Deer turn up their noses at some invasive species

"This research is important for helping conserve our forest understories and natural areas," says Kristine Averill. (Credit: Naomi Ballard via USFWSmidwest/Flickr)

The dietary preferences of deer may be promoting the spread of such invasive species as garlic mustard, Japanese barberry, and Japanese stiltgrass.

The finding comes from a new study that tested white-tailed deer preferences for seven native and eight invasive plants commonly found in the northeastern US.

“Deer avoid certain invasive plants that are increasing in abundance in natural areas, suggesting that deer are causing unpalatable species to spread,” says Kristine Averill, a research associate in Cornell University’s Section of Soil and Crop Sciences and the lead author of a study in the journal Biological Invasions.

deer "cafeteria" of invasive plants
At the Penn State Deer Research Center, ecologists offered deer a multiple-choice array of eight invasive introduced and seven native plants to determine deer feeding preferences among the species. (Credit: Kristine Averill/Cornell)

The invasive herb garlic mustard, for example, has spread throughout the United States in the last 150 years and has become one of the worst forest invaders, especially in the Northeast and Midwest. In some areas, it has become the dominant forest underbrush plant, outcompeting native plants and reducing species diversity.

Garlic mustard contains chemicals to defend against being consumed, while Japanese barberry has spines, which could explain why deer don’t eat these species.

Study results showed deer prefer some introduced invasive plants—such as Oriental bittersweet, common privet, and Morrow’s honeysuckle, as well as some native plants including Virginia creeper and red maple.

[Deer change forests from the ground up]

Within the limited number of species sampled in the study, deer favored native species overall, though they avoided native hay-scented fern, which is considered a “native invader” and is spreading in areas of forest underbrush where deer are also quite abundant.

“It’s pretty revealing that the findings we had in this study correspond to what we have been seeing in the field and confirm that deer preferences play a major role in community assembly,” Averill says.

Still, some invasive plants that deer seemed to highly prefer are increasing in abundance in natural areas, which may be because these plants produce fleshy fruits, which deer eat and then the seeds of the plant are spread in their feces, Averill says.

[Giant tortoises have a sweet tooth for invasive plants]

In the study, the researchers used captive deer at the Penn State Deer Research Center, where a herd of up to 100 deer live within fences. The researchers looked at feeding preferences of eight mature does without fawns across three seasons (late summer, early autumn, and spring). The 15 plant species were offered in pots “cafeteria style,” where deer had a choice of each variety simultaneously. A camera fitted with a motion detector and infrared for night viewing allowed the researchers to observe and record deer behaviors. The amount of each plant consumed was also measured.

Future work may study why some preferred invasive species are still so abundant in the field and may include experiments to pair two species together in feeding trials to further assess food preferences.

“This research is important for helping conserve our forest understories and natural areas,” Averill says. “It helps us understand interactions between deer and invasives and how deer might be exacerbating invasive plant problems.”

Coauthors include David Mortensen, Erica Smithwick, and Eric Post of Penn State. The United States Department of Agriculture provided partial funding.

Source: Cornell University