By: Bill Ishmael, Retired Wildlife Biologist of 30 years
White-tailed deer are the most widely-distributed and numerous big game animal in North America. Although the Badger is the official Wisconsin state “animal” symbol, the White-tailed deer is the official Wisconsin state “wildlife animal” symbol. Deer have always been a part of Wisconsin post-glacial ecosystems and recent estimates show that we share our state landscapes with about 1.5 million of them. But, like anything else, too much of a good thing can be bad.
As a prey species, deer have evolved an incredible reproductive potential, leading to rapid increases in their population, even when their preferred food sources have started to dwindle. Deer populations on the productive soils of southern Wisconsin can increase annually by a factor of 1.5 or more. So, if you have 20 deer on your property in April, there will likely be 30 by September. If most of these deer survive the hunting seasons, predation and car-kills, you could have 45 deer by the second fall. This effect of “compound interest” was clearly demonstrated at the George Reserve in southern Michigan between 1927 and 1933 when a small herd of 6 deer increased to 160 in just 6 years within this fenced preserve!
Landowners who want to improve the “health” of their land need to take in to account the impacts deer can have on their native, ornamental and agricultural vegetation as well as the impacts deer can have, indirectly, on other species of wildlife landowners wish to promote through their land management practices. Other than a few small elk herds in northern and central Wisconsin, white-tailed deer are the state’s largest wild herbivore and, through their selective browsing, deer can have a detrimental effect on the diversity and abundance of native vegetation communities wherever they exist, even where deer exist in relatively low numbers.
The average deer in Wisconsin eats about 6 pounds of food per day or roughly 1 ton of forage per year. So, if you have 20 deer on your property, about 40,000 pounds of forage will be consumed by deer each year. Deer are “sampling browsers”, eating a wide variety of vegetation, usually starting with the most palatable and nutritious foods and working their way down the list from there. Their ruminant gut allows them to quickly consume large quantities of food and then head for cover where they can “ruminate” to continue the digestion process.
In most cases, landowners won’t recognize the impacts deer are having on their native vegetation communities because the native plants are no longer there or the plants are prevented from blooming by continual browsing. You can’t see what’s not there. The best way to get a first-hand view of the impacts deer have on vegetation is to erect small deer-proof exclosures. But exclosures are expensive and need to be maintained for at least 10 years to have a demonstrable effect. An online search of “deer exclosures” will bring up dozens of photos of exclosures that clearly demonstrate the effects deer have on native vegetation. One of the many informative websites describing deer impacts to native vegetation can be found at http://www.aviddeer.com.
Through selective browsing, deer can reduce or entirely eliminate native plant species from your property, opening the door for less palatable invasive plant species such as honeysuckle, buckthorn, barberry and garlic mustard. Deer can even facilitate the spread of these invasive plants by carrying seeds in their hooves and fur. For instance, have you noticed that infestations of garlic mustard on your property most often first appear along deer trails and in deer bedding areas?
It’s important to remember too that deer diets shift dramatically through the seasons. The lush green foliage of summer and early fall may give you the impression that there’s lots of deer food out there. However, if your woodlands are overbrowsed by deer, there may not be many palatable or nutritious plants left. By late fall and early winter deer must necessarily shift their diet from green herbaceous plants to woody browse. Acorns (and other mast varieties) are a very important staple in a deer’s annual diet but acorns are the preferred food of many wildlife species and, by winter, what’s left gets buried under snow and ice. By Christmas deer have usually switched to a diet of woody browse, targeting oak seedlings, hazelnut, other native shrubs and even conifer seedlings such as white pine for the 3-4 month span of winter. Regeneration of your forests and a diverse shrub understory may be difficult or even impossible in the presence of high deer numbers.
A comprehensive approach to restoring and maintaining native vegetation on your property should include a deer management plan. Each property is different in terms of it’s capacity to sustain deer numbers at a specific level and there is no “one-size-fits-all” rule for managing deer on your property to reduce impacts on native vegetation and forest regeneration. Since large predators such as wolves, mountain lions, and bear no longer exist in southern Wisconsin (at least not in the numbers that could influence deer survival and population growth rates) regulated hunting has become the primary tool for managing deer populations. Fencing your property to keep deer out is expensive, impractical and not very aesthetically pleasing. Planting deer food plots to supplement their diet or buffer impacts on native plants species is also expensive and only serves to maintain artificially high deer numbers. Food plots occupy land that could be devoted to native plant restoration and most food plots, unless they are several acres in size, are usually devoid of food by late December anyway, leaving an artificially high number of deer “stranded’ and forced to put even more pressure on native woody browse species each winter.
Controlling the number of deer on your land, through regulated harvests during the annual fall hunting seasons, is just as important for managing the vegetation communities on your property as timber harvests, invasive plant control, tree/shrub planting or native prairie restorations. Professional Wildlife Biologists keep track of deer population estimates and hunting season harvests and, over decades, have found that, in order to cause a decline in farmland deer populations, hunters need to harvest at least 2 antlerless deer for every antlered buck harvested on their property. Harvests of 1.5 antlerless deer per buck will usually maintain the deer population while harvests of 1 or less antlerless deer per buck will result in a population increase. Landowners may need to adjust their deer harvests, up or down, based on observed impacts to native vegetation. Remember, deer have an incredible reproductive potential and populations can quickly grow to the point where controlling their numbers becomes increasingly difficult.