The Clinton administration may soon take aim at "invasive alien species." 

They're not Martians or illegal immigrants, but something that may pose a more immediate threat to the Chesapeake and other ecosystems across the nation - non-native species that can crowd out resident plants and animals.

Bill Brown, science adviser to Interior Secretary Bruce Babbitt, said scientists increasingly agree on the need for a national effort to combat these invaders, and that President Clinton could issue an executive order soon creating a panel charged with writing a national plan to deal with exotic species.

"I'd be surprised in this next quarter if we don't see this directive get issued," Brown told scientists gathered at a forum on aquatic nuisance species that took place April 21 at the Smithsonian Environment Research Center in Edgewater, MD. Brown said 500 scientists had written Vice President Al Gore urging that action be taken on the issue.

When non-native species are introduced, whether intentionally or accidentally, they have the potential to run rampant. They can spread diseases that native species have no immunity to or, without predators, they can rapidly overwhelm an area, crowding out the natives.

In the Bay, the diseases MSX and dermo, which have devastated the oyster population, are thought to have been accidentally introduced in recent decades.

While that may be the most dramatic impact, there is potential for others. Greg Ruiz, a Smithsonian scientist, presented findings at the forum showing that he had identified 160 species in the Bay that are either probably, or definitely, exotics.

Further, he said, "There appears to be a very marked increase in the rate of invasion in the last three decades."

In San Francisco Bay, exotics now dominate many ecological niches, and it is estimated a new species is introduced every 14 weeks. In the Great Lakes, the zebra mussel, introduced more than a decade ago, has so effectively filtered water that some scientists worry that it is creating a food shortage for fish. Recent research has found that when the round goby - a non-native fish - eats zebra mussels, it concentrates toxics and passes them up the food chain, creating a human health risk.

Invasive alien species aren't just a problem in the water.  Species such as purple loosestrife and phragmites are overwhelming and degrading wetlands.

"The threat of invasive species may be a close second to the outright loss of habitat," said Hannibal Bolton, of the U.S. Fish & Wildlife Service. "As invasive species become established, it is very difficult to reclaim the natural systems." He said exotic species were the largest cause of species becoming endangered after habitat loss.