ARCUS Student Award | 4th Annual Award
4th Annual ARCUS Award for Arctic Research Excellence
Submitted by: | Carita M. Bergman |
Authors: | Carita M. Bergman, John M. Fryxell, C. Cormack Gates, and Daniel Fortin |
Category: | Life Science |
Title: | Foraging strategies of subarctic wood bison: energy maximizing or time minimizing? |
Affiliation: | Department of Zoology, University of Guelph, Guelph, Ontario, Canada |
Abstract:
Many classical models of ungulate foraging are
premised on energy maximization, yet limited
empirical evidence and untested currency assumptions
make the choice of currency a nontrivial issue. The
primary constraints on forage intake of ungulates are
forage quality and availability. Using a model that
incorporates these dual constraints, we predicted the
optimal biomass of forage patches for subarctic
ungulate grazers using an energy maximizing versus a
time minimizing strategy. We tested these predictions
on wood bison (Bison bison athabascae) grazing
naturally occurring sedge (Carex atherodes).
The digestive constraint was determined by a series
of ad libitum feeding trials using sedge at
different stages of growth. Sedge digestibility
declined with biomass. Ad libitum intake of
sedge by bison declined with sedge digestibility and
thus decreased with sedge biomass. On the other hand,
short-term sedge intake rates of wood bison increased
with biomass. Incorporation of these constraints
resulted in the prediction that daily energy gain of
bison should be maximized by grazing patches with a
biomass of 10 g/m2, whereas a satisficing
bison could minimize daily foraging time needed to
fulfill its energy requirement by cropping patches
with a biomass of 279 g/m2. To test these
predictions, we used a staggered mowing regime to
convert even-aged stands of sedge to a mosaic of
patches varying in quality and quantity. Observations
of bison grazing these mosaics indicated that patches
of biomass below 120 g/m2 were avoided,
while the patches of biomass 156 and 219
g/m2 were highly preferred, with the
greatest preference for the latter. These results
indicate that bison were behaving as time minimizers
rather than energy maximizers. Daily cropping times
of free-ranging bison from the literature corroborate
our results.
Key Words: Bison bison, constraint, foraging,
grazing, intake, ruminant, sedge, ungulate