Retention Time and the Functional Response of Beavers

Abstract

We investigated the effects of interspecific variation in forage retention time on rates of food intake and energy gain by beavers. Ad libitum intake rates by beavers were a hyperbolic function of retention time in the digestive tract and a positive linear function of beaver live mass. Mean retention times of animals on mixed diets varied proportionately with diet composition. Daily intake rates by beavers provided with monospecific stands of trembling aspen or speckled alder saplings showed a monotonically decelerating (type 2) functional response to changes in sapling biomass density. Daily intake rates differed between these two forage species, such that alder with a long retention time produced lower consumption rates than aspen with a short retention time. Net rates of energy gain were also reduced when animals foraged on species with long retention time. Our results suggest that interspecific variation in retention time plays an important role in determining rates of dry matter intake and energy gain by beavers, which could influence the stability of beaver-vegetation interactions.