What twenty years of deer exclusion can do to an ecosystem

Background/Question/Methods

Many regions of the USA are currently experiencing the effects of overabundant white-tailed deer populations.  Deer can have both direct and indirect effects on local ecosystems, impacting populations of other species, including trees, shrubs, forbs, birds, and invertebrates.  Using deer exclosures set up over twenty years ago in three areas with different deer management practices (no deer hunting, doe hunting and buck only hunting), we investigated the effect of deer exclusion and management practice on the richness and abundance of saplings and forest floor invertebrates and on seed bank composition.  The three different areas are located in Dutchess County, NY.  Area one has had annual controlled hunts since 1976 (Cary Institute), area two has not been hunted since the late 1960s (Rockefeller), and area three has buck-only hunting (Mid-County).  All areas are comprised of 4 paired exclosed/unexclosed plots.  All plots were sampled for sapling density/identification and invertebrate identification and enumeration and soil nutrients during summer 2012.

Results/Conclusions

The numbers of saplings across three size classes (0.5-1.0 m, 1.0-1.5 m and 1.5-2.0 m) reflect a pattern of  'self-thinning' at all three sites, with the smallest saplings having the greatest abundance and decreasing abundance in larger size classes.  In addition, there are large differences between exclosed and unexclosed treatments for two of the deer management types, with a notable decline in abundance of saplings in the largest size class at the unexclosed plots at the Mid-County (buck hunting), and an almost complete lack of the two larger size classes of saplings at the Rockefeller unexclosed plots (no deer hunting).  Only the site with annual controlled hunts shows evidence for forest regeneration.  The richness (total number of orders) of forest floor invertebrates was lowest at Rockefeller, and diversity (H') was also significantly lower at the Rockefeller unexclosed plots, indicating that overabundant deer are having a significant impact on the diversity of forest floor invertebrates.  In contrast, seed bank data indicate that deer are not having a significant impact on either species diversity or number of seeds present in the persistent seed bank.  Together these results indicate that deer can have multiple effects on forested systems; from negligible to profound.  How systems will ultimately function under increasing deer densities is not clear and worthy of further investigation, especially in urban systems where some functions may already be compromised.