Benefits to rare plants and highway safety from annual population reductions of a "native invader," white-tailed deer, in a Chicago-area woodland.

Overabundant white-tailed deer are one of the most serious threats to woodland plant communities in the Chicago area. Moreover, the abundant deer in a highly populated area causes economic harm and poses hazards to human safety through collisions with vehicles. The artificial conditions causing the overabundance and resulting consequences qualify the white-tailed deer in the Chicago area to be considered as "native invaders". We examined the benefits of culling deer at a Chicago-area woodland preserve by comparing browse rates on four endangered plant species from years before culling began with years with culling. We also examined deer-vehicle collision and traffic flow rates on area roads from years before culling began and years with culling to assess whether population reductions may have benefited road safety in the area. All four endangered plant species (three orchid species and sweet fern) had lower browse rates in years with culls, although the decreased browsing rates were statistically distinguishable for only two of the species (grass pink orchid and sweet fern). After first verifying that traffic flow rates did not decrease from pre-cull years to years with culls, we analyzed the Illinois Department of Transportation data from area roads based on deer-vehicle collisions causing >US$500 in damage and showed a one-third reduction in deer-vehicle collisions. An economic analysis showed a cost savings during the cull years of US$0.6 million for reducing browsing to just these four monitored plant species and the reduction in deer-vehicle collisions.