How many deer are too many in Rock Creek Park?

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.