Direct and Indirect Effects of Deer in Forest Ecosystems

Ungulates can profoundly alter the structure and composition of forest communities via both direct and indirect mechanisms. Individual plant species often respond in a unique way to the direct effect of herbivory as a function of their sensitivity to browse damage, ungulate food preferences, and the density of ungulates present. Sustained browsing pressure can limit the regeneration of favored and susceptible woody plants and eliminate populations of favored or susceptible herbaceous plants. These losses, in turn, give rise to indirect effects via trophic cascades or physical habitat modification. These indirect effects affect many other plant and animal populations. In the mixed conifer–hardwood forests around the Great Lakes in North America, widespread habitat modification and the extirpation of native predators and other ungulates have acted to boost populations of white-tailed deer (Odocoileus virginianus) to historically high densities. Such densities have curtailed regeneration of several important conifers (e.g. Tsuga canadensis and Thuja occidentalis) as evidenced by demographic analysis. Deer also appear to limit regeneration of Quercus and Betula in many areas. Impacts on understory herbs are harder to assess, but baseline data from 50 years ago indicate that these communities are changing in a pattern that implicates deer: grasses, sedges, and some ferns are increasing while overall herb diversity is declining. Thus, deer are playing a keystone role in these communities. We are currently assessing an additional set of questions, including: How best can we measure and represent ungulate impacts? At which densities do deer threaten forest diversity? How do impacts depend on initial plant and ungulate densities? Which species emerge as ‘winners’ or ‘losers’ in heavily-browsed landscapes? What characteristics or traits make a species susceptible to ungulate herbivory? How do ungulates affect patterns of diversity and relative abundance in ecological communities? What are the pathways by which ungulates exert indirect effects on species? and How significant are indirect effects