Spatio-temporal dynamics of the central bison herd in Yellowstone National Park

Abstract

The spatio-temporal dynamics of Yellowstone National Park bison (Bison bison) are complex and affected by multiple abiotic and biotic mechanisms. In the Madison- Firehole area, which provides winter range for the migratory central herd, this suite of effects is particularly intricate owing to geothermally influenced habitats, severe winter climate, and variability in resource distribution. Understanding factors influencing bison spatio-temporal dynamics is of importance to Yellowstone, which is faced with managing a growing bison population that is expanding its range. I gathered data from 1997-2005 using field methods and used statistical modeling and information theoretic techniques to examine spatial and temporal patterns in bison migration, road and off-road travel, and foraging behavior in relation to abiotic and biotic factors. Numbers of bison migrating were related to density and drought severity, while snow, drought, and density affected timing of migration. The probability of bison travel and spatial distribution of travel corridors were affected by topographic and habitat attributes including slope, landscape roughness, habitat, and distances to streams, foraging areas, and forested habitats.

Streams were the most influential landscape feature affecting travel and results suggest the bison travel network is defined largely by the presence of streams. Probability of travel was higher in regions of variable topography (i.e., canyons). Pronounced travel corridors existed both in close association with roads and distant from any roads, and results indicate roads may facilitate bison travel in areas. Multiple effects influenced temporal bison travel patterns. Road travel was negatively correlated with road grooming and I found no evidence that bison preferentially used groomed roads during winter. Snowpack, density, and springtime melt were correlated with bison road and off-road travel. Bison foraging area residence times were affected by the ratio of local to landscape scale snowpack, previous foraging experiences, and local and landscape scale competition. Bison patch scale foraging behavior was predominantly affected by snowpack, with biomass and competition having minimal influence. My results indicate bison spatio-temporal dynamics are affected by multiple, interacting, scale-dependent mechanisms. Overall, factors influencing resource availability provide the primary impetus for variability in bison distribution, movements, and foraging behavior.