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"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.
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.
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