Warming indirectly simplifies food webs through effects on apex predators

Warming alters ecosystems through direct physiological effects on organisms and indirect effects via biotic interactions, but their relative impacts in the wild are unknown due to the difficulty in warming natural environments. Here we bridge this gap by embedding manipulative field experiments within a natural stream temperature gradient to test whether warming and apex fish predators have interactive effects on freshwater ecosystems. Fish exerted cascading effects on algal production and microbial decomposition via both green and brown pathways in the food web, but only under warming. Neither temperature nor the presence of fish altered food web structure alone, but connectance and mean trophic level declined as consumer species were lost when both drivers acted together. A mechanistic model indicates that this temperature-induced trophic cascade is determined primarily by altered interactions, which cautions against extrapolating the impacts of warming from reductionist approaches that do not consider the wider food web. Using a bioenergetic model and manipulative field experiment along a natural stream temperature gradient, the authors identify a temperature-induced trophic cascade where the presence of fish increases algal biomass and reduces decomposition, but only under warming.

Automated summary

This study investigates the interactive effects of warming and fish predators on freshwater ecosystems. The presence of fish and warming together result in changes in the food web and a reduction in consumer species.