White-tailed deer overabundance and the ecology of forest understories in protected areas

Large mammalian herbivores are often "strong interactors" in terrestrial ecosystems, with substantial capacity to regulate terrestrial plant communities. Evidence is growing that white-tailed deer (Odocoileus virginianus) are strong interactors in near-boreal conifer-hardwood and temperate broadleaved deciduous forests of eastern North America. In the absence of apex predators, deer populations frequently become overabundant and cause biotic impoverishment and ecosystem dysfunction. This dissertation examines ecological effects of overabundant deer on understory plants (chapters two and three), and the interactions between deer abundance and landscape structure that ultimately determine levels of herbivory and therefore impacts to plant communities (chapter four). Some of questions I address relate to the long-term effects of overabundant deer, both on ecological and evolutionary timescales. In chapter one, I use data from Public Land Survey records (from the 1840s) and from a 2002 re-survey to assess structural and compositional changes that accompanied Euro-American settlement at Sleeping Bear Dunes National Lakeshore (SLBE), Michigan. This sets the stage for chapter two, wherein I investigate the ecological legacy of a historically overabundant deer population on SLBE's North Manitou Island. The time-averaged impacts of forty years of herbivory by an overabundant deer herd apparently resulted in substantial impoverishment of the islands forest understory communities. However, compositional changes did not result in the expected dominance of species resistant to and/or tolerant of herbivory. In chapter three, I examine ways that an absence of ungulate herbivory on South Manitou Island has apparently altered the expression of various ecological and life history traits in two understory forb species, Trillium grandiflorum and Arisaema triphyllum. Populations of these two species on South Manitou Island, compared to North Manitou Island and mainland populations, appear to have a lower tolerance of herbivory (perhaps through increased allocation to current growth at the expense of longterm storage) and initiate the switch from non-flowering to flowering at a significantly larger size. Further, results of a simulated browse experiment demonstrate that T. grandiflorum is considerably more tolerant of herbivory than A. triphyllum, suggesting a possible role for long-term storage in belowground structures as an anti-herbivore defensive strategy in some long-lived understory forbs. Finally, in chapter four, I examine the landscape context of deer overabundance. Major findings include: (1) landscape composition and configuration at broad spatial scales (i.e. that of a township or county) appears to determine deer abundance; and (2) landscape configuration at a local spatial scale (i.e. that of a deer's home range) determines the level of impact associated with a particular abundance of deer.