The hunters were stalking their prey on a wooded path in Patapsco Valley State Park south of Baltimore, peering closely into the underbrush. But they weren't looking for animals.
The group of amateur naturalists was on a search-and-destroy mission for exotic plants that have invaded Maryland and are killing off native life. The problem of invasive species is drawing increased attention as globalization has brought more international trade, which has led to more seed-hopping from continent to continent.
Esther Quinn, a high school teacher from Catonsville participating in the plant hunt yesterday, pointed to a vine with razor-sharp pins prickling from its stem and lime-hued triangular leaves. It was a Devil's Tear Thumb - a plant from Asia that grows so fast it earned the nickname of "Mile a Minute."
"Evil little vines, evil," Quinn muttered.
It's not that they're evil, said Chris Brooks, the park ranger leading an educational tour yesterday called "Bad, Bad Plants" in the state park in Halethorpe. It's that a small percentage of non-native plants proliferate when they're transplanted into an area where they have no natural checks on their growth.
They outcompete local plants, steal all the sunlight and smother everything else, said Brooks. Because they're not part of the normal diet of local animals and insects, their monopolies can be deadly, he said.
The effort by state park workers and volunteers to rip out the invaders isn't a matter of xenophobia, Brooks said. It's an attempt to maintain the balance of life that allows insects to live in plants to which they're adapted, which allows birds and other native animals to eat the insects and have enough food to survive.
"We like seeing deer, chipmunks and other animals in our parks. But deer cannot digest bamboo leaves, for example," Brooks told the half-dozen local residents who joined him in an hourlong trek through the park. "We want the native plants, because they're easier for the native animals to eat."
Over the past four years, the Maryland Department of Natural Resources and local governments across the state have stepped up efforts to identify and kill the worst invasive plants, said Kerrie Kyde, invasive plant specialist for the state wildlife agency.
Nationally, about 5,000 exotic plants and animals are believed to be causing serious enough problems that they're classified as invasive. That's only perhaps 1 percent of species that have been transplanted here from Asia, Europe and elsewhere, according to experts.
"As the world becomes a smaller and smaller place, and we move things from place to place more, we increase the number of species that are moving across what would otherwise be insurmountable geographic boundaries," such as oceans, Kyde said.
Not all the invaders are worth government eradication efforts. For example, dandelions, originally from Europe, are a nuisance to homeowners who want putting-green smooth lawn - but they don't threaten other plants or animals.
On the other hand, a few exotics are monstrous. Among the worst: The dreaded Heracleum mantegazzianum, or giant hogweed. This fleshy-leafed shrub grows up to 15 feet tall and has sap so acidic that touching it can cause blisters and swelling, and contact with the eye can cause permanent blindness, according to the state wildlife agency.
The state is trying some creative tactics to fight rogue plants. It's not just using weed whackers - which can make the problem worse by scattering the alien seeds.
In Howard County, officials over the past two years have been releasing a variety of Asian weevil, Rhinoncomimus latipes, to gobble up the rampant Devil's Tear Thumb, Kyde said.
It's fighting fire with fire. Exotic species are being used to attack exotic species. The state doesn't believe the introduced insects will multiply out of control because just about all they eat is Devil's Tear Thumb, said Kyde.
Brooks, the park ranger, assembled a handful of local plant enthusiasts at a picnic table yesterday morning beside a small lake in Patapsco Valley State Park.
He held up the spiny stalk of a kudzu vine that he had ripped from a roadside area near the park's entrance. He explained that this Asian plant has crept steadily into Maryland in recent years, overwhelming trees and blocking their light.
Then he led the group on a hike along a shady path beside the Patapsco River. He pointed out the long rows of pointed leaves that mark the exotic Tree of Heaven, or ailanthus, whose dense roots proliferate underground to spread offspring.
"It creates great shade," he said, holding up a branch. "But its roots are so strong they can break up concrete and driveways."
Beside the path, he pointed out the spade-shaped leaves of a garlic mustard plant. This Eurasian shrub was imported as a spice for soups and stews during the 1800s, and some people consider it tasty boiled like spinach.
"I've had it on salad," Brooks said.
But ecologically, it's poison. The garlic mustard suppresses the growth of fungi that other plants need to break down rotting plant matter and make nutrients available so they can grow.
Helen Orem, a teacher from Halethorpe who took the tour, said that more people should be educated about exotic plants. "I think people should know the damage that non-native plants can do to our local plants," she said. "I didn't realize that they could actually kill our native trees."