Everything in nature is interconnected. Any healthy ecosystem is a harmonious balance of it’s living and non-living parts. This balance is what makes up food chains, plant and animal diversity, and population sizes. Every living and non-living part of an ecosystem has an influence on the other members that they co-exist with. Having “too much” of any one of these factors is going to have a significant impact on the ecosystem as a whole.
An overabundant population of white-tailed deer in a forest will certainly have an effect on that forest’s ecosystem. Several of these effects lead to negative outcomes that we hope to avoid. It is important to have data to monitor the impacts that deer are having in Wisconsin forests so that we may also monitor the risk for some of these negative outcomes. The twig-age data that we collect through this program allows us to monitor some of the impacts that deer are having. Here, we provide more detail on the potential risks caused by having too many deer in our forests.
Click here to learn more about the current deer management practices of the Wisconsin Department of Natural Resources.
Chronic Wasting Disease (CWD) causes lethal brain lesions in infected deer and has been spreading across deer populations in Wisconsin since 2002. Researchers are still unsure exactly how the disease is spread from deer to deer, but it is known that transmission rates increase in areas with a higher density of deer because it is more likely that these deer will come into contact with one another and spread the disease (Storm et al., 2013). Therefore, an overabundance of deer poses a deadly risk to their own wellbeing.
Storm, D.J., Samuel, M.D., Rolley, R.E., Shelton, P., Keuler, N.S., Richards, B.J., Van Deelen, T.R. 2013. Deer density and disease prevalence influence transmission of chronic wasting disease in white-tailed deer. Ecosphere 4(1):10.
Deer act as valuable hosts to tick species. If there is a higher density of deer in an area, ticks will have more hosts (and therefore, food) available to them. The tick population is then able to increase as a result. Having a higher population of ticks increases the chance that humans will come into contact with them while enjoying the outdoors. This is a concern because blacklegged ticks can spread Lyme disease to humans. Although Lyme disease is preventable and easily treated with antibiotics in most cases, the amount of Lyme disease cases reported has steadily increased over the past ten years.
Click here to learn more about how to prevent and treat Lyme disease.
Khatchikian, C.E., Prusinski, M.A., Stone, M., Backenson, P.B., Wang, I.N., Foley, E., Seifert, S.N., Levy, M.Z., Brisson, D. 2015. Recent and rapid population growth and range expansion of the Lyme disease tick vector, Ixodes scapularis, in North America. Evolution 69-7: 1678-1689.
Kilpatrick, H.J., Labonte, A.M., Stafford, K.C. III. 2014. The relationship between deer density, tick abundance, and human cases of Lyme disease in a residential community. Entomological Society of America 51 (4): 777-784.
A large population of white-tailed deer will result in higher rates of browsing on (or eating) vegetation. This has an effect on the plants that make up the understory of the forest, especially young trees and shrubs. Saplings that are favored by deer, such as maples, oaks, and birch, are kept at a short height by constant browsing. If it is too heavily browsed, a sapling may be unable to survive. These impacts on young trees have an effect on what the mature forest will look like decades into the future. A tree species that was once dominant may no longer be found. This change in the forest composition can have compounding effects on other plant and animal species that have evolved to rely on the current composition of the forest.
Kelly, J.F. 2019. Regional changes to forest understories since the mid-Twentieth Century: Effects of overabundant deer and other factors in northern New Jersey. Forest Ecology and Management 444: 151-162.
Petersson, L.K., Milberg, P., Bergstedt, J., Dahlgren, J., Felton, A.M., Götmark, F., Salk, C., Löf, M. 2019. Changing land use and increasing abundance of deer cause natural regeneration failure of oaks: Six decades of landscape-scale evidence. Forest Ecology and Management 444: 209-307.
Waller, D.M., Alverson, W.S. 1997. The white-tailed deer: A keystone herbivore. Wildlife Society Bulletin 25 (2): 217-226.
Habitat Loss for Birds
Several bird species in Wisconsin need to nest on the ground and rely on cover from the understory plants to construct their ideal nest sites and to protect them from predators (see the Wisconsin Breeding Bird Atlas for examples of ground-nesting birds). Studies have found the abundance of birds was significantly lower in habitats that were browsed by deer (Allombert et al., 2005).
Allombert, S., Gaston, A.J., Martin, J.L. 2005. A natural experiment on the impact of overabundant deer on songbird populations. Biological Conservation 126: 1-13.
Deer not only affect the understory through the plants that they eat, but also through the seeds that they disperse through their feces. One study found that deer had the potential to disperse over 500 seeds every day (Williams et al., 2008). A majority of these seeds were from invasive weedy plants that are normally found in small numbers along the edge of forests. Deer were eating these weeds and dispersing the seeds throughout the rest of the forest. This caused the invasive plants to spread far beyond their normal range and out-compete the native plants that make up the forest understory.
Williams, S.C., Ward, J.S., Ramakrishnan, U. 2008. Endozoochory by white-tailed deer (Odocoileus virginianus) across a suburban/woodland interface. Forest Ecology and Management 255: 940-947.