Captivity may broadly impede bison ecological function: space, season, and management

Robert Ritson^1,2*Justin Shannon³Kent Hersey³Kathryn Schoenecker⁴Kyran Kunkel⁵Daniel Kinka⁶Donald Beard⁷Dustin Ranglack^2,8

• ¹Wyoming Cooperative Fish and Wildlife Research Unit, University of Wyoming, Laramie, United States
• ²Department of Biology, University of Nebraska Kearney, Kearney, United States
• ³Utah Division of Wildlife Resources, Salt Lake City, United States
• ⁴US Geological Survey Fort Collins Science Center, Fort Collins, United States
• ⁵Conservation Science Collaborative, Missoula, United States
• ⁶American Prairie, Malta, United States
• ⁷Texas Parks and Wildlife Department, Austin, United States
• ⁸USDA-APHIS-WS National Wildlife Research Center Utah Field Station, Millville, United States

American bison (Bos bison) were spared from numerical extinction but considered to be ecologically extinct, partially due to spatial restrictions in their former range. A key component of the ecological function of bison is their ability to adjust their spatial behaviors at multiple scales to match the seasonal distributions and availability of resources. To understand how management influences the amount and seasonality of space bison use at various scales, we estimated home range using autocorrelated kernel density estimators (95% utilization distribution) and foraging patch size by the maximum variance in first-passage time at four temporal scales (annual, growing, nongrowing, and focal seasons) for two free-ranging and three captive bison herds. The seasonal differences in space-use within management regimes and study areas were treated as nonparametric related measures while differences amongst management regimes and study areas were treated as nonparametric independent measures. Seasonal variations in space-use were detected in both management regimes, but only free-range bison depicted this pattern at the larger home range scale (p < 0.001). Free-range bison had significantly larger home ranges across seasons compared to captive bison (p < 0.001), but captive bison had larger foraging patch sizes during the growing season than free-range bison. Study areas with captive bison did not differ in their amount of space-use at any scale or season (p > 0.05). The strongest differences were apparent among study herds in annual home range size (p < 0.001) and growing season foraging patch size (p < 0.001). Our results demonstrate that captivity likely prevents bison from using their available landscape seasonally at broad scales, which could have ecological consequences for this historically nomadic herbivore. Although sociopolitical realities prevent bison restoration at a continental scale, increasing the amount of space available to bison where possible has the potential to replicate previous ecological processes. Further research can evaluate the influence of local environmental conditions on seasonality of bison space-use and can assess effects of scale on the ecological impacts of bison.