At snail's pace

Exotic species bring slow death to native ones

Published: Saturday 04 July 2015

when a non-native species of plant or animal invades, some native species are threatened by the encroachment on their niches. Some loose the battle and go extinct. The time lapse between the introduction of the new species and the extinction of the natives can be so long that the loosing struggle can go unnoticed, says a new study.

James Byers and Lloyd Goldwasser of the University of California at Santa Barbara, usa , used a computer model to simulate the invasion of California salt marshes by an Asian mud snail ( Batillaria attramentaria ) and found that the native snails ( Cerithidea californica ) would become extinct within 90 years. In the simulation, it took at least 20 years after the Asian snail's invasion for the native snails to show any major changes in indicators of population health, such as size, growth rate and egg production. By that time, the invading snail population had already steadily increased.

The Asian mud snails hitched a ride on a Japanese oyster shipment about 70 years ago.

Delays in population changes can lead to an underestimation of the speed and impact of an invasion and inaccurate evaluations of control programmes. But Greg Ruiz, head of marine invasion research at the Smithsonian Environmental Research Centre in Maryland, usa, cautions that it is not clear how well even computer-generated data could apply to the real world. Environmental factors, such as competing organisms, temperature and rainfall, influence invader success and could complicate efforts to predict the patterns of ecology.

Invasive cost the us alone at least us $100 billion a year. Computer models could change this scenario and help biologists intervene earlier, averting disasters such as that caused by the purple loosestrife, which arrived in North America from Europe more than a century ago, and has since transformed wetlands into an uncontrollable sea of purple by crowding out native food sources for wetland wildlife.

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