Assessing the direct effects of white-tailed deer (Odocoileus virginianus) on plant invasions in forest communities

Background/Question/Methods

Invasions by exotic impact biodiversity and function in the ecosystems they invade. Anthropogenic environmental disturbances can have compounding effects on the success of invasive plants. White-tailed deer (Odocoileus virginianus), for example, have increased in range and population density since European settlement, and can have direct effects on forest plant species that may facilitate the successful invasion of exotic plants. My primary question is how the direct effects of an overabundant white-tailed deer population affect the growth and composition of native and invasive plant populations. I have established a set of seven paired fence/control plots. In four of these pairs, I have planted an assortment of 12 species, including native and invasive trees, shrubs, vines, and herbaceous species. I have monitored the trees and shrubs on an approximately monthly basis for damage from deer herbivory. I am also monitoring the remaining species with semi-annual censuses for survival and growth of each species. To address the effects of deer on the composition of the plant communities, I have performed monthly quadrat surveys of the herbaceous layer in all seven pairs of plots, beginning in late spring 2011 until the end of the late summer growing season of each year.

Results/Conclusions

An analysis of herbivore damage indicated significant differences among species (F_4,227=10.15, p<0.0001). The invasive Rhamnus davurica had significantly lower incidence of herbivory than all native trees, and the invasive Lonicera maackii had significantly lower incidence of herbivory than two of the three native trees. The community composition recorded in the first year of the study was used as a baseline for each plot. Community composition in subsequent years was compared to this baseline with a Sorensen similarity index. Mean similarity of fenced and control plots were compared with the expectation that fenced plots would show lower similarity if deer represented an important structuring force. After one year, the mean similarity did not differ between fenced and control plots (p=0.3850). Previous studies have typically focused on the effects of only a small number of species. This study will expand the number of species and range in morphologies by including 12 target species of both native and exotic origins as well as considering the composition of the entire herbaceous layer. By measuring the effects of herbivory on these planted species and the community composition, I will provide a stronger cause-and-effect link between herbivory and exotic plant invasions than previously established.