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 sward height and of bison tissue selectivity on the functional response of bison 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 pronounced 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 effectively to uncouple sward height from sward biomass. Intake rates and bite sizes obtained on simple swards were used to quantify functional responses. Without 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 tissue were significantly depressed at low sward biomass, relative to those obtained on simpler leaf-only swards. Thus, sward tissue composition is a crucial covariate for characterizing intake rates of grazing ungulates.