Introduction

In this chapter I consider how the contribution of large herbivores to community and ecosystem processes varies with increasing body size. The ecosystem features to be covered include the biomass levels sustained, energy fluxes and nutrient cycling through this biomass, and the stability of these features over time. The basic question is, how different would these patterns and processes be if megaherbivores were absent from the system?

Biomass levels

Population biomass

The biomass level that a species population sustains represents a relation between the production of food in the environment, and the ability of animals of the species to transform the food into animal biomass. In African savanna regions, vegetation production is proportional to land surface modified by rainfall, while the resting metabolic requirements of an animal per unit of mass are proportional to its body mass raised to the power minus one-quarter. Therefore, if the amount of food available in the vegetation were independent of body size, the population biomass supported per unit of land area should vary in relation to M0.25, i.e. larger species should tend to sustain somewhat higher biomass levels than smaller species.

However, two factors modify the simple relationship developed above. Firstly, the mass-specific metabolic requirements of free-ranging animals, allowing for activity costs, may be scaled in relation to a body mass exponent slightly different from −0.25. For herbivorous mammals, the best available estimate of the scaling exponent is −0.27 (from Nagy 1987, see Chapter 5), i.e. field metabolic requirements scale almost identically to basal metabolic requirements.