Diet Choice and the Funcional Response of Beavers

We investigated the effects of changing sapling availability on foraging selectivity and cutting rates by beavers in large experimental enclosures. As predicted by an energy—maximizing contingency model, the mean size of saplings cut by beavers increased with distance from the lodge and was positively correlated with sapling density. Species selectivity was also positively correlated with sapling density. Trembling aspen (Populus tremuloides) was preferred to speckled alder (Alnus rugosa), and red maple (Acer rubrum) was the least preferred species. The functional responses of beavers presented with saplings of a single species and of similar size differed from those recorded in trials with a single species of saplings of variable sizes and from trials with three species of saplings of variable sizes. Size—selective foraging by beavers reduced the maximum rate of sapling cutting at high sapling densities relative to the single—size trials, but both treatments showed similar cutting rates at low sapling densities. Species—selective foraging by beavers reduced cutting rates at low sapling densities relative to the single—species trials, but both treatments showed similar cutting rates at high sapling densities. Species—selective foraging introduced a slight inflection in the functional response curve for preferred species, which could have a stabilizing effect on trophic interactions.