Bison limit ecosystem recovery in northern Yellowstone

Abstract

American bison (Bison bison) numbers in northern Yellowstone National Park increased during the last two decades, while those of elk (Cervus canadensis) decreased. We undertook this study to assess the potential effects of bison on woody vegetation and channel morphology in the park's northern ungulate winter range. Based on differences in the number of elk and bison, body mass, and the amount of time each utilizes the northern range, foraging pressure from bison began to exceed that of elk in 2007 and is currently ~10 times greater than that of elk. In the Lamar Valley study area, stand structure data (i.e., tree frequency by diameter class) for aspen (Populus tremuloides) and cottonwood (P. spp.) indicated that the growth of seedling/sprouts of these species into tall saplings and trees has been extensively suppressed by bison herbivory. For streams that crossed the valley floor, woody riparian vegetation was absent along their banks and channels were relatively wide and deep, conditions sustained by high levels of bison use. Overall, our findings indicated that the elevated numbers of bison, via herbivory, trampling, and tree bark effects, are limiting the structure, composition, and distribution of woody plant communities in the Lamar Valley, affecting the character of the valley's stream and river channels, and, in turn, influencing habitat and food-web support for terrestrial and aquatic wildlife species. The conservation success of bison recovery now may be adversely affecting another conservation goal, the restoration and maintenance of woody riparian vegetation and riparian ecosystems.