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Invaders Have Fewer Parasites, Giving Them a Competitive Edge Over Native Animals and Plants

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European green crab underside, showing knoblike parasitic barnacle
Infested: European green crab underside, showing knoblike parasitic barnacle. Photograph by Todd Huspeni, University of California, Santa Barbara.
Invasive species—second only to habitat destruction in threatening biodiversity—have far fewer parasites and less illness to contend with than their native competitors, according to two new studies in the February 6 issue of the journal Nature. In such superpests as the European green crab, this escape from parasites means that the crab gains an unfair advantage over its competition.

"Invasive species end up with about half the parasites, or diseases, they had at home," said Kevin Lafferty, a U.S. Geological Survey (USGS) marine ecologist at the Western Ecological Research Center's Channel Islands Field Station in Santa Barbara, CA. This conclusion was among the findings of Lafferty and his colleagues Mark Torchin, Armand Kuris, and Valerie McKenzie of the Marine Science Institute at the University of California, Santa Barbara, and Andrew Dobson of the Department of Ecology and Evolutionary Biology at Princeton University.

"On average, an animal has 16 parasites at home but brings fewer 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."

Parasites are so pervasive that parasitism is the most common lifestyle on Earth, according to Lafferty, whose studies of parasites have been featured in previous Sound Waves articles (April 2002, August 2002). Many parasites don't just make animals sick, they may castrate them, change their behavior, or even kill them. 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.

In Lafferty's view, "Parasites are to invasive species what kryptonite is to Superman. Back on planet Krypton, kryptonite was a regulator, keeping Superman ordinary. Freed from kryptonite on Earth, he gained superpowers. But unlike Superman, who used his power for good deeds, invasive species can be devastating."

The scientists analyzed parasite studies of 26 invasive animal species, from snails to rats, comparing them in natural habitats and invaded habitats. Among them was the European green crab, which Torchin and colleagues traveled the world to study.

size comparison of a small European green crab and an invasive green crab, more than twice the size of the European crab
Notable difference: European green crabs in their natural habitat are smaller (left) than those in invaded habitat (right). Photograph by Jeff Goddard, University of California, Santa Barbara.
The scientists 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. Conversely, the scientists found that crabs were big and abundant in areas where barnacles were uncommon.

Green crabs have been introduced around the world, to the west and east coasts of the United States, South Africa, Australia, Tasmania, and Japan, but barnacles have never made the transfer with them. In these introduced areas, green crabs are commonly devastating pests that decimate native shellfish.

The same pattern holds true for invasive plants, according to Charles Mitchell and Alison Power of Cornell University, in a separate study. They found that the introduced plants most likely to become weeds are those that have left behind the most pathogens.

Additionally, the two studies documented that the parasites lost by invasive species are also their widespread "Achilles' heel," a weakness that can be intentionally turned against them.

In some cases, according to the scientists, bringing in parasites from a pest's native range can hinder superpests. The benefits to this organic form of pest control are sustainability, low cost, and reduced dependency on pesticides; but the scientists cautioned that biological control of pests is risky if the parasites are not specific to the target pest. "Suitable biocontrol agents should be harmless to native species, just as kryptonite is harmless to Earthlings," said Lafferty.

Related Sound Waves Stories
Parasites as Indicators of Coastal-Ecosystem Health
August 2002
Where Disease May Mean Good Health—The Role of Parasites in Natural Ecosystems
April 2002

Related Web Sites
Using Parasites to Monitor Ecosystem Health
U.S. Geological Survey (USGS)
Nature Publishing Group
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Competitive Edge of Invasive Species

Lake Mead Work Continues

Outreach Dolphin Rescue

London Interns Tour St. Pete

Congressional Briefing on Gas Hydrates

Volcanic Ash and Aviation Safety

Science Mentoring

Meetings Coastal Vulnerability

Lidar Data and Technology

International Deep-Sea Corals Workshop

Northeastern Coastal Ecosystems and Resources Workshop

Awards Shinn Wins 2002 Shoemaker Distinguished Achievement Award

Coastal and Marine Scientists Win 2002 Shoemaker Product Excellence Awards

Behrendt and Poag Elected AAAS Fellows

Normark Awarded Keen Medal

Staff & Center News A Tribute to Joe Newell

Marine Geophysics Pioneer Honored

Celebrating Careers of Five Retirees

Manheim Lectures on Trends in Scientific and Technological Innovation

Publications San Francisco Bay Earthquake Hazards

Effectivenes of Marine Reserves in Central California

Human Influence on Diatom Productivity and Sedimentation in Chesapeake Bay

Feb. / Mar. Publications List

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