Ecological Impact of White-tailed Deer on Our Forests

White-tailed deer are one of Minnesota’s most socially, economically and ecologically important critters. Their importance stems from their beauty, popularity as a prized big game animal, nutritious venison, useful hides, impressive antlers, and ability to create fun family hunting memories and boost revenue for local businesses and outdoor industries. However, deer also collide with vehicles, can damage gardens, crops and forest, and contribute to spread of parasites and disease. In this article, we’ll focus on their ecological impact or how they interact with plants and other animals in their environment. As the Minnesota Department of Natural Resources (DNR) develops the state’s first ever strategic, long-term, deer management plan, and our fall deer hunting seasons approaches, it is a timely topic.

Bounce Back Ability

Deer existed throughout the wooded river valleys and woodlands of central and southern Minnesota at the time of European settlement. In northern Minnesota, where forest habitat was much different than today, they were absent or rare. Instead, other members of the deer family were dominant – moose in the north, woodland caribou in the northeast, and elk in the prairie and prairie-forest transition zone. As settlement cleared forests for lumber and farmland, and large predator populations were reduced by humans, suitable deer habitat and deer numbers expanded geographically, increasing in the north outside of their historic range. But unregulated market and subsistence hunting eventually reduced their population so greatly, that by the 1880s, they were rare in much of Minnesota. Regulated hunting and a high reproductive capacity have enabled the population to bounce back well beyond its pre-settlement range and numbers.

Keystone Species

White-tailed deer fill the niche, or role, in ecosystems of both herbivore and prey. They are affected by, and themselves affect, their ecosystem. In forests, they have been described as a “keystone species” because their feeding activity can directly and indirectly affect many plants and animals. Their populations fare best in forests with both diverse age structure and plants, such as aspen, oak and conifer, and in neighborhoods of mixed farmland and forest. Young, early successional habitats with openings, edge and open woodland get the sunlight needed to stimulate deer food. In spring and summer, they eat grasses, forbs such as wildflowers, mushrooms, and leaves of woody plants. As summer transitions to fall, foods higher in carbohydrates are sought, including acorns, fallen fruits and some crops. In winter, deer browse on woody vegetation such buds, twigs, young bark, and conifer foliage, particularly liking white cedar, mountain maple, red-osier and other dogwoods, and hazel. Deer in turn are preyed upon by wolves, coyotes, bears, bobcats, and humans. As victims of vehicle collisions or other unfortunate circumstances, their carcasses also sustain eagles, ravens, crows, magpies, and other birds and scavengers.

Habitat Impacts

Our forests face multiple challenges, including conversion, invasive species and a changing climate. No native, wildlife, vertebrate species in Minnesota has a more direct impact on habitat health than deer. When their numbers approach or exceed the capacity of habitat to sustain them, they pose an additional challenge to our forests and themselves. Aldo Leopold, known as the “father of wildlife management” wrote in A Sand County Almanac (1949), “I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.” Their feeding habits and preferences affect the variety, quality and structure of plants in a habitat. When excessive, it harms and changes forests for decades to centuries. Plants that are tolerant to high levels of browsing, or not preferred by deer, can increase and outcompete others. Deer avoidance of white spruce, Pennsylvania sedge and garlic mustard (a non-native, invasive) for example, can make them more prevalent and spread faster. Chronic browsing kills or hinders growth of preferred plants like trillium, ginseng, orchids, Canada yew, oaks, white pine, and northern white cedar, lowering regeneration success and leaving them in low numbers or absent. Add further stress to native forest understory plants, such as invading non-native earthworms and competing invasive plants, and negative impacts are compounded. Deer also move seeds on their bodies and in their excrement, helping to establish new patches of both desirable and undesirable plants.

Wildlife Impacts

A decline in plants, their variety, and habitat quality not only means less food and cover for deer, especially during critical periods such as fawning or winter, but also for other wildlife. Cascading effects extend to insects, birds, and other mammals. Insects’ (including pollinators such as bees, moths and butterflies) ability to find the plants needed in their life cycle to feed, rest, hide, deposit eggs, and display are affected. Very specific symbiotic insect and plant relationships can be disrupted, such as between certain moths and the orchids they pollinate. Nesting sites for forest songbirds in understory shrubs and saplings, as well as the insect supply they rely on for food, can become limiting.

Deer also affect other wildlife through their ability to carry and spread disease and parasites. Ongoing research continues to point to deer as a vector for disease and parasites, such as brainworm and liver flukes. These parasites have been significant contributing factors to the poor health and mortality of moose studied in northeastern Minnesota related to the recent population decline. The expansion of deer range into what was historically moose range has increased interaction and transmission of parasites between them. To address this concern, six deer permit area boundaries in the region have changed beginning this fall to better reflect the distribution of deer and moose, allow more fine-tuned management of deer numbers, and enhance management of both species.

Finally, deer numbers also affect other wildlife through their role as prey. As the primary prey of wolves, wolf populations are directly dependent upon condition of the deer herd. When deer are scarce or little snow cover exists in winter (allowing deer to more easily run from wolves), secondary prey of wolves, such as beaver and snowshoe hares, may feel the impact as wolves have to hunt harder.

Hunting – Our Best Management Tool

To keep our forests diverse and resilient, and their habitats and wildlife healthy and sustained, deer numbers must be considered in the equation. Public hunting is our primary tool for managing our deer numbers, with the goal of maximizing their positive impact while minimizing the negative. Hunting season structures, regulations, quantity and quality of habitat (such as sufficient browse and conifer cover in winter), and severity of winter weather (snow depth and duration) are currently the most significant influences on our deer numbers. As landowners and/or hunters, we can do our part by allowing hunting access to our land and harvesting adequate numbers of antlerless deer. As part of our natural and hunting heritage, deer should remain valued and managed wisely. Both short and long term, we’ll be glad we supported the collective functioning of our forest parts for sustainability of the whole. Wishing you a safe and memorable deer season with family and friends!

For More Information on deer management and the deer plan under development in Minnesota, see the DNR white-tailed deer management web page. Thank you to Adam Murkowski, DNR big game program leader, Andrew Norton, DNR deer project leader, and several other natural resource specialists for review of this article.

Jodie Provost
Jodie is with the Minnesota Department of Natural Resources Fish & Wildlife – Forest Habitat Team.

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