Aliens are here. Devouring the forest, choking out wildflowers. Their names-balsam wooly adelgid, microstegium-sound a little funny. But the damage they can do is serious.

When humans take a plant or animal outside its natural habitat, biologists call it an alien, or nonnative species. Peter White, professor of biology and director of the North Carolina Botanical Garden, has studied the effects of alien species for years. What he and others have found leads him to be cautious about importing or exporting exotic plants or animals. Sharing, it seems, isn’t always a good thing.

Some nonnative plants that grow in the U.S. are useful. Corn, rice, wheat, tomatoes, and soybeans were all born in other countries. But other plants can grow out of control once they leave their native land, where they may have been limited by climate or particular pests.

Once aliens become reproductive wonders, they’re known as invasives. And invade they do. Johnny Randall, an assistant director of the botanical garden, points out a photo of a nonnative flower, purple loose-strife, bathing a field. “Some people might think that’s pretty,” he says. “But I don’t.”

The feathery magenta flowers look innocent enough, but, greedy for space and nutrients, they’ve crowded out all other plants, forming a monoculture. To Randall, nothing could be uglier-or worse for the environment. In wetlands, monocultures of purple loosestrife can reduce nearby duck populations because its seeds aren’t as good a food source as other plants.

Like purple loosestrife, which was introduced from Europe in the 1800s as an ornamental plant, many invasives leave their native lands courtesy of a well-meaning plant lover. Sarah Reichard, research assistant professor of urban horticulture at the University of Washington, has found that 85 percent of all damaging woody plant invaders were purposely introduced, White says. Surprisingly, botanical gardens have often contributed to the spread of invasive aliens, because they’ve long prided themselves on the number of requests they get from around the world to distribute their plants or seeds.

Other exotic invaders make the trip as hitchhikers-seeds or insects can attach to cars, trains, ships, or planes. Purple loosestrife, whose seeds are able to survive submersed in water, was partly spread by water used as ship ballasts-temporary loads used to increase stability until ships pick up cargo.

According to the National Park Service, alien species of plants and animals are the second greatest threat to the biological diversity of the U.S. And diversity is a good thing. Randall points out that insects and other small animals need a variety of native plants to feed on. Monocultures also endanger plant life; if there are only a few species around, it’s less likely that plants will adapt or evolve when the environment changes.

Exotic pests can cause damage too. The balsam wooly adelgid has killed Fraiser firs in the North Carolina mountains. This sap-sucking insect can kill a tree in five to seven years, White says. Trees are even more susceptible to pests when stressed by air pollution. Hugh Morton, a Carolina alum and owner of Grandfather Mountain, has often spoken out about the deadly mix of air pollution and adelgids.

The balsam wooly adelgid, originally from Asia, traveled to North Carolina in the 1930s attached to some balsam fir trees from New England. Since North Carolina trees have had no past history, or coevolution, with the pest, their defenses against it are low. A similar insect, the hemlock wooly ade-lgid, has now moved into North Carolina and is threatening hemlock trees in the mountains.

Other plants have been damaged by a somewhat larger exotic animal-the European wild boar. In 10 years of studying damage caused by the boars in Smoky Mountain National Park, White found that they disturb the soil and have reduced plant cover on the forest floor by 80 percent. White fenced off various areas of the park to keep the boars out, then sampled the plant growth inside and outside the fences.

The boars cause an imbalance in plant life because they eat only certain plants, such as trillium, lilies, violets, and clingman’s hedge-nettle, a rare plant that reproduces at a low rate. On top of that, the boars carry diseases that humans can catch. “That’s one reason why you can’t drink the water in that seemingly pristine park,” White says.
The damage done by invasive species has gotten widespread attention only in the last five to eight years, White says. “The previous attitude was that release of weeds was probably accidental, that somehow we weren’t responsible. But one ecologist has said, `We shouldn’t think of these releases as accidental, but as careless.’”
To avoid contributing to the problem, the Botanical Garden has restricted its seed distribution to the Southeastern U.S. The restriction is necessary, White says, because even introducing new plants to other states could interfere with native plants’ reproduction. Staff at the garden will help gardeners outside of the Southeast find a source of seeds native to their area.

The garden does grow some nonnative plants-but with caution. Their policy, which Randall drafted, promises to grow only species that won’t harm the natural world or the economy. “We’re not taking a stand against all movement of species and gene pools. Some exchanges do have benefits,” White says.

A few invasives planted long ago still plague the garden. One is microstegium, or bamboo grass, which grew five feet tall at the Mason Farm Biological Reserve this spring. In the winter, Randall says, “It’s a swath of dead grass sculpted over everything.” It’s not certain what the grass can do to surrounding plants. But Randall is afraid that it could inhibit tree-seedling growth in nearby Big Oak Woods-the largest section of undisturbed piedmont forest in the state.

But eradicating bamboo grass and other invasives takes money for herbicides and manpower. The estimated cost of fighting invasive species in the U.S. is as much as $23 billion each year. And the resulting increased herbicide and pesticide residues can hurt the environment, White says. These costs will be reduced only through cooperation and prevention from botanical gardens, nurseries, and conservation groups. “We’ve had an outstanding bill for decades,” White says. “It’s time to pay up.”

In Fran’s Footprints

Where hurricanes open a hole, invasive plants rush to fill it. Rickie White, a recent master’s student in ecology, found that in areas of the N.C. Botanical Garden damaged by Hurricane Fran, the number of species of invasive plants has tripled. These species have also increased in undamaged areas, but not as much-they’ve doubled instead of tripled.

The hurricane, White says, cut a path for such invasive plants as princess tree, tree of heaven, oriental bittersweet, and boxwood. “Many of these plants usually grow on the edges of the garden, but not in the garden,” White says. “But when trees are destroyed by a hurricane, that lets more sunlight in and gives these exotic plants a chance to infiltrate.” The increased sunlight combined with soil disturbance allows seeds to germinate that otherwise wouldn’t.

To compare damaged and non-damaged areas, White used 42 study plots scattered among the natural areas of the botanical garden. The plants in these plots were sampled and recorded in 1990. With the help of several assistants, White sampled them again in 1997 and 1998-counting and identifying every tree, shrub, and herb. “The most difficult part was that the hurricane had made the forest a jungle,” White says. “It was very dense, and lots of large trees had fallen.”

In the damaged plots, the number of invasive species had increased from an average of two species per plot in 1990 to six per plot in 1998. In undamaged areas, invasive plants had only increased to four species per plot. These results support a similar study from Florida, where the rate of exotic invasion increased following Hurricane Andrew.

Since finishing his research and master’s degree, White has begun work with the National Audubon Society, helping protect some of the remaining natural areas near Chicago. 

Peter White has been named to a national panel to study control of invasive plant and animal species. Johnny Randall recently helped organize the N.C. chapter of the Southeast Exotic Pest Plant Council, which will develop strategies for controlling invasives.