Skip navigation

Deer Discovery: Invasive Plants Get Boost from too Many Deer

By Kimbra Cutlip

White-tailed deer

A white-tailed deer in Shenandoah National Park (Flickr photo by John W. Iwanski)

New results from a long-term Smithsonian study are providing strong evidence of the dramatic impact high numbers of white-tailed deer (Odocoileus virginianus) are having upon temperate forests in the eastern United States. Since 1990, scientists at the Smithsonian Conservation Biology Institute have been monitoring two 10-acre plots in a 63-acre forest just south of Front Royal, Va. One plot is fenced to exclude deer. The other is open.

When the study began, few invasive plant species were recorded in either plot. In the last two decades, however, scientists witnessed the arrival of four Asian plant species: multiflora rose (Rosa multiflora), Japanese barberry (Berberis thunbergii), wine raspberry (Rubus phoenicolasius), and Japanese stiltgrass (Microstegium vimineum).

Wine raspberry

Wine raspberry (“Rubus phoenicolasius”) is a non-native invasive plant species in the U.S (Photo by Milo Pyne, courtesy NatureServe)

A survey of the two study plots after 25 years revealed the density of Japanese barberry, wine raspberry and Japanese stiltgrass was much higher in the open plot. The presence or absence of deer, they found, was an excellent predictor of the abundance of exotic plant species.

Interestingly there was one exception. The density of multiflora rose was higher inside the fenced plot. The researchers suggest this may be because this plant’s seeds are dispersed primarily by birds which move through the study area unimpeded. The study is published in the journal Plos One.

white-tailed deer

White-tailed deer (Flickr photo by Jen Goellnitz)

As a test case for deer’s dispersal role, the researchers looked at the native species wild comfrey (Cynoglossum virginianum), a plant with seeds known to be dispersed by deer. This species was almost absent inside the fence after 25 years.

Clearly deer are shifting the composition of plant species in eastern forests, from distributing seeds that cling to their fur and pass through their digestive systems, to selectively browsing on certain native plants rather than exotic species, the study co-authors write.

Deer also exert significant influence on the structure of the forest, reducing the understory which allows more sunlight to reach the forest floor. After 25 years of relief from grazing deer, seedling height was on average 2.25-times greater in the fenced plot than in the unfenced plot. In addition, four times more tree saplings were found inside the fenced plot.

Japanese barberry

Japanese barberry (Flickr photo by Doe, John)

The changes deer bring to the forest often favor non-native plants, but also have cascading effects on an ecosystem whether or not invasive species are present. In fact, the researchers had originally set out to study the influence of white-tailed deer on birds and small mammals, not on invasive plants.

“The plots were set up in 1990, prior to any invasive species issues in our forests,” William McShea, an ecologist at the Smithsonian Conservation Biology Institute and one of the authors of the study, explains. “We had no intention of studying invasive plant species, but invasive plants became an important management issue in eastern forests.” Over time, as the non-native plants began creeping into their study sites, the researchers realized they were watching an invasion in action. “It is a quirk of science that when you are looking for one thing something else catches your eye.”

After having observed firsthand how the presence of deer enables invasive exotic plants to spread in their Virginia study plots, the authors conclude too many deer may be boosting the invasion of exotic plants in similar forests across the Mid-Atlantic region.



Tags: , , , , ,