Long-term, indirect effects of white-tailed deer browsing on canopy-dwelling, forest birds 28 yr post-harvest

Background/Question/Methods Overabundance of white-tailed deer can dramatically alter structure and composition of forests, causing both immediate and long-term effects on biological diversity. Although immediate or short-term effects of deer browsing on vegetation and birds are well documented, long-term and indirect effects remain largely speculative. Here, we present prelimary results of an experimental study that documents long-term effects of varying density of white-tailed deer on vegetation and bird communities. In 1979-1980, known numbers of white-tailed deer were introduced into experimental enclosures in northwestern Pennsylvania and maintained at densities of 4 – 32 deer/km² for 10 yr. Portions of each enclosure were clearcut at the beginning of the study and forest vegetation has been tracked every 5 yr since. In summer 2008, we censused birds in these former clearcuts (now 28-29 yr old forests) to assess long-term effects of deer density during stand initiation on  bird abundance and diversity.

Results/Conclusions

Along the experimental gradient of 4 - 32 deer/km², increasing deer density during 1979-1990 caused in 2005 a significant linear reduction in tree species diversity (R² = 0.4133, P = 0.0043) with a concomitant increase in relative dominance by black cherry (R² = 0.4919, P = 0.0029). In 2008, red-eyed vireo, black-throated green warbler, and ovenbird were the most common species observed in all deer-density treatments. Former deer density (1979-1990) caused a significant linear reduction in total bird abundance in 2008 (R² = 0.2997, P = 0.0279) but had no overall significant effect on bird species diversity or richness (P > 0.10).  Although bird abundance and diversity were not significantly related to tree species diversity or relative dominance of particular tree species (P > 0.10), we hypothesize that the observed long-term, indirect effects of former deer density on present bird communities are mediated by insect prey abundance and prey quality as affected by deer-induced changes in forest tree composition. Future research should therefore investigate mechanisms by which deer browsing creates long-term, indirect effects on bird and insect communities.