Functional responses to resource complexity: an experimental analysis of foraging by beavers

Ecological theory has been based traditionally on univariate functional res ponses by consumers to changing resource abundance. The simplicity of this approach is clearly at odds with the complexity and variability of resource s used by real consumers, particularly terrestrial herbivores, raising doub ts about the utility of simple consumer-resource theory. In this chapter, I review the physiological and behavioural constraints on herbage intake act ing at different scales of space and time, illustrated by our experimental work with beavers in deciduous forest environments. I then consider how rat es of plant attack by beavers are influenced by behavioural responses to th e size distribution of ramets, variation in nutritional quality among ramet s, and the spatial distribution of ramets relative to the centre of beaver foraging territories, using paired experimental trials with manipulated cha nges in resource characteristics. Much of the variation in beaver functiona l and behavioural responses to resource complexity can be explained using o ptimality principles, offering hope that a robust theory of plant-herbivore dynamics may yet emerge, one that links fitness-maximizing behaviours with realistic environmental constraints.