Indirect Effects of Invertebrate Herbivory on the Decomposer Subsystem

Abstract

Invertebrate herbivores can exert important effects on the decomposer subsystem through a rang eof mechanisms. In this chapter, we review the mechanistic bases through which invertebrate herbivory may affect the quantity and quality of plant-derived resources entering the soil. We identify four main types of mechanisms through which this may occur: (1) herbivores can influence resource quantity, both in the short term through promoting rhizosphere exudation and in the long term through optimizing or reducing net primary productivity (NPP); (2) herbivores may affect litter quality, either positively or negatively through inducing plants to produce secondary defence compounds; (3) herbivores sometimes return a significant proportion of NPP to the soil as fecal material, which can have very different consequences to plant litter for decomposers; (4) in the longer term herbivores can significantly alter the functional composition of vegetation which can in turn determine the quality of litter returned to the soil. There are therefore numerous ways in which invertebrate herbivores can affect decomposers either positively or negatively, and these can exert important aboveground feedbacks. Some of the most significant effects of invertebrate herbivores in ecosystems occur during periodic population outbreaks, and the likely consequences of this for the decomposer subsystem are discussed. Usually, herbivores occur in multiple species communities, which leads to the question of how herbivore diversity affects decomposer processes; while there is a dearth of information available on the topic, there are plausible mechanisms whereby such effects could theoretically occur. It is concluded that, since all ecosystems depend upon both the producer and decomposer subsystems, a more complete understanding of ecosystem-level consequences of invertebrate herbivory can only be gained through the application of approaches that explicitly consider both subsystems, as well as the feedbacks between them.