Herbivore functional response in heterogeneous environments: a contest among models

Variation in the spatial arrangement of plant tissue modifies the functional response of herbivores. In heterogeneous environments, this variation can occur at multiple spatial scales. We used likelihood‐based approaches to examine the strength of evidence in data for models of herbivore functional response to spatial variation in plants. These models represented different hypotheses about plant characteristics controlling intake rate, including biomass, bite mass, plant density, and the composite effects of plant density and bite mass. Models were fit to observations of the food intake rate of elk (Cervus canadensis), white‐tailed deer (Odocoileus virginianus), prairie dogs (Cynomys ludovicianus), domestic rabbits (Oryctolagus cuniculus), and lemmings (Dicrostonyx groenlandicus) feeding in plant patches where plant density and plant mass ranged over at least two orders of magnitude. The model representing composite effects of bite mass and plant density had more support in the data than any competing models (Akaike weight for the composite model > 0.999). Maximum‐likelihood estimates of parameters in the composite model closely resembled independent, direct estimates, giving credence to the mechanisms represented by the model. The composite model portrayed a distance threshold (d*, in meters) distinguishing mechanisms regulating functional response. When plant spacing exceeded this threshold, intake rate was regulated by plant density. When the plant spacing was less than the threshold, intake rate was controlled by bite mass. Based on earlier work, we show that the threshold d* scales with body mass (M, in kilograms) as 7.1M⁰.⁰⁶. This relationship illustrates that: (1) spatial pattern of plants at patch scales may influence intake rate only when plants occur at very low density; (2) herbivores of very different body mass respond to heterogeneity in a remarkably similar fashion; and (3) variation in leaf‐scale characteristics controlling bite mass interact with plant spacing at patch scales to influence herbivore functional response. Corresponding Editor: M. S. Boyce