More information on how managing Deer for Sportsmen has brought biological "deserts" to our Eastern Woodlands

Deer are a normal part of the forests of Pennsylvania, however deer numbers have grown to unnaturally high levels in many parts of the state due to:


-elimination of large predators such as wolves, wolverines, and mountain lions that once roamed our forests.


-availability of abundant habitat and food sources such as agricultural fields, suburban landscaping, and edge habitat resulting from suburban development and sprawl.


Deer are herbivores, they eat plants. Too many deer means too much browse pressure on trees, shrubs, and herbaceous species. Over browsing has been occurring since the 1930's in northern Pennsylvania, in other parts of the state, it is a somewhat more recent development. Severely over browsed forests loose their intermediate vegetation layers including shrubs, seedling and sapling trees, and forest floor plants including wild flowers, grasses, sedges, and other low-growing plants. In addition, the diversity of species declines in all forest layers.

Loss of seedling and sapling trees threatens the ability of forests to regenerate, trees that die or are cut are not replaced by new trees. Forest structure is reduced to a few species of canopy trees and a ground layer of plants that deer generally do not eat such as hay-scented fern, New York fern, and a few sedges and grasses. In some areas the ground is bare. Loss of understory, shrub, and forest floor plants reduces wildlife habitat.

A highly visible stage in over browsing is the creation of a browse line which results when deer feed non-selectively on everything they can reach. Development of a browse line, characterized by the lack of green leaves to a height of about 5 feet, is evidence that deer are exceeding the carrying capacity of the area. At lower numbers, deer feed selectively in the forest, seeking out the tastiest and most nutritious plants. While even this level of browsing can be a problem for some of the most highly preferred species, it does not alter forest structure.


Deer population levels, when the earliest European settlers arrived, have been estimated at 9-11 deer per forested square mile. Today levels of 30-80 deer per forested square mile are not unusual and in some urban/suburban sites they are even higher.


Simplification of forest structure and loss of biological diversity resulting from deer overabundance threatens the health and resilience of forest ecosystems. It also diminishes the ability of our forests to carry out important ecosystem services such as air and water purification, erosion control, soil building, mineral recycling, and preservation of biological diversity.

Highly vulnerable plant species are in danger of becoming so rare that they must be classified as endangered or threatened. At least one plant, bearberry (Arctostaphylos uva-ursi) has been lost from the state due to excessive deer browsing. Examples of native plants of Pennsylvania, many of them already classified as plants of special concern, that are being severely impacted by deer include:


Shrubs
Canadian yew
Taxus canadensis
hobblebush
Viburnum lantanoides
possumhaw
Viburnum nudum
leatherwood
Dirca palustris
hearts-a-bursting
Euonymus americanus
mountain fly-honysuckle
Lonicera villosa
swamp fly-honeysuckle
Lonicera oblongifolia


Deer over browsing has gone on for so long over such large areas that there are no local seed sources of many vulnerable species. Recovery from long term over browsing will be difficult and will likely take a long time because may wild flowers do not have well developed long distance seed dispersal mechanisms. These plants are adapted to a pattern of small scale natural disturbance, such as tree falls, where clonal growth or short distance seed dispersal works well. There is concern that deer have altered our forests so profoundly that human intervention will be needed to restore a full complement of species and functions.

Over browsing by deer also worsens problems with invasive exotic species of plants as deer feed preferentially on native species allowing non-native invaders to expand and prosper. Due to the combined impact of deer over browsing and invasive species, native forest wild flowers have been replaced by stands of stiltgrass (Microstegium vimineum), garlic mustard (Alliaria petiolata), lesser celandine (Ranunculus ficaria), and Japanese honeysuckle (Lonicera japonica). Shrub layers are dominated by non-native shrubs such as Amur honeysuckle (Lonicera maackii), Morrow's honeysuckle (Lonicera morrowii), multiflora rose (Rosa multiflora), and autumn olive (Elaeagnus umbellata).

Preservation of Pennsylvania's rich diversity of native plants and ecosystems requires that we find a way to bring deer back into balance with their environment.