Functional Response During Winter of a Herbivore, The Moose, in Relation to Age and Size

This study was undertaken to provide an estimate of the functional response curve of the moose during winter. The functional response consists of two components: the intake rate while foraging and the time available for food searching. In generalist herbivores an interaction may exist between those two components, because the quality of food eaten (expressed as digestibility) may limit the period available for grazing. Thus, variation in intake rate while foraging in relation to food availability may give a biased estimate of functional response in such animals. We studied the relationship between the different components of functional response during winter for a generalist browser, the moose Alces alces, by comparing the intake rates of free—ranging individuals foraging on three Norwegian winter ranges of different quality. The intake rate was determined by collecting feces defecated during a 24—h period by radio—collared individuals. The amount of food eaten was found by multiplying the dry mass of feces with the inverse of the digestibility of the twigs eaten during that period. No relationship was found between intake rate and either snow depth or temperature. Body mass accounted for a large proportion of the variance in intake rates. The slope in the regression of intake rate on body mass (b = 0.73) was close to the one expected from Kleiber's law. However, when calves and adults were analyzed separately, a significant allometric relationship was found only for the calves. The intake rates of both age groups increased linearly with resource availability. This relationship also remained after accounting for the effects of body mass in a multiple—regression analysis. The intake rate also increased with the digestibility of the twigs eaten. However, twig digestibility was closely correlated with resource abundance in the presence data set, so the separate effects of quality and quantity were difficult to estimate. In the present data set no significant relationship appeared between resource availability and the time used for foraging. Thus, the functional response of winter—foraging moose was linearly related to the resource availability. However, we suggest that the quality of the food also should be included in the computation of functional response curves of large herbivores. Consequently, the functional response should be described in three dimensions as a function of quality and quantity of the food resources.