By scouring North Carolina's southern Appalachian forests and examining the plants and conditions found there, UNC researchers have discovered that species diversity is significantly greater for both native and exotic plants in floodplains than in the higher areas. One reason appears to be that floodwater washes away significant numbers of plants and thereby creates opportunities for newcomers. Another is that water and gravity combine to transport seeds from upstream sources and deposit them on floodplains.
Of particular concern is the observation that such floodplains, which scientists call riparian areas, contain up to 40 times more exotic species than comparable habitats on adjacent uplands. Such information is important, the researchers say, because it offers insight into how invasive plant species from other continents interact with native plants and ultimately replace them.
A report on the findings appears in the current issue of Ecology, a top scientific journal. Authors are Dr. Rebecca L. Brown, a former UNC graduate student and now a postdoctoral fellow at the Patrick Center for Environmental Research at the Academy of Natural Sciences in Philadelphia, and Dr. Robert K. Peet, professor of biology at UNC.
"Exotic species invasion is one of the major ecological problems of our time because they change habitats and could potentially cause many native species to go extinct," Brown said. "A leading theory in ecology has been that in areas with a high species diversity there should be less invasion by exotic species, but we found that's often not true."
In the study, which she conducted for her doctorate under Peet's supervision, the two examined close to 1,200 sites located at various elevations above streams and rivers. They found that flooding not only provided water and other nutrients to plants, but also created open space and distributed seeds, which allowed easy immigration of exotic species.
"As flood frequency decreased at higher elevations above the floodplains, both the number and the variety of native and exotic plants dropped," she said. "This happened even when fertility and the amount of light available was the same. It appears to be because there are fewer seeds and less disturbance as flooding decreases so that opportunities for immigration and germination are fewer."
One central question in ecology is the significance of biological diversity in ecological systems, Peet said. Human activities tend to reduce the biodiversity of natural ecosystems, and although the long-term implications of the reduction are uncertain, few are likely to be positive.
"A second central issue in ecology pertains to the transformation of ecological systems caused by the establishment of exotic species," he said. "For nearly a century, ecologists have suggested that high diversity confers a resistance to invasion by new species. Recent evidence has been mixed.
"Rebecca and I have resolved the inconsistency in the available data by showing that for those ecosystems where species immigration rates are the primary determinant of vegetation composition --such as floodplain ecosystems -- we should expect diversity of both native and exotic species to increase with increases in immigration rates. However, in stable ecological systems where competitive processes are more important in determining vegetation composition than immigration processes such as in uplands, we should still expect high diversity to confer resistance to invasion of both exotic and native species."
In other words, having numerous different types of plants in a relatively stable area makes the area somewhat resistant to newcomers. In unstable areas, such as along waterways, this isn't true, and exotics invade readily.
Support for the research came from Sigma Xi, the Southern Appalachian Botanical Society, the Nature Conservancy, the N.C. Heritage Trust, the United States Department of Agriculture Forest Service, the UNC Graduate School and the UNC Ecology Curriculum. Part of the work was done through the Carolinas Vegetation Survey, an annual voluntary effort of an army of botanists, field naturalists, students and others to describe natural vegetation across the two states.
By DAVID WILLIAMSON
UNC News Services
Note: Brown can be reached at 215-299-1151, Peet at 919-962-6942.
Journal
Ecology