Scientists in Andalucia are fighting to control an invasion of aggressive American turtles. The interlopers are threatening to wipe out native species in the very place that should offer most protection: the nature reserve of Coto Doñana, Europe's most important wetlands.

The new bullies on the block are Florida turtles - which are also known as red-eared turtles - that can weigh up to 3kg. They are smart, aggressive, twice the size of their European counterparts, and reproduce at three times the speed.

The handsome scarlet and yellow Trachemys scripta elegans was a popular pet in the Eighties and Nineties, an exotic import from the US which was sold by pet shops throughout Europe.

"The babies are beautiful, with a brilliant yellow belly and a red mask-like stripe across the face, which is particularly striking as the turtle always stretches its neck and holds its head up," says Adolfo Marco, a scientist at the Doñana Biological Station near Seville.

"But they carry salmonellainfections, their colours fade as they grow, they start to bite and make their watery habitat stink. So owners want to get rid of them, and think it kind to release them into the wild."

Spain banned the import of red-eared turtles in 1997, alarmed at the health risk they posed for children. Freed turtles found an ideal habitat in the warm Andalucian wetlands where they dug concealed and well-protected nests, and reproduced rapidly.

Thousands now inhabit the marshy lagoons, pushing the smaller European turtle - Emys orbicularis - to poorer waters. "I think we have caught the problem in time," Mr Marco said yesterday. "Much later and the colonies would have become a source whose reproduction we couldn't control. But we found that when we remove the Florida turtles, the European ones return to their old haunts. Otherwise they would not survive." Scientists have no wish to destroy the flashy predators, just remove them from the wild.

"We plan to establish special ponds for them in local parks, where people can admire them. They would have a good quality of life, and not threaten other species," said Mr Marco. "And people could bring us their unwanted turtles, instead of throwing them out."