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ABSTRACT Geographic variation in ecosystem function is often attributed to differences in climate and soil properties, with biophysical constraints assumed to dictate spatial patterns in nutrient cycling, carbon storage, and plant productivity. However, biotic interactions, particularly herbivory, also vary geographically and can generate feedbacks that influence ecosystem processes. Using a replicated three‐year field experiment, we tested how population‐level functional differences in a widespread arthropod herbivore mediate geographic variation in ecosystem function. Structural equation modeling revealed that herbivores exerted strong direct effects on plant biomass, soil carbon, and nitrogen mineralization, often surpassing the influence of historical conditions and geographic variation in climate. Moreover, functionally distinct herbivore populations had divergent effects on nutrient cycling and plant diversity, demonstrating that population‐level differences introduce novel pathways of influence on ecosystem function. These findings challenge ecosystem models that prioritize abiotic constraints and highlight the need to incorporate consumer‐driven feedbacks into ecological frameworks. 

Phenotypic plasticity is often regarded as a key mechanism for coping with environmental change, yet its adaptive potential remains uncertain in part because of inconsistencies in how environmental stressors are defined and studied, and the traits that are studied. We propose a framework that partitions global change into four distinct dimensions: mean change, variability, stochasticity, and episodic events, each of which presents unique challenges for organisms. A central determinant of plasticity's adaptive value is predictability, yet existing studies inconsistently quantify it, conflating structured environmental variation with stochasticity. We introduce standardized approaches to measuring predictability and cue reliability, ensuring that plastic responses are assessed in ecologically meaningful contexts. We then present a multiple‐trait‐based framework for evaluating the likelihood of plastic trait deployment across increasing magnitudes of global change dimensions. This framework serves as a heuristic model to guide research priorities, identify key knowledge gaps, and generate testable hypotheses about the conditions under which plasticity may contribute to persistence in the face of global change. Through a case study ofDaphnia pulex, we demonstrate how the framework can be used to identify key new research approaches and identify empirical data needed to reveal and explain emergent patterns across trait types and global change conditions. By refining predictability metrics and experimental approaches, this framework advances efforts to determine when and where plasticity can buffer populations from global change, offering a foundation for future research and conservation planning.