Forage quality and patch choice by wapiti (Cervus elaphus)

Recent models suggest that herbivores might optimize energygain by selecting patches of intermediate vegetation biomass.We tested this hypothesis in wapiti (Cervus elaphus) by estimatingdaily rates of energy gain in relation to grass biomass andby measuring patch choice in experimental pastures in whichgrass biomass was manipulated by mowing. The digestible energycontent of grasses declined with increasing biomass due to maturationalchanges in fiber and lignin content. Daily rates of dry matterintake by wapiti increased with grass biomass at a deceleratingrate, implying a Type II functional response. Linking thesevalues to published ad libitum energy intake and energy expenditureparameters, Fryxell's (1991) model predicted that the dailyrate of energy gain should be highest when wapiti feed in grasslandswith 1000–1100 kg/ha. In trials in which grass biomasswithin a mosaic of patches was manipulated experimentally between800–2900 kg/ha, wapiti preferred patches of 1200 kg/ha,close to the value predicted by the energy gain model. Our resultssuggest that the rate of energy gain by wapiti is constrainedby both grass biomass and grass fiber content, the latter ofwhich varies inversely with grass biomass. Behavioral preferencefor grass patches of intermediate biomass and fiber contentcould help explain patterns of aggregation and seasonal migrationreported previously for wapiti.