Human Impact Induces Shifts in Trophic Composition and Diversity of Consumer Communities in Small Freshwater Ecosystems

Abstract

The current global rates of human impact intensification in freshwater ecosystems and their surroundings have caused an alarming loss of freshwater biodiversity and functioning across multiple trophic levels. Particularly vulnerable to anthropogenic pressures are small water bodies (SWB). However, the majority of the evidence is limited to a single community, especially macroinvertebrates, thus leaving us uncertain about cross-community multitrophic responses of freshwater consumers to anthropogenic pressure. Furthermore, the effects of human impact on trophic composition and diversity of consumers may depend on the nature of human activities. We therefore tested the effects of different co-occurring human activities (agricultural land use, gravel exploitation, road proximity, and waste input) and of the overall human-impact intensification on trophic trait diversity and composition of different animal communities in SWB. For this, four types of consumer communities (i.e., zooplankton, benthic macroinvertebrates, epiphytic macroinvertebrates, and fish community) were sampled in a total of 58 study sites in two geographic areas in Serbia and Croatia for two years. In addition to human impact, we also tested the effects of altered water properties, macrophyte biomass, and predation. Our results clearly show that human impact induced changes in the trophic diversity and the trophic composition of different consumer communities. The direction and strength of such functional shifts depended on the nature of human impact and on the type of consumer community. Furthermore, we found that the effects of human impacts on the distribution of trophic traits across all consumer communities significantly differed between trophic groups (i.e., herbivores, decomposers, and carnivores) and trophic levels (i.e., trophic position in the food web). Our findings point toward the urgent need to consider multiple stressors and a range of animal community types in future research and management of freshwater ecosystems. These results highlight the importance of a multitrophic perspective when predicting the consequences of human impact for ecosystem biodiversity and functioning.