Top-Down and Bottom-Up Effects on Insect Herbivores Under Global Change-A Meta-Analysis.

Global environmental change is driving widespread insect declines, yet underlying mechanisms remain poorly understood, particularly when mediated by trophic interactions. Many studies purportedly test global change stressors on insect herbivores mediated via their host plants (bottom-up) and natural enemies (top-down), but we lack a comprehensive understanding of overall effects. Here, we performed a meta-analysis of 246 papers testing 18 distinct global change stressors impacting insect herbivores via trophic interactions. We found variable, often opposing impacts of global change stressors on herbivore fitness when herbivores and their host plants were exposed, while the only detected significant effect involving natural enemies was a reduced herbivore fitness under warming, potentially due to increased enemy attacks. Notably, generalists exhibited greater vulnerability to global change than specialists, challenging assumptions that dietary generalists are more resilient. Insect herbivores tested in agricultural systems were more prone to be benefited by global change, whereas closed experimental systems amplified negative outcomes. Our results underscore the importance of considering trophic interactions and context-specific dynamics in predicting ecological responses to environmental change. By integrating bottom-up and top-down mechanisms, our study provides a nuanced quantitative understanding of how global change influences insect herbivores, with important implications for biodiversity conservation in the Anthropocene.