Ungulate regulation of ecosystem processes in Yellowstone National Park: Direct and feedback effects

Ungulates are highly integrated components of grassland food webs that exert strong direct and indirect influences on ecosystem dynamics. Although wildlife ecologists and managers are aware of the impacts of ungulate herbivory on vegetation and the carrying capacity of habitats, the indirect effects of ungulates on ecosystems are rarely considered. I review research in Yellowstone National Park documenting a positive, ecologically significant feedback of ungulates on their forage. Ungulates increase aboveground production of grasslands in Yellowstone by stimulating grazed plants to allocate resources aboveground and by facilitating the rate of net nitrogen (N) mineralization and the availability of N to plants. Moreover, the migration of ungulates from winter to summer range in Yellowstone is associated with animals following the spatio-temporal pattern of nutrient-rich forage across the ecosystem. This is likely critical in the positive feedback of herbivores on their forage by providing grazed plants extended periods to recover while soil conditions are suitable for plant growth. These findings from Yellowstone, in addition to similar findings of research in other ecosystems, documenting the importance of feedback effects of ungulates on habitats, indicate the need for managers to look beyond direct influences of herbivory to understand factors controlling ecosystem processes and the dynamics of managed ungulate populations.