The Rapa whelk reproduces by laying a mat of 130 or more egg cases on some seafloor surface. Each case, which resembles a fat strand of lemon-yellow wool in a shag carpet, can hold 300 to 450 eggs. Once the larvae emerge from these cases, they float in the water for up to 2 weeks before settling to the bottom and developing into a hard-shelled snail. (Mann/VIMS)

The problem with all of them is that when they emigrated, they left behind those predators that had evolved to keep their populations in check. Now, living in an environment largely devoid of natural controlling agents, they are free to spread like the proverbial weed.

Currently, some 6,500 species of alien animals, plants, and microbes have established self-sustaining populations within the United States, according to a new survey of U.S. biological resources (SN: 9/25/99, p. 199). In a few cases, horticulturists intentionally imported these species to beautify the landscape, to improve the quality of food supplies, or to prey on previously introduced aliens. In other instances, hobbyists imported fish, birds, reptiles, or other pets that eventually got loose.

Scientists, too, are responsible for a notable disaster here and there. For instance, the infamous gypsy moth now defoliating forests throughout much of eastern North America trace to a couple of escapees from a late-19th century research lab. There, a breeder was investigating the insects as a source of genes for a better silk worm.

However, "there have been far, far, far more invasions in the oceans" than in the terrestrial environment—and than most people realize, notes James T. Carlton of the Maritime Studies Program at Williams College & Mystic Seaport in Mystic, Conn. The reason, he argues, is that marine invaders stay effectively hidden from the human world until they are fairly well established.

He also cites a few other factors that disguise the real magnitude of marine invasions:

• There are few controls on the movement of organisms between the seas. For instance, ballast-water intake and discharge controls have met strong opposition—even though ballast water is recognized a major conduit for the dispersal of organisms around the world, Carlton notes.
• "Fewer and fewer scientists can distinguish one organism from another," Carlton says. At one time, he notes, biologists were adventurers, trekking the wilds, and getting up close and personal with countless members of a species. Not only did this allow them to appreciate and recognize the natural variation within a species, but it also permitted them to quickly distinguish organisms that didn’t belong—the aliens. Today, he argues, "fewer and fewer of us are leaving our computers and going to explore the shores." A situation has now developed where "we are ill prepared to understand the strength of an invasion signal"—like repeated catches of an unusual whelk in the Chesapeake—"no matter how loud it is," he says.
• Few individuals understand the magnitude of havoc that a single alien species can wreak. Carlton observes that there are invading species in U.S. waters that are infiltrating ecosystems and are poised to raise havoc that would rival the zebra mussel’s. However, he laments, "unless I can put this new species on a milk carton or give it poster-child status, most people refuse to take notice of it"—until some industry that’s important to them suffers economic consequences. And though such alarming economic impacts often do occur, they tend to show up only after an invader has become so entrenched that it’s virtually impossible to rout.
• The increased movement of goods and people around the globe is "scaling up the load of species in motion," Carlton says.
• Recent improvements in water quality are making many ports increasingly hospitable to foreign invaders. Indeed, "this is the number one hypothesis—the street wisdom, if you will—for why the zebra mussel and so many other invasions have taken hold in the Great Lakes just within the past decade," Carlton told Science News Online. In the 1960s, he muses, Lake Erie was so polluted that an alien species would be as likely to croak as to thrive. Now, however, as coastal water quality continues to improve, alien species may find U.S. waters to be like a resort spa.

The bottom line, Carlton contends: "It is and will remain far easier to take a multi-billion dollar picture of the surface of Mars—or hold a piece of the moon in our hands—than to effectively control most marine invasions."