Normally, it begins with the unexplained absence of frogspawn. Then comes the slow but steady disappearance of dragonfly larvae, fish and ducklings. In extreme cases, there are vicious attacks on small dogs.

Around Britain, the placid calm of urban ponds and watercourses is being disturbed by a rapacious new menace – legions of abandoned pet terrapins.

Conservationists have issued a warning that hundreds of boating lakes, canals and waterways in towns and cities are infested with terrapins and small turtles which were bought as pets while brightly-coloured babies barely bigger than a 50p coin but dumped by owners unable to cope as they grew to mature carnivorous adults the size of a dinner plate.

The trend began in the early 1990s when thousands of red-eared terrapins, each capable of living up to 30 years, were bought by young fans of the Teenage Mutant Ninja Turtle cartoon. But ecologists have warned of a more recent second wave of releases which is seeing additional species, including the aggressive snapping turtle, dumped in the wild.

Although native to warmer climes such as America's Mississippi valley, the terrapins and turtles readily take up residence in Britain's parks and wetlands where they have a ready food supply, including young waterfowl.

Experts have seen examples of ponds stripped of wildlife by a population of just two or three terrapins.

Such is the scale of the problem that 51 terrapins and turtles, from five different species, were recently removed from a single pond in a north London park after the local authority called in a specialist trapper. Two years ago, a colony of 150 of the creatures was removed from the 25 ponds on Hampstead Heath and re-homed at a sanctuary in Tuscany.

The result is a double headache for conservation groups as they try to control the problem by trapping and removing the unwanted invaders but struggle to find new homes for the captives because of their longevity (some species can live for up to 50 years) and the costs of running a dedicated aquarium. One sanctuary receives unwanted animal at a rate of six a week.

John Baker, of the Amphibian and Reptile Conservation (ARC) Trust, said: "When these animals are bought as babies they seem attractive pets. But they grow to a significant size and people think it is OK to take them to their nearest body of water and release them into places where they prey on native species and can spread disease.

"The additional problem is what to do with them once we find them. The law says they cannot be returned to the water and sanctuaries are often reluctant to take them. Caring for a terrapin is a major undertaking – they live for decades and we don't want to see them put down. People really need to be more responsible about buying them in the first place."

As committed scavengers without natural predators in Britain, terrapins and turtles find themselves at the top of the food chain in urban ponds and watercourses, chomping their way through a menu of native species that includes newts, fish, toads, frogspawn, larvae and, for the largest and most aggressive specimens, the occasional duckling or juvenile moorhen and coot.

Of particular concern is the common snapping turtle, a powerful American species, which has a vicious bite and is known for its aggression. One of the creatures was captured in the trawl of Clissold Park in Stoke Newington, which netted 51 critters, while another was suspected of carrying out attacks on several dogs and a Canada goose in east London.

Rebecca Turpin, London officer for the ARC Trust, said: "We should not underestimate the impact that these animals can have. They can decimate a pond. I personally know of several where there is no wildlife left because of a few resident terrapins.

"They can go through the native species pretty quickly if the conditions are correct."

The influx of red-eared terrapins to Britain in the early 1990s was halted by legislation banning imports of the species, but it has been replaced in the pet trade by a number of new types, including the yellow-belly slider, the Cumberland, the diamondback and the European pond turtle. Individual specimens can be bought for as little as £10.

Experts have consoled themselves with the fact that Britain's climate means that although the terrapins and turtles can survive, they are unable to breed because cooling temperatures in the autumn do not leave fertilised eggs enough time to hatch.

But the evidence in recent years is that a small numbers of juveniles has survived and prospered, raising the prospect of an established population across the British Isles.