Deer Impact on Local Forests

April 23, 2009
Joan Ehrenfeld
Trail Walker


Deer Impact on Local Forests


Support Your Local Friendly Hunter

by Joan Ehrenfeld

New Jersey and southern New York are overrun with deer: most ecologists and forestry biologists consider the deer population the single greatest threat to the ecological health of our forests. Throughout much of our region, the forests lack an understory-one can see long distances among the trees, or walk off-trail for long distances with only minimal impedance from shrubby undergrowth. During the summer, there is commonly a conspicuous "browse line"-green vegetation down to about four feet off the ground, and then a sharp straight boundary with a brown landscape beneath this height. This is not natural; in fact it is a dramatic sign of serious ecological problems.


The deer browse line is visible in the woods. Photo by Joan Ehrenfeld.


In short, our forests lack the seedling and sapling trees that provide the forest with continuity and the ability to recover from wind and ice, insects and pathogens. And, because deer munch on herbs as well as woody plants, they eat the native wildflowers as voraciously as they do young trees and shrubs, greatly depleting their populations.

Deer had been nearly extirpated from New Jersey at the end of the 1800s. In 1901, a total of 20 deer were harvested by hunters, and deer hunting was completely prohibited. By 1913, deer had been imported from outside the state to re-stock private reserves and parks, and populations started to recover. Consider today's harvest: during the 2005-2006 hunting season, 59,657 deer were harvested in New Jersey (and somewhat fewer during the past two seasons), out of a population of about 140,000 deer. In addition, thousands of deer are killed by cars; the New York State's Dept. of Transportation estimates that there are 60,000 to 70,000 deer-vehicle accidents annually; North Jersey Deer Crash Coalition says 7,000 deer-car crashes are reported each and estimates twice that number are unreported. So, with all these dead deer, why are they a problem?

Deer are fecund creatures, producing as many as three (but usually fewer) offspring each year. The adults mate in late October through November in our area, and the young are born in the spring (May to June). They eat a wide variety of plants, both leafy herbs and woody twigs, and also fruits and nuts, such as acorns. Woody tissues, including twigs, young seedlings, and bark, are their main foods during the winter. They lack teeth on their upper front jaws, and so they cut off twigs unevenly, leaving a ragged edge, unlike other browers such as rabbits or small mammals, which cut off twigs sharply and cleanly. This fact allows observers to easily tell when browsing damage to plants was inflicted by deer or by other animals.

The lack of predators is often cited as a reason for the exploding populations. This is true, in part: predators tend to eat vulnerable young animals, removing them from the population before they can reproduce, whereas cars and hunters tend to remove the larger, older animals that have already produced numerous young. But another factor is at work, too.

Deer particularly like the edges between forests and fields; they are not creatures of dense woods. These habitats offer the widest variety and abundance of all the different types of vegetation during the year. Indeed, some of their favorite foods are crop plants-corn, soybeans, and other crops-making them one of the greatest problems on farmland. It is not uncommon for farmers in our area to lose 20% or more of their crops to deer.

But it is the patchy landscape of bits of forest and field, lots of back yard, and small remnant farms that make up our suburban landscape that creates the perfect environment for deer to thrive, and so they do.

It is this love of forest edges that contributes as much to the deer overpopulation problem as the lack of predators. When our region was first actively settled by Europeans, in the mid-1600s, most of the landscape was continuous forest, with only small openings around Native American settlements. It has been estimated that New Jersey had about 70,000 deer in 1500, or about 9 per square mile, over pretty much the entire area of the state (7,500 square miles). Today, the population of 140,000 deer is squeezed into a forest area of about 2,800 square miles, yielding an average density of 50 per square mile. And this figure is misleading, as deer are not uniformly distributed. In some of our natural areas, deer densities are well over 100 animals per square mile. Because deer tend to stay in small "home ranges" of less than a square mile, a lot of deer in one place can mean a lot of ecological damage, to farms and house landscaping as well as forests.

This is not healthy for the deer, and it is catastrophic for the forests. The lack of understory not only eliminates wildflowers, shrubs, and young trees, it removes the habitat required for many of our migrant forest birds that nest in the shrubs and on the forest floor, and it also removes the habitat for salamanders and small mammals that need dense cover and a thick layer of dead leaves on the forest floor. The whole forest community suffers when deer are over-abundant.

The hopeful news is that deer populations have been declining, slowly. In New Jersey, the population peaked at 204,500 deer in 1995; the estimate for 2006 was 131,782. New programs, like special permits for farmers, programs to assist communities in developing deer management plants, a "hunters helping the hungry" program that allows hunters to take additional deer to supply community food pantries, and liberal limits on taking "antlerless" deer are helping.

But we are still a long way from having healthy forests in New York and New Jersey, and indeed throughout the eastern deciduous forest. Extensive research on non-lethal means of controlling populations has shown them to be far from effective and exceedingly costly. So, support for your friendly local hunter is probably the best way of helping our forests, and all the other critters that depend on them for a place to live.


Joan Ehrenfeld is Professor, Department of Ecology, Evolution, and Natural Resources at Rutgers University.