DIET CHOICE AND THE FUNCTIONAL-RESPONSE OF BEAVERS

We investigated the effects of changing sapling availability on foragi ng selectivity and cutting rates by beavers in large experimental encl osures. As predicted by an energy-maximizing contingency model, the me an size of saplings cut by beavers increased with distance from the lo dge and was positively correlated with sapling density. Species select ivity was also positively correlated with sapling density. Trembling a spen (Populus tremuloides) was preferred to speckled alder (Alnus rugo sa), and red maple (Acer rubrum) was the least preferred species. The functional responses of beavers presented with saplings of a single sp ecies and of similar size differed from those recorded in trials with a single species of saplings of variable sizes and from trials with th ree species of saplings of variable sizes. Size-selective foraging by beavers reduced the maximum rate of sapling cutting at high sapling de nsities relative to the single-size trials, but both treatments showed similar cutting rates at low sapling densities. Species-selective for aging by beavers reduced cutting rates at low sapling densities relati ve to the single-species trials, but both treatments showed similar cu tting rates at high sapling densities. Species-selective foraging intr oduced a slight inflection in the functional response curve for prefer red species, which could have a stabilizing effect on trophic interact ions.