Effects of overabundant deer in the lower Midwest on native biodiversity and interactions with invasive species
The density of white-tailed deer (Odocoileus virginianus) in many areas of the eastern United States is at record levels due to land use changes and extirpation of large predators. Overabundant deer can have negative effects on woody vegetation but less well understood are effects on other aspects of forest systems including interactions with invasive species. In a large southern Indiana forest preserve where deer densities were estimated by pellet counts to be over ten times higher than surrounding areas, we manipulated deer browsing by creating a series of 15m x 15m fenced exclosures over several years with an adjacent control plot of equal sizes. A series of vegetation and soil characteristics have been monitored annually in exclosure and paired control plots over multiple years. In the second experiment, we specifically evaluated how deer browsing and the presence of an invasive annual grass, Microstegium vimineum, affected the survival and growth of native tree seedlings in multiple sites where 1m x 1m plots were planted with one-year old tree seedlings. Half of each plot was fenced to prevent deer browsing and half of the plots had the invasive removed with a grass-specific herbicide in a split-plot design.
In the large exclosure experiment, there was no recruitment of native tree seedlings in any control plots even while there was abundant tree seedling recruitment inside exclosures. Preexisting tree seedlings also grew faster inside exclosures, as did invasive shrubs, indicating that deer browsing was significantly suppressing invasive shrubs. The mean height, diversity and density of spring ephemeral species were also significantly higher in exclosures, and soils were significantly less compacted than in control plots. In the second experiment with 1m x 0.5m exclosures, tree seedling survival was higher in plots where Microstegium was removed and in exclosure plots where deer browsing was prevented. Further, seedling biomass was greatest in exclosures where Microstegium was removed, but there was no effect of exclosure with Microstegium present. In total, our results suggest that deer browsing reduces tree seedling establishment and helps limit invasive shrubs, but that the invasive grass may potentially provide refuge from browsing for tree seedlings.