Invasive plants, animals freed of natural enemies

Invasive plants and animals that hitchhike across the globe have an advantage in their new homes because they leave their enemies behind, two new studies conclude.

In one study, biologists in California compared the parasite burdens of invading animals with those of native species in the new territories. In another paper, ecologists in New York compared similar data for plants. Both teams found invasive plants and animals carry fewer numbers and types of parasites than native species.

In the animal study, biologist Mark Torchin of the University of California, Santa Barbara and his colleagues analysed data from 26 randomly selected invading species, from mollusks to mammals.

In general, they found the introduced animals had half as many parasites as their native counterparts.

"On average, an animal has 16 parasites at home, but brings less than three of these to new areas that it invades," said Torchin.

Parasitism is the most common lifestyle on Earth, the researchers say. Not only can parasites make animals sick, they may also castrate them, change their behaviour or kill them.

By leaving their parasites behind, invasive species gain an advantage over their new, native competitors. In a related study on plants, researchers found that the introduced plants that left behind the most pathogens are the ones that are most likely to become weeds.

They looked at 473 randomly chosen plants that the U.S. Department of Agriculture considers invasive species from the Mediterranean region. The plants included Russian thistle and leafy spurge.

• FROM JULY 29, 2002: Scientists defend Canadian ecosytems from invasive species

On average, Cornell University ecology Prof. Alison Power and postdoctoral researcher Charles Mitchell found the invasive plants in the U.S. had 77 per cent fewer diseases compared with the same species in their native European habitats.

Invading plants that acquired the most pathogens in their new homes were less likely to become noxious agricultural weeds.

"These results suggest that invasive plants' impacts are a function of both release from and accumulation of natural enemies, including pathogens," the Cornell researchers said.

Both studies appear in Thursday's issue of the journal Nature.