Grazing impacts of White-tailed Deer or "Culling Bambi to save the forest"

Large herbivores, such as white-tailed deer, can significantly determine the structure and composition of forest plant communities. In several National and Provincial Parks located on southwestern Ontario shorelines, deer overgrazing has been a major cause of habitat degradation. Few studies of herbivory have examined the link between herbivory-induced or mediated changes in environmental variables, and vegetation change. In this study we used ordination analyses (DCA, CCA) to examine the effect of changes in understorey light levels on plant communities in Rondeau Provinical Park and other Carolinian forests. In 1996, plant communities in forest stands with a past history of high deer densities (50 deer/km2) were compared with deer exclosures and other long-term ungrazed sites. Ordinations showed that older exclosures were similar to ungrazed sites and dominated by native plant species. Both differed significantly from grazed sites, which were dominated by non-native species. Newer exclosures and stands where deer densities were reduced differed from both grazed and long-term ungrazed sites, suggesting that these plant communities are following an intermediate or alternate recovery trajectory. Interestingly, despite the differences in plant community composition there were no significant differences in species diversity among treatments. Thus, measurements of diversity or species richness alone are inadequate descriptors of plant community change. From 1987-91 over 50% of trees were lost from permanent plots in Rondeau. The main reason is that deer overgrazing has prevented tree regeneration, causing increased understorey light levels. We suggest that in this more open forest, the normal recovery of native plant species is prevented, and the trajectory of some plant communities over time, may have been significantly altered. The major implication for park managers is that sustained deer herd reductions alone are not enough to ensure the recovery of vegetation - management for accelerated canopy closure may also be desirable.