Monday, August 6, 2007 - 2:10 PMOOS 6-3: "Re-wilding" gone wild: Overabundant deer herds and the ensuing legacy of impoverished forest biodiversity in Pennsylvania
Alejandro A. Royo, Todd E. Ristau, and Stephen B. Horsley. USDA Forest Service
The history of white-tailed deer (Odocoileus virginianus) management in eastern North America typifies a contemporary example of ‘rewilding’ of a large ungulate herbivore following large-scale exploitation in the 19th century. In Pennsylvania, deer reintroductions and subsequent overprotection resulted in a rapid recovery of deer populations from near extirpation in 1895 to nearly one million animals by the 1920’s. For nearly a century, this overabundant herd profoundly influenced understory plant composition, providing a significant opportunity to investigate the long-term impact of overbrowsing on forest biodiversity and successional patterns. Here, we summarize several research projects from northern hardwood forests in Pennsylvania to highlight the numerous deleterious impacts on forest diversity generated by overbrowsing. Observational and experimental work repeatedly demonstrates that overbrowsing directly decreased understory plant abundance, growth, and reproduction, at times driving browse-sensitive species to local rarity and restricting their occurrence to inaccessible refugia. Prolonged overbrowsing also shifted forest understory composition to dense, nearly monodominant layers of highly browse-tolerant plants across large portions of Pennsylvania. This radical shift in understory composition altered subsequent plant-plant competitive interactions as this stratum strongly suppressed germination and survival of shade intolerant tree species. Finally, this widespread habitat modification indirectly modified other herbivore-plant interactions, including intensifying granivory by small mammals. Overall, we demonstrated that the legacy of a century-long hyper-abundant deer herd is a depauperate forest community with strengthened competitive and granivory regimes. These changes to interspecific interactions are so pronounced that considerable efforts will be required to reverse the current momentum toward continued biotic impoverishment.