FORAGE QUALITY AND PATCH CHOICE BY WAPITI (CERVUS-ELAPHUS)

Recent models suggest that herbivores might optimize energy gain by selecting patches of intermediate vegetation biomass. We tested this hypothesis in wapiti (Cervus elaphus) by estimating daily rates of energy gain in relation to grass biomass and by measuring patch choice in experimental pastures in which grass biomass was manipulated by mowing. The digestible energy content of grasses declined with increasing biomass due to maturational changes in fiber and lignin content. Daily rates of dry matter intake by wapiti increased with grass biomass at a decelerating rate, implying a Type II functional response. Linking these values to published ad libitum energy intake and energy expenditure parameters, Fryxell's (1991) model predicted that the daily rate of energy gain should be highest when wapiti feed in grasslands with 1000-1100 kg/ha. In trials in which grass biomass within a mosaic of patches was manipulated experimentally between 800-2900 kg/ha, wapiti preferred patches of 1200 kg/ha, close to the value predicted by the energy gain model. Our results suggest that the rate of energy gain by wapiti is constrained by both grass biomass and grass fiber content, the latter of which varies inversely with grass biomass. Behavioral preference for grass patches of intermediate biomass and fiber content could help explain patterns of aggregation and seasonal migration reported previously for wapiti.