Behavioral responses to environmental change alter direct and indirect trait-mediated interactions

Abstract

We used a field-based experiment to explore the consequences of altered environmental conditions on a predator–prey–resource system. The foraging choices of grasshoppers on plant resources were measured in the presence and absence of predator spiders and in shaded and unshaded conditions. We show that shading conditions affected the spiders, their direct behavioral interactions with the grasshoppers, and the indirect behaviorally mediated interactions between the spiders and the plants. In unshaded conditions in response to spiders, grasshoppers reduced their energy intake by 90%, reduced their time spent feeding by 68%, and switched to a largely grass diet that should increase mortality, while in shaded conditions spiders induced no behavioral shift. In no shade, spiders had an indirect positive effect on forb plants as evidenced by a trophic cascade pattern in plant biomass in the one-, two-, and three-trophic-level communities. But in shade, there was no indirect interaction of the spiders on the plants. Our results demonstrate the challenge of predicting the effects of environmental change on complex, real-world ecosystems and highlight the critical need for conducting experimental manipulations in the proper context with full complements of species.