Natural regulation of herbivorous forest insect populations.

From basic principles of positive/negative feedback control and classical density-dependent/independent population theory we outline a general explanation for the regulation of herbivore populations in forested ecosystems. We identify three fundamental patterns of behavior; (i) populations regulated close to equilibrium ("tight" regulation) by fast-acting negative feedback processes, (ii) populations regulated by delayed negative feedback processes ("loose" regulation) which often exhibit wide-amplitude density cycles, and (iii) populations characterized by positive feedback processes over certain density ranges that may exhibit divergent (metastable) behavior. We then discuss the effects of these different kinds of herbivore population behavior on the dynamics of forest ecosystems and explore some elemental models of herbivore/forest interactions. Finally, using the spruce bark beetle, Ips typographus, as an example, we demonstrate how the theory can be used to explain the dynamics of specific herbivore populations and to develop general strategies for managing destructive herbivore outbreaks.