COS 8-9
Influences of white-tailed deer on plant community assembly in understories of northern temperate forests
The objective of this work is to test how a keystone herbivore, the white-tailed deer, mediates the apparent importance of niche and neutral effects in the assembly of understory plant communities. According to the niche differentiation model, species have individualized responses to biotic and abiotic variables and, thus, different competitive abilities in a given environment. Neutral factors include stochastic processes such as speciation, birth, death, and immigration (dispersal). An array of plots was established in each of four sugar maple-dominated sites across northern Wisconsin, USA. In each plot, we measured various environmental variables and recorded percent cover of understory plants, number of woody stems and evidence of deer herbivory. Video camera traps captured the amount of time deer spent in each plot and how long they foraged. We used multiple regression, non-metric multidimensional scaling and partial Mantel tests to evaluate the influence of environmental and biotic variables and spatial distance on understory community composition and groups of species divided according dispersal modes. Variation attributed to spatial distance provided evidence of neutral effects while variation explained by other factors indicated niche differentiation.
Results/Conclusions
Preliminary analyses indicated that, overall, the role of environmental differentiation became stronger as the dispersal distances of plants increased. For example, the presence of grasses and sedges as a guild appeared to be driven by the availability of light, although the niches of individual species varied with elevation and soil compaction as well. For short-distance dispersers like Trillium grandiflorum, however, neutral effects played a larger role. Additional key factors driving understory community assembly at our northern forest sites included soil texture and richness, as well as population densities of small mammals. Across the four sites, deer population density varied nearly ten fold. Deer impacts on community assembly were especially strong in open canopy plots, where foraging activity was concentrated. High deer pressure magnified differences in the importance of niche vs. neutral effects between dispersal groups by serving as a filter that increased habitat stress, reducing the number of species that were able to thrive.