Functional Response of Herbivores in Food‐Concentrated Patches: Tests of a Mechanistic Model

Abstract

Type II functional responses are frequently observed in herbivores feeding in patches where plants are concentrated in space. We tested a mechanistic model of regulation of intake rate of herbivores foraging in food—concentration patches (Laca and Demment 1992, Spalinger and Hobbs 1992) that accounts for asymptotic, Type II responses. The model is based on the hypothesis that competition between cropping and chewing regulates instataneous intake rate in response to changes in the size of bites obtained by the forager. We tested this hypothesis and examined the ability of our model to account for observations of intake rate of 12 species of mammalian herbivores ranging in body mass over 4 orders of magnitude. We measured short—term intake rates of mammalian herbivores feeding in hand—assembled patches of plants. We varied bite size by changing plant height and density in patches offered to herbivores, and observed dry matter intake rates in response to this variation. Averaged across species, our model accounted for 77% of the variance in food intake rate (P <.001 for all species). Predictions of maximum intake rate closely resembled observations of processing capacity, demonstrating that processing rather than cropping sets an upper limit on short—term intake. Tests of model mechanisms provided strong support for the hypothesis that competition between cropping and chewing is responsible for the Type II functional response seen in herbivores feeding in food—concentration patches. The model was able to consistently predict intake rates observed in 16 previous studies. These results indicate that plant characteristics regulating bite size (e.g., leaf size and geometry, spinescen) frequently control instantaneous rates of food intake by mammalian herbivores.