9 - The numerical response: rate of increase and food limitation in herbivores and predators

Summary

Introduction

Herbivores and predators: types of consumer–resource systems

The resources used by animal populations are either non-consumable or consumable (Caughley & Sinclair 1994). While the absolute level of non-consumable resources is generally not influenced through its use (e.g. shelter), the level of consumable resources is (e.g. food). The most comprehensive classification of the relationship between resources and animals is that developed for grazing systems by Caughley & Lawton (1981). They accounted for the degree to which herbivores interact with their food resources and interfere with each other's capacity to access those resources. Interactive grazing systems are those in which herbivore consumption influences the rate of renewal of food plants, which in turn influences the dynamics of the herbivore population itself. Interactive grazing systems are further differentiated into interferential systems in which herbivores can affect each others capacity to assimilate food plants, and laissez-faire systems in which they do not. Non-interactive grazing systems are those in which herbivore feeding has no influence on the rate of renewal of food plants and, hence, no reciprocal influence on the dynamics of the herbivore population. Non-interactive grazing systems are differentiated between reactive systems in which rate of change in herbivore abundance is a function of food plants, and non-reactive systems in which herbivore population dynamics are largely independent of food availability. We argue that this classification encompasses the range of mechanisms that link most animal consumer systems to their food resources and so is applicable to both herbivores and predators.