FUNCTIONAL-RESPONSES OF ADAPTIVE CONSUMERS AND COMMUNITY STABILITY WITH EMPHASIS ON THE DYNAMICS OF PLANT-HERBIVORE SYSTEMS

A comparatively recent focus in consumer-resource theory has been the examination of whether adaptive foraging by consumers, manifested thro ugh the functional response, can stabilize consumer-resource dynamics. We offer a brief synthesis of progress on this body of theory and ide ntify the conditions likely to lead to stability. We also fill a gap i n our understanding by analysing the potential for adaptively foraging herbivores, which are constrained by time available to feed and diges tive capacity, to stabilize dynamics in a single-herbivore/two-plant r esource system. Because foraging parameters of the adaptive functional response scale allometrically with herbivore body size, we parameteri zed our model system using published foraging data for an insect, a sm all mammal and a large mammal spanning four orders of magnitude in bod y size, and examined numerically the potential for herbivores to stabi lize the consumer-resource interactions. We found in general that the herbivore-plant equilibrium will be unstable for all biologically real istic herbivore population densities. The instability arose for two re asons. First, each herbivore exhibited destabilizing adaptive consumer functional responses (i.e. density-independent or inversely density-d ependent) whenever they selected a mixed diet. Secondly, the numerical response of herbivores, based on our assumption of density-independen t herbivore population growth, results in herbivores reaching densitie s that enable them to exploit their resource populations to extinction . Our results and those of studies we reviewed indicate that, in gener al, adaptive consumers are unlikely to stabilize the dynamics of consu mer-resource systems solely through the functional response. The impli cations of this for future work on consumer-resource theory are discus sed.