Tree diversity promotes functional dissimilarity and maintains functional richness despite species loss in predator assemblages
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The effects of species loss on ecosystems depend on the community’s functional diversity (FD). However, how FD responds to environmental changes is poorly understood. This applies particularly to higher trophic levels, which regulate many ecosystem processes and are strongly affected by human-induced environmental changes. We analyzed how functional richness (FRic), evenness (FEve), and divergence (FDiv) of important generalist predators—epigeic spiders—are affected by changes in woody plant species richness, plant phylogenetic diversity, and stand age in highly diverse subtropical forests in China. FEve and FDiv of spiders increased with plant richness and stand age. FRic remained on a constant level despite decreasing spider species richness with increasing plant species richness. Plant phylogenetic diversity had no consistent effect on spider FD. The results contrast with the negative effect of diversity on spider species richness and suggest that functional redundancy among spiders decreased with increasing plant richness through non-random species loss. Moreover, increasing functional dissimilarity within spider assemblages with increasing plant richness indicates that the abundance distribution of predators in functional trait space affects ecological functions independent of predator species richness or the available trait space. While plant diversity is generally hypothesized to positively affect predators, our results only support this hypothesis for FD—and here particularly for trait distributions within the overall functional trait space—and not for patterns in species richness. Understanding the way predator assemblages affect ecosystem functions in such highly diverse, natural ecosystems thus requires explicit consideration of FD and its relationship with species richness.
KeywordsBEF China Biodiversity Ecosystem function Invertebrate Trophic interaction
We thank the administration of the Gutianshan National Nature Reserve and the BEF China team for their support. Data on basal area of trees were provided by Martin Baruffol and Martin Böhnke. We are grateful to Sabine Both and Alexandra Erfmeier for providing data on herb layer plant diversity. The comments of two anonymous reviewers helped to improve the manuscript. Funding by the German Research Foundation (DFG FOR 891) and the National Science Foundation of China (NSFC 30710103907 and 30930005) is gratefully acknowledged. O. P. acknowledges the support of the German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, funded by the German Research Foundation (FZT 118).
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