More than what they eat: uncoupled biophysical constraints underlie geographic patterns of herbivory

Herbivory rates have classically been hypothesized to decrease from the tropics towards higher latitudes because the more benign abiotic conditions in tropical systems foster greater ecosystem complexity including greater intensity of biotic interactions. However, attempts to quantify latitudinal patterns of herbivory often fail to support this hypothesis. While biases have been offered as explanations for null results, here, we argue that framing the question of latitudinal variation in herbivory around nutrient and energetic constraints of insect herbivores and plants may provide mechanistic explanations of latitudinal herbivory patterns. As a case study, we focused on sodium as an uncoupled nutrient between herbivore and plant communities: sodium is a key limiting micronutrient for herbivore neural and muscular development while present at orders of magnitude lower concentrations in plants. We compared sodium deposition with latitude, mean annual temperature (MAT) and actual evapotranspiration (measure of primary productivity, AET) in their ability to predict consumed percentage leaf area from published datasets. Leaf percent herbivory increased with sodium deposition and MAT and decreased with latitude but was unrelated to AET. Sodium had comparable effect size and predictive ability to either MAT or latitude. Additionally, herbivory was highest in locales with both high sodium deposition and high MAT. Our hypothesis that geographic variation in herbivory is driven by an interaction of unrestrictive temperature environments (high MAT) and limiting nutrient supply to herbivores (high sodium deposition) was strongly supported. We propose that greater generality, predictability and theoretical development on geographic variation in herbivory will arise from a refocus on the biophysical constraints (e.g. productivity, micronutrient availability, leaf mass consumed) that ultimately control consumer interactions rather than latitude per se. This refocus is likely to open new hypotheses for the evolution of defense syndromes across plant populations and communities based on the specific geography of limiting nutrients.