A new study by researchers at the University of Pittsburgh has found that an invasive plant species in the forests of the northeastern United States is getting major help from exploding deer populations.

The particular invader in question is the garlic mustard plant, which was brought over to Long Island by colonists back in the 1860s, and has been making inroads ever since.

Rachel Spigler, one of the authors of the study, published this past week in the journal PNAS, said many invasive species are restricted to forest edges. "But this invader has been very successful in getting into the interior of forests. As a consequence, it's of very high concern for disrupting the native community and the native plant species," she said.

While ecologists worry about deer overpopulation as much as invasive species, until now it wasn't clear if the problems were linked. To find out, scientists compared land plots where deer either roamed free or were blocked by fences in a woodland area near the suburbs of Pittsburgh.

Spigler, who is now a professor of biology at Temple University, says the team tracked the health of plants for six years.

"You can really visually see the difference when trillium, one of the main species that we studied, is in bloom in these fenced areas," she said. "It's just beautiful."

Trillium is a native species with large, charismatic white flowers that deer love to eat. By contrast, in the plots where deer could enter and eat the plants, the number of native species was cut in half. Based on their findings, the team estimated that annual growth rate of trillium drops from 20 percent to 4 percent if deer are in the environment.

The takeover of the garlic mustard plant at the expense of native species, however, isn't necessarily permanent.

"We see definitely that the invasive species where deer are present are growing explosively, but once you remove those deer you can actually reverse the invasion process and they start to go extinct," said Spigler.

As a result, Spigler said limiting deer populations, which are often at least twice as high as the historic level, is important for the overall health of forests.