Fig. 1: Hypothesized effects of insect herbivory on ecosystem element cycling in a broadleaved forest.

Although herbivores exert a wide variety of other direct and indirect effects, our study focused on one major direct effect of herbivores—removal of foliar matter. Briefly, insect foliar herbivory (H) constitutes an important pathway for labile carbon and nutrients to move from green foliage to the soil—via excreta, cadavers, unconsumed leaf fragments, early abscised leaves, and leachate. Once these products of H enter soil, contained nutrients alter a range of processes that support soil microbial communities and plant growth. Foliar production (FP), while not tested here, would be negatively affected by H at the individual target plant level, but neutrally or positively affected by H at the stand level as adjacent non-target trees, composed of herbivore resistant genotypes or species, benefit from access to additional fluxes of growth limiting nutrients. Similarly, greenleaf nutrients (F_E) would decline at the target plant level as a result of H, but for similar reasons as FP, would remain unchanged or even increase at the stand level. RE_E represents the difference in element content between green and freshly senescent leaves, the quantity of which would be absorbed by the tree prior to senescence. H_c (FP x F_E x H) represents the gross amounts of elements consumed by insect folivores, and H_i (L_EH + H_c – L_E) refers to the additional (net) element inputs from insect folivores due to release of nutrient rich green leaf material prior to resorption¹⁰. In all cases, the subscript E refers to elements. Arrow sizes denote the relative size of the flux. Herbivory-related calculations are fully described in Supplementary Table 1. Tree silhouette adapted from NikhomTreeVector/Shutterstock.com.