Invasive Species Gain Advantage by Shedding Parasites

By Cat Lazaroff

WASHINGTON, DC, February 6, 2003 (ENS) - Invasive species gain a competitive advantage in their new homes because they leave behind their native parasites and diseases, argue two new studies in today's issue of the journal "Nature." This escape from illness gives non-native pests - now second only to habitat destruction in threatening biodiversity - a head start in displacing native species, the researchers said.

Globalization of commerce, particularly by ships and air traffic, transports hitchhiking plants and animals around the world, and in many cases they become pests in their new locations. Two teams of researchers - one targeting animals and the other focusing on plants - have drawn the same conclusion regarding the reason for the success of these invaders: freedom from disease.

Zebra mussels are now widespread in the Great Lakes and inland rivers of the United States, where they smother native mussels and clog intake pipes. (Photo courtesy University of Wisconsin Sea Grant Institute)

Lafferty and his colleagues, Drs. Mark Torchin and Armand Kuris, and Valerie McKenzie, at the Marine Science Institute at the University of California - Santa Barbara (UCSB), and Dr. Andrew Dobson at the Department of Ecology and Evolutionary Biology of Princeton University, analyzed parasite studies of 26 invasive animal species, from snails to rats, comparing them in natural habitats and invaded habitats..

In general, the team learned, the introduced populations had only half as many parasites as native populations of the same species.

"On average, an animal has 16 parasites at home, but brings less than three of these to new areas that it invades," said Torchin, the lead scientist of this study. "In the new region, parasites are not well matched to novel hosts, and only about four parasites will successfully attack an invading species."

The European green crab on the left has had its growth hampered by parasites. The larger crab on the right had no such hindrance. (Photo by Jeff Goddard, courtesy UCSB)

By leaving parasites behind, introduced species may have an advantage over less fit native competitors, which remain fettered to their own full complement of parasites.

"Parasites are to invasive species what kryptonite is to Superman," Lafferty explained. "Back on planet Krypton, kryptonite was a regulator, keeping Superman ordinary. Freed from kryptonite on earth, he gained super powers. But unlike Superman who used his power for good deeds, invasive species can be devastating."

Among the animals the researchers studied was the European green crab. It has had a devastating effect on U.S. fisheries along the Atlantic coast, and is now threatening fisheries in the Pacific Northwest as it moves from California toward Puget Sound in Washington.

The green crab eats Dungeness crabs, rock crabs, mussels, oysters and clams. In northern New England, the advent of the green crabs has been associated with the demise of the softshell clam fishery.

This European green crab has a knob like parasitic barnacle attached to its underside. (Photo by Todd Huspeni, courtesy UCSB)

The team found that in Europe, the green crab's native home, parasitic barnacles castrated the crabs. Where the barnacles were common, the crabs were small and rare.

But in areas where barnacles were uncommon, crabs grew large and abundant.

The green crab has also invaded Australia, Japan, Tasmania and South Africa, but barnacles have never made the transfer with them. In all of these areas, green crabs have become devastating pests that decimate native shellfish.

The same pattern holds true for invasive plants, found Drs. Charles Mitchell and Alison Power of Cornell University. In the second of today's studies, they report that the introduced plants most likely to become weeds are those that have left behind the most pathogens.

Leafy spurge uses toxins to halt the growth of other nearby plants, displacing native vegetation. It is found across the U.S., except in southeastern states. (Photo courtesy King County Department of Natural Resources)

On average, they report, invasive plants in the U.S. have 77 percent fewer diseases compared with the same species in their native European habitats. Viral diseases are harder for plants to escape because the viruses can travel, systemically, in the plants or in their seeds, the ecologists note.

"We're coming to realize we should be grateful for our native plant pathogens," said Mitchell, a postdoctoral researcher in Cornell's Department of Ecology and Evolutionary Biology. "Invasive plants cost an estimated $33 billion a year to the U.S. economy, but the damage would be worse were it not for our native fungi and viruses that control invasive plants to some degree."

Purple loosestrife is crowding out native plants in U.S. wetlands, marshes and meadows. In Michigan, biologists are trying to control the invader using beetles imported from Europe, the plant's native habitat. (Two photos courtesy Michigan Sea Grant)

"We continue to receive free service from our natural ecosystem in the form of pest control," Mitchell said. "The natural process of pathogen infection is helping to prevent invasive plants from becoming worse pests to humanity - without any subsidy from humans."

The two studies suggest that the parasites lost by invasive species could also be their Achilles' heel: a weakness that can be turned against them.

One of the beetles that may be useful in controlling purple loosestrife.

But the scientists cautioned that biological control of pests is risky if the parasites are not specific to the target pest. In some cases, insects introduced to control invasive plants have been found to prefer native plants. Once these insects are released, they cannot be easily removed from their new habitat.

"Suitable biocontrol agents should be harmless to native species, just as kryptonite is harmless to Earthlings," said Lafferty.