FACULTY NOTES

You can ask students to work individually, in pairs, or in small groups. It might be helpful to provide students with the background information in addition to the figure. You may also want to note that while this figure uses bar graphs to illustrate temporal trends, line graphs are generally preferable for this purpose.

Brief notes on methods should students inquire: The researchers measured vegetation by counting and identifying to species all woody plants >1m in height and <4cm in diameter within three 24x24m quadrats at each site. The researchers captured birds by mist-netting. Every year at each site, they strung 25 fine mesh nets between trees for 3 days during the month of June. Each day at dawn and dusk, researchers removed birds caught in the nets; recorded their species, sex, and reproductive condition; and then released them. They recorded data for 25 bird species of which 3 are reported in Figure 2.

Answers for Student Questions:

Chipping Sparrow numbers declined in exclosure sites over the 9 years. Chipping Sparrows were negatively related to deer exclosure. Indigo Bunting numbers initially increased in exclosure sites and then decreased back to similar levels as those in control sites (with the exception of 1998). Indigo Buntings were initially positively related to deer exclosure but this relationship did not persist over time. With the exception of a spike in 1991, Ovenbird numbers generally remained constant between exclosure and control sites through most of the study; however, Ovenbird numbers increased in exclosure sites during the final years of the study. Initially Ovenbird abundance appeared unrelated to deer exclosure. The data of 1997-1998 suggested that a positive relationship could exist.

The key to explaining these patterns is succession. Students will need a hint to realize this. As McShea and Rappole (2000:1168) concluded:

“Release from deer browsing caused rapid successional changes in the forest 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 the time span involved. For example, Indigo Buntings responded immediately to removal of deer but then declined at exclosure sites until the ninth year, when an ice storm opened the canopy and resulted in a second pulse of herbaceous vegetation and a second pulse of birds.”

The main point of this activity is that deer’s impact on forest vegetation also affects other animals. In this case, Figure 2 shows that excluding deer from protected forests changed the relative abundance of 3 bird species. Whether the impact of deer is good or bad is largely in the “eye of the beholder.” Is one bird species of greater value than another species? Rappole and McShea found that several resident birds in their study sites, such as Tufted Titmouse, Blue Jay, Northern Cardinal, and Carolina Wren, showed marked decreases in abundance after removal of deer. These species tend to have stable or increasing populations in national bird surveys and are not normally of management concern. Migrant birds that foraged either in the understory or higher in the canopy responded positively to the increases in vegetation density and diversity that followed deer exclosure. Many of these species are of greater conservation concern than the resident birds (McShea and Rappole 2000).

The take home messages from this example are:

1. interactions at one trophic level (deer herbivory) influenced another (birds), and


2. humans decide what to manage for (e.g., conservation of specific rare bird species) and must understand complex ecosystem interactions to achieve management goals.

Student Assessment: Concept Map

Respond to short-answer questions included in student instructions. You can also ask students to draw a concept map illustrating the relationships that they believe are occurring over time between deer, herbaceous and woody forest vegetation, and birds in the experimental forest plots.