First deer densities are notoriously difficult to

  • No School
  • AA 1
  • iamfakeeee
  • 10

This preview shows page 8 - 9 out of 10 pages.

First, deer densities are notoriously difficult to measure (Burnham et al. 1980), certainly more difficult than the coverboard and stem density counts we used to measure vegetation density. Second, deer management by state agencies operates at a county or district level (Knox 1997) that might contain multiple units of conservation land with multiple conservation objectives. Third, bird species respond to vegetation volume, not deer density. Control sites located in close proximity to each other maintained either high or low densities of birds because of differences in vegetation volume, not densities of deer. Soil and presumably moisture traits at each site de- termined the rate at which the vegetation responded to reduced deer densities. Productive sites can tolerate higher deer densities, whereas sites with low soil poten- tial and/or no canopy opening will respond slowly to deer reduction. DeGraaf et al. (1991) showed that vege- tation parameters, in their case forest thinning, took pre- cedence over deer densities in predicting bird numbers. Lowering deer densities is one means to increase vegeta- tion density and diversity, but there can be no target deer density; rather, vegetation measurements will de- termine when the deer densities at that site are suffi- cient to allow a vegetation response. It is difficult to provide a specific vegetation index that would gauge deer effect. A long-term index would measure the density and diversity of understory shrubs, but changes in these values will occur over 5–10 years, not on the annual basis needed for deer management de- cisions. An annual index should not be based on seed- ling densities, because these values showed great annual variation (personal observation). It is possible to use an index based on the proportion of browsed twigs for fa- vored tree species (Balgooyen & Waller 1995). There is often variability in the relative abundance of preferred trees, and this may dilute the sensitivity of the index. Plants within the Liliaceae or Orchidaceae families are common throughout the eastern United States, and both the number of plants and the proportion flowering are sensitive to changes in deer densities (Balgooyen & Waller 1995; Augustine & Frelich 1998; Fletcher 1999). Shifts in the abundance and diversity of the bird com- munity at our sites were dynamic, with birds responding to annual changes in site condition. Release from deer browsing caused rapid successional changes in the for- est understory as vegetation progressed from grasses to forbs to Rubus spp. to woody saplings. These changes corresponded to a shift in bird species composition from Chipping Sparrows to Indigo Buntings to Hooded Warblers to Ovenbirds. This successional process, in combination with site differences, makes it difficult to say whether or not a particular species will increase in response to lower deer densities, because the answer depends on the site characteristics and time span in- volved. For example, Indigo Buntings responded imme- diately to removal of deer but then declined at exclosure
Image of page 8
Image of page 9

You've reached the end of your free preview.

Want to read all 10 pages?

  • Fall '19
  • Biodiversity, White-tailed deer, Measurement of biodiversity

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture