The effect of tissue complexity and sward height on the functional response of Wood Bison

1. Little is known about the interplay between tissue complexity and tissue selection and its effect on the ungulate functional response. Effects of s ward height and of bison tissue selectivity on the functional response of b ison Bison bison athabascae Rhoads were examined. 2. The height of stem present in a sward was varied. Grazing depth of bison was negatively related to height of stem, and this relationship was more p ronounced in tall swards. Bison preferred leaf to stem tissue at all sward heights, though preference of leaf was positively related to sward height. 3. While controlling for selectivity, a factorial design was used effective ly to uncouple sward height from sward biomass. Intake rates and bite sizes obtained on simple swards were used to quantify functional responses. With out tissue selection, sward height had no effect on the functional response . Sward biomass explained 90% of the variation in intake rates. 4. Rates of bison food intake on complex swards with both stem and leaf tis sue were significantly depressed at low sward biomass, relative to those ob tained on simpler leaf-only swards. Thus, sward tissue composition is a cru cial covariate for characterizing intake rates of grazing ungulates.