Scientists in the US have produced the first quantitative forecast of future rates of biological alien invasions. They hope their method will forewarn and forearm vulnerable countries.

For alien aggressors such as the Colorado beetle - a potato pest that could decimate British crop yields - international trade is a gateway for invasion. The pests hitch lifts on cargo such as live plants or raw logs, containers and transport vehicles, spreading quickly and easily, often finding no natural enemies in their new habitat.

Invasive species are responsible for putting at risk nearly half the species on the US threatened and endangered lists and cost the economy billions of dollars a year.

In a study published in the journal Conservation Biology, Jonathan Levine of the department of organismic biology, ecology and evolution, 51¹ú²úÊÓƵ of California, LA, and his colleague Carla D'Antonio at the department of integrative biology, 51¹ú²úÊÓƵ of California, Berkeley, forecast a 16-24% rise in the number of non-indigenous species entering the US by 2020. To put it simpler - 18 mollusc species, 34 plant pathogens and 492 insect species.

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"Our predictions may be particularly important because invasions are to some extent avoidable," Mr Levine told The THES. "Quantitative forecasts are required for educated planning."

The researchers use bio-mathematical models to relate past merchandise trade to the number of biological invasions. They link the figures to the estimated rise in US imports to predict future rates of invasion. The scientists claim their approach should be applicable worldwide.

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Yvonne Buckley of The Natural Environment Research Council's Centre for Population Biology, Imperial College London, is confident of the value of such forecasts. She said: "Predicting invasions will help make policy decisions and allocate the right amount of money to deal with them.

If you are aware of the problem in advance you can make contingency plans."