A war is raging on the riverbanks of the Thames. Not that you would ever guess if you took a stroll alongside them. Here, where the river bends and heads toward Oxford, all seems calm, quiet, green, and serene. Only the occasional splash of a sycamore seed – helicoptering down from the lanky trees that lean over the river – disrupts the soft, steady flow of the deep greenish Thames.

But don't be fooled. Hidden in the reeds are two guerrilla armies fighting for dominion over this watery kingdom. On one side stands a battalion of sleek and cocky American invaders, who have set up camp and forced many to flee. On the other side stands a band of brave and plucky Brits, determined to win back the banks.

Welcome to the clash between the American mink and the British otter. Forget "The Wind in the Willows" – this is "The War of the Willows," a battle of otter resistance to mink colonization.

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"I'm sure the American mink is a beautiful creature – in America," says David Macdonald, professor of wildlife conservation at Oxford University. "Here, I'm afraid, it is a pest."

Earlier this month, he and his team published a report titled "The State of Britain's Mammals 2006," in association with the animal charity, the Mammals Trust. It had some good news for otter-watchers: after 30 years of waning numbers, and even near-extinction, these slick swimmers and fierce fishers are making a comeback. They're taking on the new carnivores on the block: American mink, the descendants of escapees from fur farms around Britain.

"We're seeing evidence that large carnivores like otter will brutalize smaller carnivores like mink in order to drive them away," says Professor Macdonald.

Brutalize?

"Yes – they will use heavy-handed tactics – or should that be heavy-pawed tactics? Anyway, they will attack smaller carnivores that pose a threat." His team has even had reports of gangs of otters pursuing minks down riverbanks, literally scaring the invaders away.

The number of native otters declined catastrophically in Britain in the 1960s and '70s, largely as a result of a pesticide called dieldrin. It was used on farms, and all too frequently it found its way into the diet of fish. Otters ate the fish. Dieldrin turned out to be lethal to otters, an thousands died.

Mink moved in on former otter territory. These native North Americans were imported to Britain in the early 20th century to supply the fur industry. In the 1970s and '80s, just as otters were dying out, large numbers of mink escaped or were set free by animal rights activists. It didn't take them long to stick their flags in the banks once ruled by otters, finding themselves unchallenged by any other competitive carnivore.

Until now. The otters are back.

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"Who do I want to win? The otters, of course," says Malcolm Shaughnessy, mooring his longboat – named Walking on Water – at the south entrance to Oxford.

"The mink do not belong here. The otters have a right of return," he declares, as if talking about refugees displaced by war.

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