Excessive deer populations hurt native plant biodiversity

Garlic mustard is crowding out native plants like trillium in the forests of Pennsylvania. But don't blame the garlic mustard, blame the deer -- that's the conclusion of Carol Horvitz, professor of ecology at the University of Miami.

Horvitz is the lead author of a new study demonstrating how excessive numbers of deer deplete forest biodiversity.

"Our findings show that there is a link between disruption of the native animal community and invasion by non-native plant species," explains Horvitz. "Similar links maybe found in other ecosystems between disrupted fauna and declining diversity of flora."

Garlic mustard is an invasive plant that has run rampart is forests across the country, snuffing out many native species. But Horvitz and his research partners were able to show that garlic mustard can't outcompete others on its own, it relies on the help of deer.

Deer can't eat garlic mustard. And with deer populations exploding, garlic mustard has taken advantage of forests floors cleared of competing plants by the grazing mammals.

To demonstrate the deer's effect on garlic mustard prolificacy, researchers planted two fields of the invasive species -- one left open to hungry deer, the other fenced off. Tracking the fields over a number of years, the ecologists found the protected field saw trillium take back the majority of its original territory, while mustard continued to dominate with the help of their antlered friends.

"When people walk in the woods where deer are overabundant, they don't realize what's missing," said Susan Kalisz, the University of Pittsburgh ecology professor who lead the field research that enabled Horvitz's new study. "They don't know what used to be growing there. They don't know that species are being lost and replaced by invaders."

Deer populations are roughly ten times larger than levels prior to the arrival of Europeans in North America.

To improve biodiversity in forests, the researchers say, deer populations must be reduced, while natives are restored and invasive plants are rooted out.

"It's not simple," Kalisz said. "Deer management policies vary from state to state and deer don't respect political boundaries."

"Yet, deer exact a toll not only on forest species, but also farms, orchards, and even your car and your car insurance rate," Kalisz added.

The study is more evidence of the compounding and unexpected impacts a single species can have on the ecosystem. A video detailing the positive impacts of that the reintroduction of wolves have had in Yellowstone recently went viral -- popularizing the concept that a small change can have major environmental and ecological impacts.