Nematode distributions as spatial null models for macroinvertebrate species richness across environmental gradients: A case from mountain lakes.

Alejandro Palomo,Walter Traunspurger,Guillermo de Mendoza,Jordi Catalan

doi:10.1002/ece3.2842

Abstract

Nematode species are widely tolerant of environmental conditions and disperse passively. Therefore, the species richness distribution in this group might largely depend on the topological distribution of the habitats and main aerial and aquatic dispersal pathways connecting them. If so, the nematode species richness distributions may serve as null models for evaluating that of other groups more affected by environmental gradients. We investigated this hypothesis in lakes across an altitudinal gradient in the Pyrenees. We compared the altitudinal distribution, environmental tolerance, and species richness, of nematodes with that of three other invertebrate groups collected during the same sampling: oligochaetes, chironomids, and nonchironomid insects. We tested the altitudinal bias in distributions with t‐tests and the significance of narrow‐ranging altitudinal distributions with randomizations. We compared results between groups with Fisher's exact tests. We then explored the influence of environmental factors on species assemblages in all groups with redundancy analysis (RDA), using 28 environmental variables. And, finally, we analyzed species richness patterns across altitude with simple linear and quadratic regressions. Nematode species were rarely biased from random distributions (5% of species) in contrast with other groups (35%, 47%, and 50%, respectively). The altitudinal bias most often shifted toward low altitudes (85% of biased species). Nematodes showed a lower portion of narrow‐ranging species than any other group, and differed significantly from nonchironomid insects (10% and 43%, respectively). Environmental variables barely explained nematode assemblages (RDA adjusted R2 = 0.02), in contrast with other groups (0.13, 0.19 and 0.24). Despite these substantial differences in the response to environmental factors, species richness across altitude was unimodal, peaking at mid elevations, in all groups. This similarity indicates that the spatial distribution of lakes across altitude is a primary driver of invertebrate richness. Provided that nematodes are ubiquitous, their distribution offers potential null models to investigate species richness across environmental gradients in other ecosystem types and biogeographic regions.

Highlights

Biodiversity patterns across environmental and spatial gradients have been a focus of ecological research during the last decades (Chase, 2014; Gaston, 2000; Heino, 2011)
Our results indicate that the nematode species of the mountain lakes of the Pyrenees, unlike other invertebrates, are widely distributed across the altitudinal gradient, and do not show any altitudinal bias in species distributions (Figures 2, 3, and 4)
Nematode species do not respond to altitudinal environmental factors, in contrast to the other groups analyzed, as indicated by the explained variance by the environment (Table 1, Figure 5), which was higher in the other groups than in nematodes (0.02)

Summary

| INTRODUCTION

Biodiversity patterns across environmental and spatial gradients have been a focus of ecological research during the last decades (Chase, 2014; Gaston, 2000; Heino, 2011). Because the altitudinal environmental gradient strongly affects oligochaete and insect species distributions, and previous studies indicate that some nematode species are widely tolerant of environmental changes (Jacobs, 1984; Michiels & Traunspurger, 2005), the environmental tolerance across altitude was expected to be higher for nematodes than for the other invertebrate groups. If this were so, any similarity of species richness patterns between nematodes and other groups could be interpreted as a consequence of similar spatial constraints in the metacommunity dynamics, rather than similar species-environment relationships. We hypothesized (i) that nematodes are scarcely affected by the altitudinal environmental changes compared to other invertebrate groups; and, (ii) that, despite the difference in environmental influence, similarities in species richness patterns between nematodes and other groups indicate that spatial constraints play a major role governing species richness distributions across altitude

| METHODS

Findings

| DISCUSSION