Invasive plants get help from enemies

Native predators control invaders better than transplanted ones


Ask anyone who's tried to cut down pesky bamboo shoots or rid his garden of purple loosestrife: Invasive plants can be a major hassle.

Scientists have long believed that invasives thrive as well as they do because they have escaped the natural predators in their native homes. But a study released today says that's only part of the reason.

Scientists at Georgia Institute of Technology say that invasive plants have succeeded because they are sharing their new homes with the enemies that evolved along with them.

The researchers examined 63 studies documenting the effects that native and nonnative grazing animals have on more than 100 native and invasive plants. They studied grasslands, forests, deserts, marshes and other habitats.

Their conclusion: When animals such as sheep, goats, cattle and horses are introduced to new habitats, they are less effective than native animals, such as bison and elk, at combating the spread of invasive plants.

The study, published in today's edition of Science, was not aimed at finding ways to control bamboo or kudzu, invasive Asian imports that are choking waterways and backyards, said senior author Mark Hay, an ecologist at Georgia Tech.

But the results show how humans and animals have played a major role in spreading invasive plants, he said.

The findings also bolster arguments for reintroducing more animals such as bison and wolves to Western states, and beavers to East Coast wetlands. Human influences are too pervasive for conservation officials to merely set land aside and hope for the best, Hay said.

"We act as if we were to leave things alone they will get better. But with six billion people living on the Earth right now, that's not going to work," Hay said. His co-authors are John D. Parker, a researcher at Cornell University, and Deron E. Burkepile, one of Hay's graduate students.

The researchers say the pattern can be traced to the 1600s, when European colonists began altering the landscape by replacing native bison and elk with imported cattle, sheep, horses, pigs and goats.

Since early times, the animals have been eating up the native grasses, damaging habitats and opening the door for European grasses that were among the earliest invasive plants here. The pattern continues to this day, scientists say.

Natural enemies

"The notion has always been that these organisms are spreading because they've escaped their natural enemies, but the problem is that they're following their natural enemies," Hay said.

The study offers an interesting perspective on the spread of invasives, said Erica Fleishman, a conservation biologist at Stanford University. "I think the bottom line science is strong and it sends an important message," she said.

But she and other experts say the research doesn't address several forces that have played a major role in the spread of invasive plants, such as globalization of the economy, climate patterns and human settlement.

"They have an interesting tidbit of a story. But there are co-varying factors which they didn't consider very well," said Tom Stohlgren, an ecologist with the U.S. Geological Survey who has written extensively on the spread of invasives.

Human settlement has been a major factor in the spread of invasive plants, he said. Invasives also tend to grow in temperate areas where native plants thrive, he said.

The study's authors say they wanted to focus on a specific piece of the puzzle by contrasting the effects of nonnative and native plant-eating animals on invasive plants. "The power of this paper is we've studied lots and lots of species and lots and lots of habitats, and it's the same everywhere," Hay said.

Plants face new competitors when introduced into new habitats and will sometimes die off, Hay said. They generally thrive only because other exotic plants and animals have paved the way for their arrival.

He compared the struggle with a family moving into a new neighborhood. "When you're in the fifth grade and you move to a new school, what's the probability you're going to be the toughest kid in the class?" he said. "You face new competition."

Study guidelines

One issue was how the study authors define native versus nonnative plants and animals. The crops and livestock that provide about 98 percent of the U.S. diet - including corn, wheat and rice, as well as cattle, poultry and other livestock - were all introduced by European colonists.

For purposes of the study, the researchers considered seed crops and plants classified as noxious by state and federal regulators as introduced plants. Nonnative animals included any brought to the Americas by Europeans, such as horses that came over with the Spanish in the 1600s.

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