Influence of white-tailed deer and forest fragmentation on invasive introduced plants
Plant communities and plant invasion by introduced species are shaped by multiple interacting factors including geographic constraints, abiotic, and biotic factors. For example, in the Northeastern United States, native white-tailed deer (Odocoileus virginianus Zimm.) and forest fragmentation independently facilitate plant invasion, yet how these factors interact is unknown. In this work, plant-herbivore interactions, surrounding landscape configuration, and native plant diversity were investigated to assess their association with introduced plant invasion across the region. First, floristic composition data from 24 deer research sites distributed across the Northeastern US were used to investigate the effect of deer on introduced versus native plants at species and community levels. Second, to better understand opposing deer effects at the plant species level, controlled multiple-choice preference trials were conducted using captive deer. Relative preference was determined for eight invasive introduced plants and seven native plants that are currently widespread and frequently occurring in the region. Third, the relative importance of deer presence and density, multiple landscape fragmentation metrics, and plot-level native plant diversity were investigated for their roles in plant invasion patterns in forest understory communities across the Northeastern US.
Deer strongly altered species composition and facilitated an increase in the abundance of several invasive introduced plant species, while inhibiting the richness and abundance of many natives and reducing Shannon diversity (H’). Deer did not affect introduced plant richness or abundance at the community level, but they increased the degree of plant invasion due to strong declines in native plant abundance. Deer preference trials helped explain plant invasion patterns observed in the field; even though deer consumed more native plant biomass overall, preferences varied strongly at the level of species. Overall, deer appear to indirectly facilitate the invasion of introduced plants that are relatively unpalatable due to preferential selection and consequent reduction of more palatable plants.
At the landscape level, the degree of forest fragmentation was positively correlated with deer density and plant invasion. As deer density increased with increasing fragmentation, the percentage of introduced species in the plant community increased. Several interactions among landscape attributes, deer presence, and native plant diversity were identified. Overall, to fully understand the factors that influence plant invasion, the context of herbivore abundance, surrounding landscape, and native plant species diversity must be considered.