8 - Complex numerical responses to top-down and bottom-up processes in vertebrate populations

Summary

Introduction

The intrinsic rate of growth of animal populations (r_max) is a speciesspecific character that is determined by a trade-off between reproductive capacity and survival. In simple form, given a finite amount of resources such as food and time, a species can evolve adaptations that either enhance reproduction and result in lower survival, or increase survival at the cost of lower per capita reproduction. These life-history features are related to body size in a wide range of animal species from protozoa to mammals, with r_max negatively related to body size (Blueweiss et al. 1978; Caughley & Krebs 1983; Sinclair 1996).

The species-specific adaptation, r_max determines how species respond to environmental impacts. In a given environment, both large and small species experience the same negative environmental effects, and the degree to which the species are adapted to resist decline or tolerate them is reflected by r_max. Body size buffers large mammals against environmental disturbance compared with smaller mammals, and this contributes to the greater apparent stability of large-mammal populations. Therefore, in mammals, population variability is inversely related to body size when considered over absolute time. However, when corrected for generation length, there is no relationship between population variability and body size. This implies that all species show the same intrinsic degree of population variability. Thus, when lifespan is taken into account, small species do not experience any more severe extrinsic perturbations than larger species (Sinclair 1996).