SAVANNAH, GA. -- About the hoofed mammal gardeners love to hate, there's good news and the other kind from Cornell University researchers who study plant-eating habits of the white-tailed deer.
Speaking at the Ecological Society of America (ESA) annual meeting Aug. 3-8 in Savannah, Ga., Mark Vellend will report his discovery that a significant role in seed dispersal is played by deer browsing on vegetation wherever they wish and depositing seeds, in their pellet-like feces, to germinate and produce new plants up to two miles away.
"The good news is that deer might facilitate the spread of native plants to habitats recovering from disturbance, where the seeds otherwise wouldn't be able to reach," says Vellend, a Cornell graduate student of ecology and evolutionary biology. Along with Cornell undergraduate Jonathan Myers, the graduate ecologist spent countless hours dissecting deer pellets.
The bad news, says Vellend: "Unfortunately, many of the seeds we're finding in deer feces are from noxious weeds, including four of the top 20 invasive plants of greatest concern in New York state. The multiflora rose, for example," he says of a pretty plant that creates impenetrable brambles.
Ecologists have long known about the myriad ways seeds are dispersed from plants in eastern North America: Ants carry some seeds and so does the wind, while birds and other vertebrates drop indigestible seeds in their feces. And certain plants with ballistic capabilities can shoot seeds several feet or even yards away.
Many of those methods, however, fail to account for the relatively rapid spread of some plant species when unused farmland returns to woodlands. Or the resurgence of woodlands when Ice Age glaciers scoured landscapes of all vegetation millennia ago and then receded, leaving barren soil in their wake.
The study by Vellend and his colleagues was the first comprehensive test of seed dispersal by white-tailed deer, which are known to researchers by their scientific name, Odocoileus virginianus , and to long-suffering gardeners by unprintable epithets. Once a rare sight in suburban neighborhoods, deer populations have reached unprecedented levels in many parts of eastern North America. Wildlife biologists credit a variety of factors (the return of abandoned agricultural fields to woodland, suburban sprawl and a shortage of predators, both human and wild) for the deer explosion.
A typical cluster of deer pellets examined by Vellend and colleagues contained more than 30 germinable seeds. They found that seeds are dispersed by deer throughout the year. The majority of seeds found had no obvious adaptations for dispersal -- that is, if the deer hadn't carried them to new locations, they probably wouldn't be there. Some seeds were from trillium, the three-petaled woodland flower that is a protected species in some areas. Other seeds came from plants that need no protection and threaten to overwhelm native plants.
At the ESA meeting, Vellend will report: "White-tailed deer represent a significant and previously unappreciated vector of seed dispersal across the landscape, likely contributing an important long-distance component to the seed shadows of hundreds of plant species and providing a mechanism to help explain rapid rates of plant migration."
As for the rapid rate of plant disappearance, the Cornell ecologist offers a ray of hope to gardeners: "If eaten at the right time, plants may reappear next season, thanks to seed dispersal, though not likely in your own yard. Maybe your neighbor's yard."
Vellend also was assisted in his deer study by research associate Sana Gardescu and Professor Peter L. Marks in Cornell's Department of Ecology and Evolutionary Biology. The study was supported, in part, by the A.W. Mellon Foundation, the McIntire-Stennis program, the Cornell University Biological Sciences Honors Program and a STAR Fellowship from the U.S. Environmental Protection Agency.