Botanist Elizabeth Wells hiked into a field of grasses and sedges south of Leesburg last month and began identifying the invaders.

The white-blossomed teasel, a waist-high weed native to Europe.

A Japanese grass known as arthraxon, which probably arrived on North American shores after hitchhiking aboard ships in crate packaging; stealthy and rugged, the plant then moved inland.

Over there, Wells squinted off in the distance, were the beggar's-ticks, another alien species from Europe, which bear a resemblance to the black-eyed Susan.

And on she went, walking and calling out the names of exotic plants, nonchalantly pulling ticks off her jeans. "We don't have thistle out here," she said, referring to an invader species that is common in Loudoun. "It can't take the wetness."

This field near the Oatlands Plantation is one of three artificial wetlands totaling 65 acres that were built and are owned by Toll Road Investors Partnership II to replace the natural ones that were destroyed during construction of the Dulles Greenway.

The U.S. Army Corps of Engineers--the government agency that handles permits for such "wetland mitigation" projects--would allow the Greenway to be built only if its developers agreed to build artificial wetlands.

The construction of these replacement wetlands began in 1994. Each tract was bulldozed and a sort of bowl scooped out, with the topsoil placed to one side. A pond was constructed at the bottom of the bowl; the topsoil was replaced; and the seeds of natives grasses and trees were planted.

Wells, a botany professor at George Washington University in Washington, is studying the tracts as one component of the Loudoun Environmental Indicators Project, undertaken by a phalanx of scientists in 1998. Eventually, the project will document the state of the environment in Loudoun and attempt to predict its future.

The goal of Wells' research is to take inventory of the plants that establish themselves on the man-made wetlands and compare them with plants that grow on natural wetlands on the edge of the Banshee Reeks Reservoir adjacent to Goose Creek. By mid-October, at the end of this year's growing season, she will have documented the success of the native plants and how much ground they covered.

"I'll be able to let the [Army] Corps know whether these artificial wetlands succeed in growing native vegetation or whether they fail and just have vegetation typical of farm fields," Wells said.

In the competitive world of nature, the native and the nonnative plants struggle for a foothold on the fragile wetlands--and the battle for dominance is intense on newly created wetlands. Wells refers to the exotic, nonnative plants as "the invaders."

"The aliens are crowding out things [native plants] that would be there and that animals and insects would eat, so the animals are deprived of food," Wells said. "That is land on which food they would eat normally would grow."

She ticked off the names of other exotics she has seen on the property. Spotted knapweed with its purple flowers. The ubiquitous Queen Anne's lace, an herb brought over by English colonists for use as a medicinal. The Japanese stilt grass, "one of the worst grasses in wetlands."

"We have it here and it just completely blankets everything where it grows, and it's very close coverage so that nothing else can grow," she said.

But there is some good news for native species. Spotted around the wetlands so far by Wells and members of the Loudoun Wildlife Conservancy and other groups have been frogs and toads, snakes and hosts of insects. Great blue herons, red-tailed hawks and other birds have discovered the cornucopia of food, Wells said.

The wetlands' builders also scattered "a tremendous amount" of native seeds for grasses and other plants, and these have been successful, Wells said, especially the native sedges and rushes, which are nourished by springs on the property.

Native tree species--swamp dogwoods, sycamores, willows--and at least one native shrub were also planted, and they are thriving.

"Grasses and sedges are the first things to establish themselves on the wetlands," Wells said. "It's good to see them doing so well."