferences and desiccation/water regime preferences. These characteristics are present in several databases such as GlobTherm, which is dedicated to cold or heat tolerance, and are crucial to predict species responses to environmental changes. For instance, tolerance may be

defined as the quantile of the distribution in which the species is still present while its fitness is not optimal.

Ecotoxicological responses can be considered among traits related to the tolerance of organisms to environmental factors or physiological traits related to biomarkers of exposition (e.g. the expression of stress-related proteins). Individual tolerance to toxic chemicals, such as, pesticides, define the fitness of an organism and, in consequence, the probability of a population to persist in a polluted environment (e.g. Geiger et al. 2010, Sánchez-Bayo & Wyckhuys 2019). Presently, tolerance is described separately for each pollutant that organisms may encounter and finding general patterns/traits should be addressed in the future.

## 4.3 Elemental and molecular traits

In a few cases (n=5), the respondents mentioned stable isotope, elemental, or molecular composition of organism as traits. These characteristics can be measured at an individual level, are often taxon-specific (Potapov et al. 2019a), and are informative to predict both responses and impacts of soil invertebrates (Potapov et al. 2019b). Some example studies include stable isotope and fatty acid compositions in trait-type analyses to understand the evolution and ecological responses of soil invertebrates (Chen et al. 2017, Zhou et al. 2022). These traits such as stable isotope composition is relation to fitness of soil invertebrates because they clarified clarified basal food resources and allow a more comprehensive understanding of the structure and functioning of soil food webs (Potapov et al. 2019b). We thus propose to include these characteristics in the commonly used functional traits.

## 4.4 Functional groups and trait syndrome

Trophic guilds, feeding groups, or other ecological or morphology-based categories were mentioned 20 times by the respondents. Can they be considered functional traits? Brousseau et al. (2018) argue that they cannot because they typically describe a combination of traits rather than a functional attribute per se. It is therefore more of a trait syndrome than a trait as such. Rarely defined (but see Meyer et al. 2022), this notion appears several times in the literature (e.g. Chin et al. 2023) to illustrate a simultaneous response of functional traits to a disturbance. A trait syndrome is also a combination of species traits in a community. The widespread ‘functional group’ approach (Wong et al. 2019) assigns species to different functional groups a priori, based on observed or assumed characteristics or functions
<table><caption>**Table 1.** Traits *sensu lato* listed by the questionnaire respondents. The responses out of the definition (e.g. ecological groups and community parameters) are removed and listed in the text (raw traits contains 202 terms of "traits", some terms were quoted several times).</caption><thead><tr><th>Category</th><th>Traits</th><th>Number of citations</th></tr></thead><tbody><tr><td><strong>Behaviour</strong></td><td></td><td></td></tr><tr><td></td><td>mobility (vertical or horizontal)</td><td>6</td></tr><tr><td></td><td>feeding behaviour</td><td>5</td></tr><tr><td></td><td>hunting strategy/type</td><td>3</td></tr><tr><td></td><td>nesting behaviour</td><td>2</td></tr><tr><td></td><td>burrowing behaviour</td><td>1</td></tr><tr><td></td><td>avoidance behaviour</td><td>1</td></tr><tr><td></td><td>orientation behaviour</td><td>1</td></tr><tr><td></td><td>feeding habits/preferences</td><td>8</td></tr><tr><td><strong>Ecological niche traits</strong></td><td></td><td></td></tr><tr><td></td><td>habitat, microhabitat, soil layer preference</td><td>14</td></tr><tr><td></td><td>water regime preference (hygrophily, tolerance to water logging or flooding)</td><td>6</td></tr><tr><td></td><td>drought/desiccation resistance/tolerance</td><td>5</td></tr><tr><td></td><td>vertical distribution</td><td>5</td></tr><tr><td></td><td>temperature tolerance/preference</td><td>4</td></tr><tr><td></td><td>sensitivity (e.g. to chemicals)</td><td>3</td></tr><tr><td></td><td>acidity tolerance/preference</td><td>2</td></tr><tr><td></td><td>halotolerancy</td><td>1</td></tr><tr><td></td><td>preference for given C/N ratios</td><td>1</td></tr><tr><td></td><td>frost tolerance</td><td>1</td></tr><tr><td></td><td>light sensitivity</td><td>1</td></tr><tr><td><strong>Morphology</strong></td><td></td><td></td></tr><tr><td></td><td>body length/size/width</td><td>59</td></tr><tr><td></td><td>body colour/pigmentation/patten</td><td>19</td></tr><tr><td></td><td>body weight/mass</td><td>18</td></tr><tr><td></td><td>body shape/form/modification</td><td>11</td></tr><tr><td></td><td>leg/appendages length</td><td>10</td></tr><tr><td></td><td>mouthpart morphology/type/strength</td><td>10</td></tr><tr><td></td><td>eye type/size/development/number</td><td>7</td></tr><tr><td></td><td>wing presence/morphology</td><td>4</td></tr><tr><td></td><td>postantennal organ</td><td>4</td></tr><tr><td></td><td>scales</td><td>3</td></tr><tr><td></td><td>presence of defensive structures/chemical defence</td><td>3</td></tr><tr><td></td><td>dispersal ability/mode</td><td>3</td></tr><tr><td></td><td>furca length</td><td>2</td></tr><tr><td></td><td>pseudocelli</td><td>2</td></tr><tr><td></td><td>leg morphology</td><td>2</td></tr><tr><td></td><td>other</td><td>9</td></tr><tr><td><strong>Phenology</strong></td><td></td><td></td></tr><tr><td></td><td>time to maturity</td><td>1</td></tr><tr><td></td><td>reproduction type/mode</td><td>9</td></tr><tr><td></td><td>reproduction speed</td><td>1</td></tr><tr><td></td><td>longevity</td><td>1</td></tr><tr><td></td><td>reproductive season</td><td>1</td></tr><tr><td></td><td>life time</td><td>1</td></tr><tr><td></td><td>fecundity</td><td>1</td></tr><tr><td><strong>Physiology</strong></td><td></td><td></td></tr><tr><td></td><td>respiration type</td><td>2</td></tr><tr><td></td><td>metabolic rate</td><td>2</td></tr><tr><td></td><td>assimilation efficiency</td><td>2</td></tr><tr><td></td><td>metal detoxication</td><td>1</td></tr><tr><td></td><td>Mt-gene expression</td><td>1</td></tr><tr><td><strong>Elemental and molecular traits</strong></td><td></td><td></td></tr><tr><td></td><td>stable isotope composition</td><td>2</td></tr><tr><td></td><td>fatty acid composition</td><td>2</td></tr><tr><td></td><td>stoichiometry</td><td>1</td></tr></tbody></table>
of interest. However, in reality, many assumptions behind such groupings have no real measurements in the background (Hedde et al. 2022). For example, for earthworms or springtails, classifications can be based on morphological criteria (e.g. ecomorphological groups, figure 1) to determine spatial distribution in the soil, without the values limits or links between these values and their vertical distribution being clearly established. If functional groups could be seen as trait syndromes, they need to be better defined first. Traits can help define these groups, as has been demonstrated for the revised system of the ecological groups of earthworms defined by using several traits (Bottinelli & Capowiez 2021). In the meantime, functional group and trait approaches should be considered in parallel, being conceptually different.

## 4.5 Taxonomic composition and structure

More surprising was the presence in the list of traits of community indicators such as the Shannon index, biodiversity, or composition (mentioned 5 times). Indices of taxonomic diversity are *not* functional traits since they refer to the distribution of individuals across taxonomic groups. Therefore, we recommend that taxa-based indices should not be included in the functional trait approach, as this can cause serious confusion between taxonomic-based and trait-based approaches. Taxonomic-based approaches need to complement trait-based approaches, but conceptually they are different.

## 5. List of databases

The second aim of this study was to list trait data sources, traits and their use by soil ecologists. Currently, most trait data are distributed among private datasets/spreadsheets and online data portals with different access levels. Here we refer to 'database' as an overarching term, referred to any management of ecological data (Jones et al. 2006). This includes: (i) single-user databases such as spreadsheets (e.g. in Open Office, Microsoft Excel) or desktop relational database systems (e.g. in Microsoft Access, Filemaker Pro). Ecologists frequently use these formats (e.g. SoilBioStore) because they are relatively easy to set up. However, they do not allow for easy data sharing for several reasons: they are often built for a specific project which limits data standardisation, and sometimes they are built with software tools that become obsolete with time. Multiuser databases also have different levels of data standardisation. (ii) Databases published in data

repositories, such as Figshare or Dryad, that integrate multiple project-specific databases. Despite being open for use, they have similar data integration drawbacks like spreadsheets or desktop databases because they do not force data standardisation and often have too generic metadata to search for specific traits or taxa. (iii) Structured databases where detailed information about the data of interest is present (metadata) (Michener 2006, Michener et al. 2007) and data are structured according to a certain format. They improve scientific data understanding and management (e.g. information on context, protocols, semantics, data structure) and can facilitate data use beyond the original goals of their collection. Metadata can be used for a specific-project database or to connect them favouring data integration and interoperability. In ecology, sets of metadata have been adopted by the community of ecologists (e.g. EML, Ecological Metadata Language) but there are many other metadata standards, which are not automatically compatible and that trait data standardisation requires additional efforts by data providers.

In total 12 databases were mentioned in the questionnaire and many are single-user databases. The most used databases cited in the questionnaire were: BETSI (20 times), Ecotaxonomy and Nempaplex (5 times each). Through literature search and expert consultations, we identified 15 other databases, resulting in 27 databases that include data on soil invertebrates traits (Table 2). Note that some databases are not displayed as trait databases but are used as such (e.g. Tardigrada Register).

The availability of databases varies greatly. Access to single-user databases (e.g. Microsoft Excel spreadsheets or Microsoft Access databases) is often at the author's discretion. However, some authors provide data openly via a website or in R (e.g. Egrowth, SoilBioStore), or as an article supplement (e.g. Ellers et al. 2018, Hishi et al. 2019, Makkonen et al. 2011). Notably, however, most of trait databases used in publications are not provided in any format. This is of concern because of poor data standardisation across studies as each author formats data and defines traits in his/her own way. Moreover, this hampers reproducibility of the study, violating foundations of scientific research publication. We thus call for open sharing of trait data (Gallagher et al. 2020) via, e.g., a data paper format, which is now offered by many journals and provides benefits for the data author regarding of publication and citations.

Most multi-user databases have open data but require registration before these data can be accessed. Only a few databases provide the definitions associated with the traits, although some are based on an editable thesaurus (e.g. Myriatrix, MilliBase and Ecotaxonomy). There were also difficulties with access due to offline
<table><caption>**Table 2.** Information on the 27 databases (including single-user spreadsheets, spreadsheets published in data repositories, and structured databases) hosting data on functional traits of soil invertebrates. References list includes database papers if existing or examples of database use.</caption><thead><tr><th>Database name</th><th>Taxonomic groups</th><th>References</th><th>Open data</th><th>Number of traits/taxa if known</th><th>Example of traits</th></tr></thead><tbody><tr><td colspan="6"><strong>Multitaxa databases</strong></td></tr><tr><td>BETSI - Biological and Ecological Traits for Soil Invertebrates</td><td>multiple soil invertebrate taxa</td><td>Pey et al. 2014a 2014b</td><td>Open data</td><td>Up to 300 traits (up to 1300 taxa)</td><td>body length, adult activity, reproduction type</td></tr><tr><td>CESTES - metaCommunity Ecology: Species, Traits, Environment and Space</td><td>multiple soil invertebrate taxa</td><td>Jeliazkov et al. 2020</td><td>Access upon special request</td><td>14 traits (72 taxa)</td><td>ability to burrow, body mass, body length</td></tr><tr><td>CRITTER - Canadian Repository of Invertebrate Traits and Trait-like Ecological Records</td><td>multiple soil invertebrate taxa</td><td>Brousseau et al. 2018, Handa et al. 2017</td><td>Open data after registration</td><td>50 traits (400 taxa)</td><td>mouthpart type, Body shape, Diet Activity</td></tr><tr><td>Ecotaxonomy</td><td>multiple soil invertebrate taxa</td><td>Potapov et al. 2019, Sandmann et al. 2019</td><td>Open data</td><td>Over 500</td><td>D13C bulk, feeding mechanism, furca shape</td></tr><tr><td>ECOTOX Knowledgebase</td><td>multiple soil invertebrate taxa</td><td>Olker et al. 2022</td><td>Access upon special request</td><td>-</td><td>single chemical environmental toxicity data</td></tr><tr><td>Edaphobase</td><td>multiple soil invertebrate taxa</td><td>Burkhardt et al. 2014</td><td>Open data after registration</td><td>-</td><td>morphometric data</td></tr><tr><td>GlobTherm - a global database on thermal tolerances for aquatic and terrestrial organisms</td><td>multiple soil invertebrate taxa</td><td>Bennett et al. 2018</td><td>Access upon special request</td><td>Up to 2000 taxa</td><td>heat tolerance, cold tolerance, acclimatisation</td></tr><tr><td>M.P. Berg (VU University Amsterdam, unpublished data)</td><td>Isopoda & Collembola</td><td>Bokhorst et al. 2012, Ellers et al. 2018, Makkonen et al. 2011, Widenfalk et al. 2016</td><td>Closed, except a few datasets published with papers</td><td>10-20 traits</td><td>body length, drought resistance, macro-habitat width</td></tr><tr><td>NEON - National Ecological Observatory Network</td><td>multiple soil invertebrate taxa</td><td>Stachewicz et al. 2021</td><td>Access upon special request</td><td>Up to 2700 taxa</td><td>body length, head width, antennae length</td></tr><tr><td colspan="6"><strong>Single-taxon databases</strong></td></tr><tr><td>Dataset Oribatida</td><td>Acari (mites)</td><td>Minor et al. 2017</td><td>Closed</td><td>3 traits</td><td>maximum body length, feeding guilds, and reproduction modes</td></tr><tr><td>Spiders Functional Trait Dataset</td><td>Araneae (spiders)</td><td>Macías-Hernández et al. 2020</td><td>Open data</td><td>12 traits (506 taxa)</td><td>body length, vertical stratification, foraging strategy</td></tr><tr><td>Carabids.org - a dynamic online database of ground beetle species traits (Coleoptera, Carabidae).</td><td>Carabidae (ground beetle)</td><td>Homburg et al. 2014</td><td>Open data after registration (but unavailable for several years)</td><td>6 traits (up to 1000 taxa)</td><td>body size, hind wing development, hibernation, reproduction time</td></tr><tr><td>COLTRAIT</td><td>Collembola (springtails)</td><td>Salmon et al. 2014</td><td>Closed</td><td>25 traits</td><td>body size, reproduction type, body shape</td></tr><tr><td>SoilBioStore</td><td>Collembola (springtails)</td><td>D'Annibale et al. 2017</td><td>Open data</td><td>18 traits (132 taxa)</td><td>colouration, mouthpart, moisture preference</td></tr><tr><td>Taxonomy, Distribution, and Trait Data Sets of Japanese Collembola</td><td>Collembola (springtails)</td><td>Hishi et al. 2019</td><td>Open data</td><td>13 traits</td><td>body length, furca length, anal spine</td></tr><tr><td>Ant Profiler - A database of ecological characteristics of ants</td><td>Formicidae</td><td>Bertelsmeier et al. 2013</td><td>Open data after registration (but unavailable in 11/2021)</td><td>24 traits</td><td>habitat, diet, body size, colony</td></tr></tbody></table>
<table><caption>Table 2 continued.</caption><thead><tr><th>Database name</th><th>Taxonomic groups</th><th>References</th><th>Open data</th><th>Number of traits/taxa if known</th><th>Example of traits</th></tr></thead><tbody><tr><td>GlobalAnts - a new database on the geography of ant traits</td><td>Formicidae</td><td>Parr et al. 2017</td><td>Open data after registration</td><td>26 traits (up to 1913 species)</td><td>colony type, worker number, pilosity, mandible length</td></tr><tr><td>Shelled Gastropods of Western Europe</td><td>Gastropoda (snails)</td><td>Falkner et al. 2001, Astor et al. 2017</td><td>Paying CD/book except datasets published with papers</td><td>At least 9 traits (up to 270 taxa)</td><td>shell shape, age at maturity, humidity preference</td></tr><tr><td>Egrowth -A global database on intraspecific body growth variability in earthworm</td><td>Lumbricina (earthworms)</td><td>Mathieu 2018</td><td>Open data</td><td>1 trait (51 species)</td><td>body mass</td></tr><tr><td>Dataset Chilopoda</td><td>Myriapoda</td><td>Bonato et al. 2018</td><td>Open data</td><td>6 traits</td><td>maximum body length, number of ocelli</td></tr><tr><td>APHIA et MilliBase</td><td>Myriapoda</td><td></td><td>Open data</td><td>-</td><td>stage, body size, functional group</td></tr><tr><td>NINJA</td><td>Nematoda</td><td>Sieriebriennikov et al. 2014</td><td>Open data</td><td>-</td><td>feeding types</td></tr><tr><td>Mulder & Vonk (2011)</td><td>Nematoda</td><td>Mulder and Vonk 2011</td><td>Closed</td><td>5 traits (up to 30000 individuals)</td><td>body length, width, and estimated mass of nematodes</td></tr><tr><td>NEMAguilts</td><td>Nematoda</td><td>-</td><td>Open data</td><td>-</td><td>trophic functional traits</td></tr><tr><td>NEMAPLEX</td><td>Nematoda</td><td>-</td><td>Open data</td><td>4 traits (up to 8500 taxa)</td><td>body mass, feeding type</td></tr><tr><td>Dataset protists</td><td>Protists</td><td>Fiore Donno et al. 2019</td><td>Open data</td><td>3 traits</td><td>feeding mode, morphology, and locomotion mode</td></tr><tr><td>Tardigrada Register</td><td>Tardigrada</td><td>-</td><td>Open data</td><td>-</td><td>cuticle, buccal, legs &claws</td></tr></tbody></table>

time of consultation (e.g. carabids.org, Nov. 2021) or even disappearing access interfaces from the web (Ant Profiler, Nov. 2021). Existing multi-user databases are not always interoperable with e.g. DarwinCore or other informatic language standards (Wieczorek et al. 2012). Although we should note the efforts made in the user interface Ecotaxonomy or the assistance programmes for the coding of functional traits in BETSI, we have to admit that the general interoperability and other open science principles (Wilkinson et al. 2016) are still in their infancy in trait-based research on soil invertebrates.

Many databases focus on a single taxonomic group. Although several databases aimed at covering multiple taxonomic groups (e.g. BETSI), they usually still focus on a few groups or even a single group. Among the groups covered the best in the existing multi-taxa databases we find springtails, earthworms and carabid beetles, which are also well represented in the single-group databases. On the other hand, four databases are available for nematodes but this group is not included in multi-taxa databases. The number of traits defined in the databases varies from just one (e.g. Egrowth) to over 300 (e.g. BETSI, Ecotaxonomy). However only a few most common traits are well informed in the latter.

Very few databases have addressed the issue of intra-specific or even intra-individual variability (but see Egrowth for body length). The reason is undoubtedly very prosaic: taking individual measurements takes time. To compensate for this, some databases, such as BETSI, integrate data from different literature sources, making it possible to concatenate trait data from different countries, at least on the European continental scale. It has been shown that this intra-specific variability, particularly in body length, can play a role in diversity/ function links, both from measurements taken from the literature (Bonfanti et al. 2018) and experimental measurements (Chassain et al. 2023). However, there is still a long way going to analyse trait data on continental or global scales, and even more so with an intraspecific analytical approach.

While we consider databases on microorganisms (see Cébron et al. 2022) which is out of our scope here, an integrative approach across microorganisms and invertebrates can be proposed in the future (Romillac & Santorufo 2021). Overall, we conclude that trait data on soil invertebrates are currently very heterogeneous and dispersed over a multitude of sources, varying in the level of purpose, standardisation, and access level. This situation
is unfortunate and we call for integration of the databases
in the future to aid cross-taxon and cross-continental
studies on functional traits in soil animal ecology.

6. Conclusions

This review reflects on the development of trait
approaches in soil ecology over the last decade. A
wide diversity in the understanding of the concept has
led to a diversification in the research field, but also to
poor integration and standardisation across studies.
We advocate that additional effort should be made by
researchers to clearly define the traits that they use and
the concepts that they apply. Based on recent publications,
the questionnaire among specialists and our own
experience, we propose a broad definition of functional
traits for soil invertebrates and outline what should not or
must not be considered a trait (e.g. ecological groups or
community parameters). We suggested including colony-
level traits for social insects to make the functional trait
approach more universal across taxa. We also identified
a new category of ‘elemental and molecular’ traits
derived from the body composition but measured at the
organismic level and informing on the processes at the
organismic level. The ecotoxicological characteristics
of species are currently neglected in ecological trait
databases even if they are crucial for understanding and
predicting the responses of populations and communities
in an ever more contaminated world. Plenty of data on
the functional traits of soil invertebrates exist. However,
most data are not easily accessible and distributed across
multiple databases with different interoperability and
access levels. Better integration across databases and
acceptance of open science principles among researchers
will promote standardisation, dissemination and
development of trait-based approaches in soil ecology.

7. Acknowledgements

The authors would like to thank all the members of COST
Eudaphobase and especially the third working group. A.P.
was funded by the Deutsche Forschungsgemeinschaft
(DFG, German Research Foundation) – Projektnummer
493345801 and FZT 118, 202548816. S.L. was supported
by national funds (O.E.), through FCT/MCTES, as
well as for financial support to CESAM by FCT/
MCTES (UIDP/50017/2020 + UIDB/50017/2020 +
LA/P/0094/2020), through national funds. Sara Di
Lonardo was supported by the National Recovery

and Resilience Plan (NRRP), Mission 4 Component 2
Investment 1.4 - Call for tender No. 3138 of 16 December
2021, rectified by Decree n.3175 of 18 December 2021
of Italian Ministry of University and Research funded
by the European Union – NextGenerationEU (Project
code CN_00000033, Concession Decree No. 1034 of 17
June 2022 adopted by the Italian Ministry of University
and Research, CUP B83C22002930006, Project title
‘National Biodiversity Future Center - NBFC’.

7.1 Funding

This work was supported by the COST Action
EUdaphobase ‘CA18237: European Soil-Biology Data
Warehouse for Soil Protection’.

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