Space Invaders

Invasive species find an advantage over native species: freedom from former diseases

08-01-2003 // Roger Di Silvestro

INVASIVE SPECIES, like the zebra mussels smothering out native mussels in the Great Lakes, generally get an unfair advantage in new lands because they leave behind the predators that help hold them in check in native habitats. Now scientists have discovered that invasives have another advantage on their side—loss of their characteristic parasites.

So says Mark Torchin, a researcher at the University of California–Santa Barbara Marine Science Institute. "On average, an animal has 16 parasites at home, but brings less than three of these to new areas that it invades. In the new region, parasites are not well matched to novel hosts, and only about four parasites will successfully attack an invading species." So the invader ends up with only about half as many parasites as it had in its native habitat.

One of the species that Torchin and his colleagues studied was the European green crab, which has invaded U.S. fisheries along both the Atlantic and Pacific coasts. In Europe, green crabs are held in check by parasitic barnacles that neuter them. But the barnacles don’t occur in U.S. waters, so here the crabs grow large and numerous. The result: The crabs are wrecking the soft shell clam fishery in northern New England and could jeopardize clams, mussels and oysters on the West Coast, as well as rock crabs, bait fish and Dungeness crabs.

Loss of pathogens also benefits invasive plants. Cornell University researchers Charles Mitchell and Alison Power report that the most successful of the more than 470 invasive plants in the United States carried few parasites with them from home and were resistant to North American diseases. On average, invasive plants in the United States have 77 percent fewer diseases than do the same species in their native habitats.

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