Decline in niche specialization and trait β-diversity in benthic invertebrate communities of Central European low-mountain streams over 25 years.

Biotic homogenization is one of the key aspects of the current biodiversity crisis. Here we analyzed the trends of three facets of niche homogenization, i.e. niche specialization, trait α-diversity and spatial β-diversity, over a period of 25 years (1990-2014) using a large dataset of 3782 stream benthic invertebrate samples collected from central European low-mountain streams. We studied a set of traits describing the ecological niche of species and their functions: body size, feeding groups, substrate preferences, flow preferences, stream zonation preferences and saprobity. Trait composition changed significantly during the study period, and we identified an overall increase in niche homogenization. Specifically, community niche specialization significantly decreased by 20.3% over the 25-year period, with declines ranging from -16.0 to -40.9% for zonation-, flow-, substrate-preferences, body size and feeding traits. Trait diversity did not change significantly, although we recorded significant decreases by -14.2% and -10.2% for flow- and substrate-preference and increases by 5.8% and 22.6% for feeding traits and zonation preference over the study period. Trait spatial β-diversity significantly decreased by -53.0%, with substrate-preference, feeding groups and flow-preference traits declining from -61.9% to -75.3% over the study period. This increased niche homogenization is likely driven by the increase of down-stream typical taxa, which are favored by warming temperatures. Further, it is in apparent contradiction with the recorded increase in abundance (+35.9%) and taxonomic richness (+39.2%) over the same period. Even such increases do not safeguard communities from undergoing niche homogenization, indicating that recovery processes may differ with regard to community taxonomic composition and traits. Our results emphasize the complexity of community responses to global change and warrant caution when founding conclusions based solely on single community metrics.