Deer Impacts on forested communities and Canada Yew Populations at Apostle Islands National Lakeshore, Wisconsin

Abstract

Abundant white-tailed deer (Odocoileus virginianus) present a significant management problem in forested landscapes of North America. Conserving vulnerable communities requires quantifying herbivory levels and the density of the herbivore populations. The current study was conducted in 2006 to investigate the status of white-tailed deer throughout Apostle Islands National Lakeshore and their impacts on forest communities as well as Canada yew (Taxus canadensis) populations. Deer densities were estimated using fecal pellet surveys across islands of known deer occupancy. Using a defecation rate of 34.4 pellet groups deer-1 day-1, population estimates on Sand (8.23 deer km2) and York (7.68 deer km2) islands were considerably greater than Basswood (1.82 deer km2) and Oak (1.56 deer km2) islands. The variation in deer density reflects measurable differences in forage availability and browse quality between islands. Browse intensity and electivity indices revealed that northern white cedar or hemlock were selected for browsing by deer on Basswood and Oak islands, yet when Canada yew was abundant (on other islands) they were selected against. Dimensional analyses of browsed and unbrowsed stems were used to assess annual browse biomass production and utilization of Canada yew by white-tailed deer. A species-specific allometric relationship of stem diameter and shoot biomass was developed for Canada yew and used to predict the total browse biomass consumed at the stem level. Estimates of annual browse production and utilization from York and Sand islands revealed that deer herbivory accounted for 53% and 74%, respectively, of new browse biomass losses. Deer populations and herbivory levels on Sand and York islands are not sustainable for continued growth and reproduction of Canada yew populations, which are regionally uncommon. Landscape composition and species susceptibility to herbivory should be important considerations in the management of deer populations across Apostle Islands National Lakeshore.