White-tailed deer (Odocoileus virginianus Zimmermann) browsing effects on tallgrass prairie forbs:: Diversity and species abundances

We examined the effect of white-tailed deer (Odocoileus virginianus Zimmermann) browsing on diversity (Shannon Index) and abundance of tallgrass prairie forbs at a site in northeastern Illinois over a period of ten years (1992-2001). Deer densities in the study area varied from 32-50 deer km(-2) between 1992 and 1997 and declined to 7-9 deer km(-2) following initiation of hunting. In a plot protected from deer browsing, abundances of browse-sensitive species increased and unpreferred and browse-tolerant species decreased. These changes initially enhanced diversity, but later, diversity declined as dominance of browse-sensitive species increased and evenness declined. On an unprotected plot, diversity declined from 1992 through 1997 as evenness decreased, resulting from declines in abundances of browse-sensitive species and increased abundances of unpreferred and browse-tolerant species. However, diversity increased on the unprotected plot after 1997 as deer density was reduced and abundances of preferred browse-sensitive species increased. We conclude that diversity of prairie forbs will be maximized at some intermediate level of deer browsing. Our results support the intermediate disturbance hypothesis, which posits that diversity is maximized at intermediate levels of disturbance.