Nelson Issue Brief: Deer – Hunting, Ecology and Chronic Wasting Disease

Monitoring Deer Density Impacts on Forests

Deer populations rebounded from near extinction in many parts of Wisconsin in the late 1800s as habitat conditions improved, hunting was limited, and natural predators became scarce. Deer now thrive in landscapes that mix young and old forests with crop fields, suburbs, and other edge habitats. As deer herds reached and surpassed record densities in the early 2000s, we saw outbreaks of diseases like CWD in deer and Lyme disease in humans, as well as mounting numbers of deer-vehicle collisions. Impacts on crop fields, forests, and ornamental plantings have also increased. Are current deer densities too high?

My research team looks at how deer affect forests. We count the numbers and sizes of tree seedlings at sites in the Northwoods of Wisconsin and the western Upper Peninsula of Michigan. We also use fences that exclude deer to see how plants respond in the absence of deer. We were alarmed to discover how few seedlings of slow-growing conifers like Canada yew, eastern hemlock, and northern white cedar now survive repeated browsing from deer.

Trail cam images of deer next to an exclusion fence and the result of deer browsing on forest plants

The trail cam photo at left shows how an exclusion fence keeps deer from browsing on forest plants. The photo at right shows shows the effect deer have on the rest of the forest.

Deer also prevent most yellow birch, white pine, many maples, and most oaks from surviving through the critical one- to five-foot high stage, termed the “molar zone” as these are the leaves that deer can reach to eat. Through these strong and cumulative effects, deer are shifting the composition and structure of forests across Wisconsin.

Most of a forest’s diversity, however, is found in the understory — the shrubs and plants of the forest floor — rather than in its trees. We compare different Apostle Islands that have known deer population histories, and for which we have good historical data on forest understory conditions collected by UW-Madison students and faculty in the 1950s. These studies show that deer grazing promotes grasses, sedges, and some ferns while eliminating many of our prettier lilies, orchids, and other wildflowers.

Overall, diversity has declined in a process termed biotic homogenization. There are exceptions; for example, on the Menominee and Ojibway Indian reservations, deer populations hardly affect tree regeneration or understory and wildflower diversity. Tribal management strategies result in more mature forest conditions and lower deer densities.

Our research finds that deer act as a “keystone herbivore,” greatly affecting the composition and dynamics of forests in Wisconsin and throughout the Midwest. The first step to addressing deer density is to devise rapid, cheap, and effective ways to monitor and assess deer. Our new “Twig Age” method can be used in both professional and “citizen science” monitoring programs. It provides accurate, real-time information on where and when deer have impacts, allowing policymakers to adjust forest and deer management in informed and effective ways.

Learn more about deer-related research at UW-Madison