Many plants that once delighted gardeners have also run amuck when they escaped into the wild. Purple loosestrife--a European native popular as an ornamental plant in the early 1800s-- has invaded wetlands in 48 states at an estimated cost of $45 million a year for control and loss of forage crops. It is crowding out 44 native plants and endangering the wildlife that depend on them.

Topping off the list are the tiny creatures that nobody invited --microbes, insects, worms--that often sneak in with their traditional hosts but soon found more bountiful prey to occupy themselves. "It's too late to send these organisms back," says Pimentel.

But it is not too late to tighten safeguards against accidental introductions. Because of the ease of travel and increased agricultural commerce, it is easier than ever before for species to establish beachheads in distant lands. Most of the alien organisms in the U.S. arrived in the past 70 years.

"The true challenge lies in preventing further damage to natural and managed ecosystems," Pimentel told the AAAS meeting. "We have a long way to go before the resources devoted to the problem are in proportion to the risks. We hope that this assessment will advance the argument that resources spent on preventing the introduction of potentially harmful species will be returned many times over in safeguarding our environment."hosts but soon found more bountiful prey to occupy themselves. "It's too late to send these organisms back," says Pimentel.

But it is not too late to tighten safeguards against accidental introductions. Because of the ease of travel and increased agricultural commerce, it is easier than ever before for species to establish beachheads in distant lands. Most of the alien organisms in the U.S. arrived in the past 70 years.

"The true challenge lies in preventing further damage to natural and managed ecosystems," Pimentel told the AAAS meeting. "We have a long way to go before the resources devoted to the problem are in proportion to the risks. We hope that this assessment will advance the argument that resources spent on preventing the introduction of potentially harmful species will be returned many times over in safeguarding our environment."

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