A Chevy Chase man last month found a small herd of deer outside his house near Rock Creek Park. He said he couldn’t get them all into a single photo frame. (ROY BOWMAN)

Last year, the National Park Service announced a plan to use sharpshooters and birth control to reduce the deer population in Rock Creek Park. The NPS wants to shrink the deer population from 80 animals per square mile to 20 to protect the plants and other wildlife in the park. (The park covers about four square miles.) The cull is currently on hold, pending a legal challenge by the animal rights group In Defense of Animals. That offers us an opportunity to discuss how wildlife management experts determine an ideal deer population.

Let’s first pause to appreciate the historical irony of this situation. When European settlers arrived in America, both deer and the plants they eat were abundant. Over-hunting and clear-cutting of vegetation drove the population to such low levels that, a century ago, a deer sighting was an exciting event. Hunting was a challenge. Pennsylvania deer hunters, for example, killed fewer than 2,000 deer per year in the early 20th century.

Hunting restrictions and regrowth of plant life brought deer back, and now deer are so plentiful that they are crowding out native plants and other species. Estimates vary, but most ecologists believe that there are now more deer in the eastern United States than there were before European settlement. Pennsylvania hunters kill more than 300,000 annually. In 2011, drivers in that state struck more than 100,000 deer with their vehicles.

So now we’re trying to reduce the deer population. But how do deer management experts set a target deer density?

In the past, it was pretty simple. “When the deer population in an area exploded, you simply watched how the vegetation changed,” says Karl V. Miller, a professor of wildlife ecology and management at the University of Georgia.

The trouble with such before-and-after studies is the number of extraneous variables. Human encroachment on the land or the sudden introduction of invasive species made it difficult to isolate deer density as a cause of changes.

Researchers then developed “exclosure” studies, in which they fenced an area off from the surrounding forest and compared changes between the areas outside and inside the fence over time. The current state of the art is the enclosure study. Scientists create a series of comparable, enclosed areas. In each plot, they control deer density to see how zero, 10, 20 or 40 of the animals per square mile affect a site.

Researchers look primarily for changes in the density and composition of vegetation. A thriving over-story — the tree canopy high above the deer’s head — combined with dwindling understory and mid-story layers indicates that deer are harming the environment. A lack of vegetation within the reach of a deer is a clear indication that plant life is suffering.

Effect on trees, songbirds

Native trees tend to fare especially poorly when deer proliferate. The seedlings of hemlocks, maples and oaks grow slowly, remaining vulnerable to hungry deer through critical periods of their development. Invasive species, which grow quickly despite constant deer browsing, perform well.

Many invasive species have another important advantage: Deer don’t like them.

“We classify all plants as first-, second- and third-choice plants for deer,” says James Kroll, director of the Institute for White-tailed Deer Management and Research at Stephen F. Austin University in Nacogdoches, Tex. “When there is an overabundance of deer, the first-choice plants disappear, while the third-choice plants do well. Ferns, which deer do not prefer, thrive in areas of high deer density. Large parts of the forests of West Virginia and Pennsylvania are now covered in hayscented fern.” The native trees, such as maples and hemlock, are dwindling in some areas.

Kroll notes that, when he moved to East Texas in 1972, there was a common evergreen covered with beautiful red fruit known as the strawberry bush. Deer love them, and Kroll rarely sees them anymore.

Animal life can also suffer from high deer density. Deer browsing tends to destroy songbirds’ preferred trees, and the elimination of the understory robs songbirds of the insects that sustain them.

Recent enclosure studies suggest that 20 deer per square mile, the National Park Service’s target for Rock Creek Park, is a sustainable level for a forested area in this region. In northern forests with shorter growing seasons, the target is typically slightly lower, around 10 to 15. New forests, which grow quickly from farmland, can support up to 50 deer per square mile.

Even at 20 deer per square mile, though, you can’t expect the forest to recover immediately. According to studies cited by the Park Service, tree seedlings have declined significantly in Rock Creek since 1991, and in many parts of the park seedlings below deer height have been virtually wiped out. It takes years, or even decades, for seedlings to grow to full height, and soil compaction and disturbance from deer tramping also delay new plant growth.

What about the effect of deer abundance on humans, such as spreading tick-borne diseases and car accidents? Most experts say it’s difficult to factor them into a numerical deer target.

“The number of Lyme disease cases a community will tolerate is subjective,” says Jay Boulanger, who studies deer overabundance at Cornell University. In addition, although it stands to reason that cutting deer will cut the rate of tick-borne diseases, there hasn’t been adequate research linking a target deer density to reduced transmission, and there are many other animals that carry Lyme ticks.

The birth control option

Once you decide that culling is a good idea, you must consider how to do it. Some researchers and many animal rights advocates favor delivering contraception to does either orally or by darts. The tactic has worked to reduce deer herds before, particularly on Fire Island in New York, where a deer contraception program that began in 1993 has brought annual births there down from 300 to a mere 30. But the majority view in the deer management field is that it’s usually less effective than shooting.

“After you dart a deer, how do you know that you’re not darting the same deer over and over?” Miller asks. He also argues that a herd of does on birth control presents a danger for drivers. If they do not become pregnant, they spend more time in heat, and bucks constantly chase after them. That increased movement, he argues, could create more collisions between cars and deer. The debate between shooting and contraception, however, is a major controversy in deer management.

No matter which control method is chosen, the proliferation of deer presents a dilemma for animal lovers. What do you do when a species’ health and growth depends on eliminating members of another species? This ethical aspect lies beyond the purview of ecological science, and is something our society simply has to grapple with.

The current deer situation recalls the familiar warning “You break it, you own it.” Whether or not the Park Service goes forward with culling, there’s no question that humans are already managing deer by profoundly altering the landscape where they live and wiping away natural controls.