Scaling up from functional response to numerical response in vertebrate herbivores

Considerable progress has been made in the last decade towards understandin g the relationships between vertebrate herbivores and their food supply. Me chanistic approaches to analysing the constraints on food intake, and the c onsequences for population dynamics, are replacing the classical theoretica l descriptions of predator-prey dynamics. The challenge of the former appro ach is to discover what our mechanistic understanding can reveal about proc ess and pattern in plant-herbivore relationships. This chapter describes th e modelling of the processes of food intake and diet selection, from the le vel of the individual bite, up to daily nutrient intake, metabolism, energy balance, reproduction and mortality, thus integrating the mechanisms under lying population dynamics. Two examples, of a temperate and a savanna grazi ng system, are used to show how far mechanistic modelling can be used to ex plain the relationship between vegetation and herbivore abundance and the p hysiological basis of overcompensatory population dynamics.