DOI: 10.5061/dryad.6djh9w1c2
title: Namibian fairy circles: Hostile territory for soil nematodes

description: Fairy circles are rings of grass (2-12 m diameter) with centers of bare
 soil that are found in some arid grasslands. Above- and belowground
 chemical and biological processes have been explored in an ongoing debate
 about the ultimate causes of this pattern. Belowground biota, including
 beneficial and pathogenic nematodes in the soil food web, may both
 influence and respond to the formation of fairy circles. For example,
 root-feeding nematodes have the potential to enhance plant water stress,
 promoting the spatial organization into rings to minimize competition. We
 studied the soil nematode communities associated with Stipagrostis fairy
 circles along a 900-km range in the Namib Desert of Namibia in southern
 Africa. Nematode abundance and diversity were highest in soils beneath the
 vegetation rings that define fairy circles and in soils in the vegetated
 matrix surrounding the rings, demonstrating the positive impact of
 plant-derived resources (i.e., roots and organic matter) on nematode
 communities. Network analysis of nematode communities showed that in many
 ring soils, fungal or root hair-feeding Aphelenchoides, Ditylenchus, and
 Hexatylus frequently co-occurred among other plant-parasitic taxa. In
 contrast, soils from the bare centers of fairy circles had lower organic
 matter content and were nearly defaunated. Fairy circle centers appear to
 be a hostile environment for soil nematodes, reflecting a resource-limited
 soil food web that may contribute to the persistent absence of vegetation.
 Bacterial-feeding Acrobeloides were over-represented in communities in the
 centers, which could reflect an association with termites or ants, whose
 activity has been proposed to play a role in fairy circle formation. These
 findings show that nematode communities respond to the unique
 environmental conditions created by fairy circles, and nematode
 assemblages found in center, ring, and matrix soils also may contribute to
 sustaining the pattern. 
description: Nine fairy circle sites in Namibia were sampled, spanning a
 900-km north-south range that encompasses most of the distribution of
 fairy circles in Namibia. Soils were collected between 24 March – 31
 October 2020.  Six representative circles were selected at each site
 within an approximately 1-ha area. At each circle, four subsamples were
 collected and combined from the center of the circle (“center”) to a depth
 of 0-10 cm using a plastic scoop. Similarly, four subsamples were
 collected and combined from the soil beneath the grass composing the ring
 of vegetation defining the margin of the fairy circle (“ring”), and four
 subsamples were collected and combined from the inter-circle matrix
 (“matrix”), at least 1 m from the ring.  A total of 162 samples were
 collected (nine sites x six rings x three positions). Soils were sieved
 (2-mm mesh) to remove rocks and transferred to plastic bags.
 <strong> </strong> Soil moisture was
 determined gravimetrically (24 h at 105°C). Soil organic matter was
 measured as loss on ignition (LOI, 360°C, 2 h). Suspensions of 10-g
 air-dried soil in 30-ml deionized H<sub>2</sub>O were mixed
 and equilibrated for 30 min before solution pH was measured with a Pocket
 Pro+ pH Tester (Hach, Loveland, CO). Using the same solution, electrical
 conductivity was measured as an indicator of salinity using a conductivity
 meter (Traceable<sup>®</sup>, Cole-Parmer, Vernon Hills,
 IL). Nematodes were extracted over 72 h from 120 g soil
 using a Baermann funnel technique (60 g soil x 2 funnels per sample,
 combined when drawing off the solution). Sample volume was reduced to
 approximately 500 µl, and nematodes were fixed in 5% hot:cold formalin
 solution. Nematodes were counted and identified to the lowest taxonomic
 level possible using a Zeiss inverted microscope. Nematode abundance data
 were used to calculate nematode density (# 100g<sup>-1</sup>
 soil).
description: # Namibian fairy circles: Hostile territory for soil nematodes
 [https://doi.org/10.5061/dryad.6djh9w1c2](https://doi.org/10.5061/dryad.6djh9w1c2) ## Description of the data and file structure We studied the soil nematode communities associated with *Stipagrostis* fairy circles along a 900-km range in the Namib Desert of Namibia in southern Africa. The objective of this study was to investigate the relationship between the fairy circle vegetation pattern and soil nematode diversity and abundance. Soils were collected between 24 March – 31 October 2020.  ### Files and variables #### File: TreonisFairyCircleNematodes.csv **Description:** Soil nematode communities associated with *Stipagrostis* fairy circles in the Namib Desert of Namibia in southern Africa.  ##### Variables * GPS:  Coordinates for site * site: Name of field site * position: location of sample within fairy circle vegetation pattern (center = un-vegetated center, ring = grass surrounding center, matrix = vegetation in between rings * Soil moisture (g/100g): determined gravimetricaly * OrganicMatter (% LOI): Organic content measured as loss on ignition * pH: measured on a soil slurry * EC (uS/cm): Electrical conductivity of a soil slurry * Nematodes/100g: Number of nematodes extracted by Baermann funnel expressed per100 g of soil * taxonomic richness (#): The number of different morphospecies present in sample * Acrobeles (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Acrobeloides (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Chiloplacus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Drilocephalobus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Elaphonema (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Mesorhabditis (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Nothacrobeles (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Panagrobelus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Paracrobeles (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Pelodera (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Plectus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Aphelenchoides (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Aphelenchus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Ditylenchus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Hexatylus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Dolichodoridae (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Dorylaims (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Discolaimus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample. * Carcharolaimus (#/120g):  The number of this taxa present in the 120 g of extracted soil sample.