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In What Ways Can Deer Affect Other Wildlife?
Deer and other wildlife species have shared the forests of eastern North America for many centuries, and under most circumstances, these habitats are shared with little conflict. However, competition for favorite food items like acorns can impact population changes in some wildlife species. Other ways that one wildlife species changes habitat for other species include removing important plant species, changing soil chemistry, changing habitat structure by breaking limbs or plants. Most of these negative changes occur when one species becomes overabundant relative to the others at that site.
Forests have many different habitats. Some species – mice, chipmunks, some birds -- spend their life cycle on or near the forest floor, eating seeds or insects or the tiny organisms that live in the soil. Other species – some birds and insects -- live on shrubs and low-growing plants. Still others live almost their entire lives among the foliage and open spaces near the tops of trees. All of these species can affect the quality of habitat for other species by their direct and indirect effects on the food and cover found in the forest.
If deer impact in a forest is low and habitat quality is high, there is usually ample food and cover for other wildlife species who prefer that forest type.
But as deer impact increases, deer can begin to affect the quality of habitat for other species. This is perhaps most true in oak forests. One study in Pennsylvania found there were 19 percent fewer acorns outside a fence, where deer had access to them, compared to the number inside a fence where only other animals could reach them1. In Virginia, another study showed that deer removed 10 to 30 percent of the acorns in a good seed year, and that they were choosy – these acorns were more likely than the acorns they left behind to be free of weevils. Deer consumed 16 to 67 percent of the weevil-free acorns2.
Scientists use many methods to understand deer, habitat, and other wildlife species. In Virginia, McShea and Rappole noticed that as the density of plant life on the forest floor increased, so did the abundance and number of species of birds that use those plants as habitat. Then they installed movement-tripped cameras in the forests with high- and low-density forest floor plants and found twice as many deer using the forests with low understory density3. We can’t be sure that deer caused the lower understory density, but that is one possible explanation.
Because most plants start their lives as small seedlings on the forest floor and within the reach of deer, deer can literally consume the important habitat for other species. In Pennsylvania, deCalesta compared forests that had been thinned 12 years earlier, and then grew back with different densities of deer, ranging from 10 to 64 deer per square mile. In the low deer density enclosures, birds that nest in the mid-canopy – birds like eastern wood pewee and indigo bunting -- were far more abundant. At the higher deer densities, trees that created mid-canopy habitat were missing, because deer had eaten them when they were seedlings, and the habitat for those bird species never developed4. In Massachusetts, DeGraaf and others found a similar result. Birds that gleaned for food in the tree canopy were less abundant, and there were fewer species of those birds, in forests that had high densities of deer for many decades. Rufus-sided towhees were an example of a species that was more abundant in low deer density areas.
But the changes that deer make in forest habitat can be positive for some species as well. In the Massachusetts study, hermit thrushes were more abundant in the high deer density areas5.
Sometimes, when deer impact is high enough to give plants they don’t like a big advantage in their competition with plants they prefer to eat, this can change animal habitat as well. Deer very rarely eat fern, especially hay-scented fern, and in many places were deer impacts have been high, large areas of the forest have a beautiful carpet of fern at mid-summer. Royo, working in such forests in Pennsylvania, wondered if small mammals would behave differently under fern cover than they did out in the open, presumably visible to other predators. He found that small mammals ate a higher proportion of the seeds they found under fern cover than seeds that were in the open. He also found that small mammals were choosier under fern cover. When he put equal numbers of large and small seeds under fern cover, small mammals favored the larger seeds as food6.
1 Steiner, K.C. 1995. Autumn predation of northern red oak seed crops. In Proceedings 10th Central Hardwood Forest Conference, USDA Forest Service General Technical Report NE-187. 489-494.
2 McShea, William J.; Schwede, Georg. 1993. Variable acorn crops: responses of white-tailed deer and other mast consumers. Journal of Mammology 74(4): 999-1006.
3 McShea, William J.; Rappole, John H. 1992. White-tailed deer as keystone species within forested habitats of Virginia. Virginia Journal of Science 43(1B): 177-186.
4 deCalesta, David S. 1994a. Effect of white-tailed deer on songbirds within managed forests in Pennsylvania. Journal of Wildlife Management 58(4): 711-718.
5 DeGraaf, Richard M; Healy, William M.; Brooks, Robert T. 1991. Effects of thinning and deer browsing on breeding birds in New England oak woodlands. Forest Ecology and Management 41:179-191.
6 Royo 2005 Thesis