Bison Forage Utilization in the Upper Madision Drainage, Yellowstone National Park

Abstract

We monitored bison (Bison bison) use of forage on winter ranges in the Madison, Gibbon, and Firehole drainages of Yellowstone National Park (YNP) to determine how bison used wintering areas in these drainages and to determine if use patterns had detectable impacts on standing biomass. Bison were observed in wintering areas in the three drainages during all seasons of the year. Probability of bison occupying individual sites on a given day within the area during summer, autumn, winter, and spring, 1996-1997 ranged from 0-40 percent, 0-29 percent, 0-91 percent, and 0-67 percent, respectively. Sites identified as historic high use areas had high frequency of use (78 and 91%) during the 1996-1997 winter. Two sites identified as historic medium winter use had lower frequency of use (56%), and sites identified as historic low use areas had no observed use during the 1996-1997 winter. Exclosure cages placed at the six sites allowed us to estimate forage removal during summer 1996 through summer 1997. Over all sites and sampling periods, plots exposed to grazing had 23 percent less biomass (dry weight) than plots protected from grazing. Bison use at individual sites was weakly correlated (r=0.28, P=0.05) with estimated forage utilization and more strongly correlated with estimated grams of forage removed (r = 0.55, P <0.01). Despite high variability in vegetation biomass within sites, we did identify one site, Terrace Springs, as heavily impacted by bison foraging during summer. A reduction of >50 percent in bison numbers in the Madison, Gibbon, and Firehole drainages between summer 1996 and summer 1997 did not reduce foraging pressure on the Terrace Springs site.