How Do White-Tailed Deer Change Ecosystems, Anyway?

Credit: Robert Woeger, Unsplash

Some facts about deer 

An adult white-tailed deer, Odocoileus virginianus, that might weigh between 100 and 150 pounds, must eat about 8-12 pounds of fresh forage every day. They eat a wide range of plants, from flowers to shrubs, to tree saplings, and in oak woodlands, acorns. All of these plants share the characteristic of a certain softness: deer lack upper front teeth, so their browsing involves a sort of mashing and tearing unlike the cutting and biting employed by many other herbivores, large and small, from rabbits to cows. It’s easy to identify deer-browsed areas, once you know the signs. Often there are browse lines at about four feet, below which everything looks as though it’s been trimmed—mature trees and bushes lack lower limbs and leaves, saplings remain stunted, if not eaten to the ground, and flowering plants have lost buds and flowers. In addition, there might be few flowering plants or shrubs, and a preponderance of grasses, sedges and ferns. 

Besides texture, deer are selective in other ways, favoring young, nutritious plants that lack the kinds of vile flavors and/or toxins many plants employ in their defense. The list of plants they’ll happily consume is long: your beloved non-native garden plants and arborvitae, and in the natural landscape, just to mention a few, trilliums, Jacob’s ladder and other spring ephemerals, jewelweeds, Michigan lilies, oak, willow, and hackberry seedlings, and on and on. One deer can happily eat 24-36 oak seedlings in a day, and later in the summer, acorns might make up 50% of its diet. As a result, for example, deer overabundance in the eastern US has been shown to negatively impact populations of the white-footed mice who compete for those acorns, which in turn leads to increased populations of the invasive gypsy moth, since white-footed mice also eat gypsy-moth larva. Blue jays, squirrels and other animals that rely on oak mast will also have trouble. Thus, a large deer population will put real pressure on other animals; we often think of predators as dangerous to other animals, but here is a case of dominance through vegetarianism. 

What deer really love are small spring ephemerals and wildflowers that are tender and blooming in the early summer. A single bite might deprive these plants of flowers and leaves, enough so that they cannot easily or effectively regrow or reproduce themselves that season. When browsed, these plants often stay smaller over time and have trouble reproducing themselves. The true ephemerals have a limited amount of time to grow, flower and set seed before the woodland canopy closes, at which time many species go dormant until the following year. After being grazed, even if they do reemerge, it might be several years before they flower again. 

Trees and shrubs are vulnerable throughout the season. Deer are creatures of habit and will return repeatedly to a place where they have found good forage. Oaks and other preferred tree saplings will get eaten to the ground. I’ve watched deer foraging in an area full of young hackberries that have stayed at about three feet tall over several seasons; whenever they sprout new leaves, they get browsed. Eventually they’ll die. In this way, deer can impact successional stages of woodlands. 

Unlike, apparently, cattle and horses, deer have an ability to know what plants to avoid, on account of toxicity in addition to taste or prickly leaves. You won’t find deer eating poisonous plants such as white snakeroot (the plant that causes milk sickness in humans when cows eat it), milkweed, or butterweed. Nor will they often eat members of the mint family such as obedient plant, agastache, or late figwort. Rough-leaved plants such as false sunflower, they’ll mostly avoid as well. Longer still is the list of invasive non-natives that they’ll have nothing to do with: creeping Charlie, plantain, moneywort, lesser Celandine, garlic mustard, dame’s rocket, buckthorn, Japanese barberry, Asian honeysuckle, lily-of-the-valley and others. Nor will they eat most grasses, sedges and ferns.

In a balanced, woodland ecosystem, a small number of deer in a large area will help create and maintain biodiversity and ecosystem health by creating appropriate levels of disturbance. Their eating habits wouldn’t matter so much, except that, as I’ve written here, in the absence of predators they’ll reproduce to excess and destabilizing systemic feedback loops will develop. More than roughly ten deer per square mile, and the ecosystem begins to shift. Overpopulation leads to overgrazing of preferred plants, leaving openings like holes in a worn patchwork quilt, in turn opening up space for browse-tolerant or resistant plants to dominate and invasive plants to establish themselves. This has been extensively studied; computer models have even been developed to predict, for example, the “time-to-extinction of forest herb population as a function of initial abundance.” There are numerous cascading effects on other plants; animals such as birds, bees, butterflies, and small mammals; and even some parasitic fungi.

The effects on other animals

As the population pressure continues, other animals, along with the deer, try to rely on the fewer remaining palatable native plants, while non-natives proliferate. The more that inedible-to-deer and invasive plants prevail, the more pressure it puts on other remaining native plants until extirpation occurs; the deer begin to starve and stray widely into surrounding human-occupied areas. Other, knock-on effects can intensify over time. Recent studies have shown that as deer change the plant species composition of a given area, they alter microclimates, and affect light availability and spatial patterning on the woodland floor. Further, trampling may compact the soil, disturb fine root networks, and disrupt mycorrhizal relationships, causing changes in soil chemistry, and biotic and physical properties. These changes in turn can lead to further alterations in plant community composition as the feedback loops are continually reinforced, gravely disrupting the ability to thrive of other animals that have relied on a particular set of ecosystem elements, patterns, and relationships. 

As with plants, maximum animal diversity occurs in the presence of a moderately sized deer population. Deer overabundance means small mammals will lose forage and cover. Birds, which rely on vertical layers for nesting and foraging, will find themselves bereft. What will a shrub-nesting bird do when the shrubs and favored berries disappear, possibly subsequently replaced by non-native shrubs whose berries are not nourishing? 

And then there are the insects, all those myriad species whose job it is to run the world. I’ll never forget the moment I realized that deer eating flowers meant that native bees would have a hard go of it. Could overabundant deer be a factor in the decline of native bees? Possibly. A short consultation with “Flora of the Chicago Region” or Illinoiswildflowers.info will confirm the numerous species of native bees, large and small, that rely on pollen and nectar offered by the wildflowers popular with deer. While bumblebees, who tend to be generalists that make use of a wide variety of blooms, might be able to adapt, some solitary bees are oligolectic, or dependent, on a particular species of flower. Further, in some species, females might rely on one particular species, males on another. Other solitary bees that emerge at certain times in spring and summer might find fewer nectar and pollen resources than they need to provision their nests. In these cases, deer browsing bears a direct relationship to the survival of these species. 

Long term effects

As overbrowsing continues in an open woodland, the biodiverse tapestry of hundreds of native species inevitably degenerates to a mix of grasses, sedges, ferns, members of the mint family and non-native plants. There might be an absence of young trees and a dearth of bushes. If this happens for long enough, tipping points are reached, leading to a strange, simplified state that cannot recover naturally, even assuming the deer are somehow removed. 

Once this has occurred, restoring the woodland to anything like its previous biodiverse state requires massive human intervention, which will be difficult—and shifting baselines may mean that the local humans might not even realize such a simplified landscape is not normal or desirable. This has happened in woodlands in Pennsylvania, where the ground-layer might be filled with a monoculture of hay-scented ferns instead of the prior multitudes of plant species. Recently I was surprised to see in a native gardening book a recommendation—with photos—that this kind of landscape could be used as a model for gardeners recreating a woodland on their property.

Here in northern Illinois, conditions may not be quite that dire, yet I have seen the changes in a woodland savanna where I’ve walked for twenty-five years. In recent springs there have been muddy bare spots, trampled and grazed down to the ground, where years ago one could see lovely spring ephemerals such as trilliums, bloodroot, and Jacob’s ladder and later summer flowers such as Joe Pye weed (which deer theoretically are supposed to leave alone—until they get hungry enough), and oak saplings were present. Besides the grasses and sedges that have welcomed the lack of competition, all kinds of ground level non-natives are racing to fill these spots, which will help keep native regeneration low. These plants, such as creeping Charlie, plantains, money wort and other spreading, creeping garden weeds with which we are all familiar, are usually a nuisance, in restoration terms—but in this situation, where they would normally have been kept a reasonable level by a strong community of native plants, they are proliferating, just as they do in your home flower beds if you space the plants too far apart. 

Studies utilizing exclosures have shown that when deer were kept from certain areas, some native plant species expanded their distribution, while exotic plant abundance dropped. However, there is no guarantee that recovery within the fenced areas is a return to the prior, biodiverse state. Depending on how long conditions had been worsening before exclosure, the recovering plant guild could very well be missing species.

Management strategies

Large predators such as wolves are indisputably good at helping control the number of deer. It turns out that having two major predators is even better, and brings greater balance to the ecosystem. In glacier National Park, for example wolves and cougars worked synergistically. Not only did they control deer and elk, but mesopredators, such as raccoons and skunks, returned to balanced levels as well. In northern Illinois, I suppose this would be coyotes and humans.

A hands-off, "let-nature-take-its-course" strategy doesn’t work in the absence of predators and human hunters. Plant species continue to decline until tipping points are reached. This situation is the despair of land managers, conservationists, and restorationists who know that to do their work in the absence of meaningful deer management and control is to undertake a near hopeless task. When invasive shrubs such as buckthorn are removed, recovery cannot proceed when deer eat whatever grows back naturally or is planted. The result can be depauperate, invasive-species filled woodlands punctuated by trees, shrubs and patches of native plants enclosed in wire cages, illustrating a desperate effort to help save plant species that should populate the entire woodland. An acquaintance recently told me the story of how a restoration group had fenced off a large area, in which they had nurtured the woodland understory into recovery. A fine, biodiverse community of native forbs that have become rare in the woods at large was flourishing. Early this summer a single doe and fawn somehow were able to get in, and browsed until the whole area was decimated.

In the past, deer-focused, "wildlife management” strategies have been tried, aiming to maintain large deer herds for the benefit of hunters. There has been an idea that a “maximum sustained yield” of 50% of carrying capacity might work. However, it has been found that deer begin to negatively affect and reduce biodiversity at 25% of carrying capacity. Maintaining deer herds to exclusively benefit human purposes is detrimental to the ecosystem as a whole and is unsustainable, because maintaining overpopulation will lead to an ecosystem that not only fails to support other species, but the deer, as well. This is an example of the type of mistakenly human-centered planning that continues to plague many fields, including climate change mitigation, resiliency planning, urban planning, agriculture and so on.

The best management practices constitute “ecosystem management,” which is backed up by scientific studies and could be considered a gold standard. This holistic approach is becoming increasingly used by private landowners, and also some enlightened state and county wildlife and forest management departments. In this form of management, not only are deer herds monitored and an appropriate population level determined (such as 10 deer/square mile over X number of square miles), but other indicator species, particularly plants, are also monitored. These could be flowering plants such as trilliums or other spring ephemerals that tend to decline in the presence of grazing pressure, and, conversely, the increase of species like grasses, sedges, ferns, and, of course, deer ticks, that increase with deer overpopulation. The latter is an important way to gauge ecosystem decline when browse-sensitive species have disappeared. Other indicators could be general conditions like bare, overgrazed ground, the increasing presence of weedy non-natives that easily colonize in those conditions, and “browse lines” in which shrubs and trees have been “trimmed” below about four feet. Deer population management would proceed when a “decline (or increase) of X%” indicates the initiation of deer control.

What does recovery of woodlands take? Clearly, if at all possible, deer should be managed before tipping points are reached, because at that point recovery to anything resembling the prior state might actually be impossible, or only achievable with intensive human intervention. The basic prescription requires a reduced deer population for an extended period of time, coupled with replanting, reseeding, and control of non-native invasive plants. Yet, since ecological restoration can only arc with time’s arrow towards new states of dynamic equilibrium, there is no going back to what once was. The ecosystem will become, inevitably, something different from what it would have been, had deer been kept at appropriate levels all along. In addition, global warming and climate disruption are already warping the patterns of relationships in local ecosystems, so it is difficult to accurately predict how our woodlands will behave in even the near future. Yet all climate change mitigation schemes rely on the existence of healthy natural areas. With careful, attentive management and care, a given landscape can recover a great deal of biodiversity, though composition and complexity will change as conditions change. So much depends on what we do, right here, right now.

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