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Ever since the Teenage Mutant Ninja Turtles craze of the late 1980s, there have been sightings of terrapins "the size of dinner plates" dipping in and out of English canals.

The animals were native to Florida but they are just one of many alien species making their home in England.

There is also increasing evidence that more unusual species are establishing colonies here, perhaps because of the warmer weather. So who are the aliens among us?

Turtle power

Image caption The Teenage Mutant Ninja Turtles craze of the 1980s resulted in hundreds of people buying pet terrapins

"Dinner plate" sized or not, the sight of a terrapin emerging from a canal is hardly the stuff of nightmares.

People are fascinated by canal terrapins. Mark Robinson, Canal & River Trust

Sightings have recently been reported in the Erewash Canal on the Derbyshire-Nottinghamshire border, but the Midlands climate is not thought to be warm enough for them to breed.

Those spotted were probably unwanted pets, bought at the height of the Teenage Mutant Ninja Turtles craze, and dumped when they grew too large.

They live for up to 40 years, and could have been swimming happily in the canals ever since.

But the Canal and River Trust has launched campaigns to capture red-eared terrapins from a number of waterways, including most recently the Ashby Canal on the Leicestershire-Derbyshire border.

There are fears the reptiles, which eat dragonflies, fish, frogspawn and even small waterfowl, might have a damaging effect on the local ecosystem.

Nick Brown, of Derbyshire Wildlife Trust, said: "These creatures are lovely when small but grow huge and very predatory and have been reported pulling down ducklings."

Image copyright Canal & River Trust Image caption A juvenile terrapin was found in the Regent's Canal last October

In London's Regent's Canal, where temperatures are marginally closer to the terrapins' Florida homeland, a juvenile was captured in 2013..

Mark Robinson, the Canal and River Trust's national ecologist, said: "I think in areas like London where you've got warmer temperatures because of the buildings acting as a heat reserve, coupled with warmer winters, it's possible [they are breeding]," he said.

He admitted, however, that the terrapins draw visitors: "People are fascinated by seeing them and like seeing them - I'm not sure they have a huge negative effect."

Canal snakes

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The banks of the Regent's Canal are also home to a small population of exotic serpents that are thought to be breeding.

The discovery of Aesculapian snakes, from southern and central Europe, led national newspapers to claim earlier this year they could strangle small pets.

But exotic species enthusiast Mark Hows rubbished the suggestion, saying the shy, non-venomous reptiles, which number about 30 in London, were "scared of people".

"They are incredibly hard to see," he said. "People have walked within a few feet of them and not seen them.

The snakes are filling an ecological niche Mark Hows, Exotic species enthusiast

"Trying to get a photograph of them is quite tricky."

He believes they were deliberately released into the wild in Regent's Park - but not by London Zoo, which is in the park grounds.

In Wales, a colony has been well established since the 1960s when they escaped from a zoo in Colwyn Bay.

"The key is what ecological niche they are filling and whether that's already filled by another species or not," said Mr Hows.

"We don't have many adders in London, so they are filling that niche."

He said their usual prey - rats - are abundant in London, so beloved cats and dogs are probably safe.

A bad smell in the Forest of Dean

Image copyright Mark Hows

When a young skunk was captured in the Forest of Dean in 2009, it gave credence to a theory that the North American mammals, renowned for their terrible smell, could be breeding there.

Mark Hows, who logs exotic species on his website and often goes out looking for them, photographed a number of skunks in the forest the same year.

He explained they made "very good pets" before an operation to remove their infamous scent gland became illegal.

But could they upset the ecological balance within the Gloucestershire forest?

"They are fairly omnivorous scavengers," said Mr Hows.

"They will eat bird eggs and chicks, which could be an issue, and we have other animals like badgers that might compete with them."

But he said they could potentially pull in visitors.

"There are wild boar in the Forest of Dean - they're quite a tourist lure now," he said.

"People come down to see them so they might also come down for the skunks."

Settling in

Dozens of non-native species are already successfully breeding in Britain.

Image copyright Google/Getty/Mark Hows/Tom Buckley Image caption Pictures courtesy of Google/Getty/Mark Hows/Tom Buckley

Breeding populations of wallabies have been confirmed across the UK, including in Highgate Cemetery, north London

have been confirmed across the UK, including in Highgate Cemetery, north London The capital is also home to tens of thousands of parakeets , helped to breed by the mild climate and plentiful gardens

, helped to breed by the mild climate and plentiful gardens Yellow-tailed scorpions - not quite as fearsome as they sound - are well established at Sheerness docks in Kent

- not quite as fearsome as they sound - are well established at Sheerness docks in Kent Beavers have started breeding in the River Otter in Devon after being reintroduced. They were a common sight 400 years ago but were hunted to extinction

have started breeding in the River Otter in Devon after being reintroduced. They were a common sight 400 years ago but were hunted to extinction Another species doing well after reintroduction is the wild boar - there are thought to be more than 800 boar in the Forest of Dean.

Invaders on the horizon

So which animals not yet in England pose the greatest risk to our ecology? A report published earlier this year has established the biggest potential threats to our shores from invasive species.

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High on the list is the raccoon, a North American mammal which has already established large populations in Germany and France.

A 2013 report by the International Union for Conservation of Nature said raccoons around Berlin were thought to be killing off amphibians and birds, including herons, by feasting on their eggs.

But the risk is not confined to animals - the report said the agile climbers had been "disturbing citizens" by living and breeding in lofts.

"This has caused a drop of property value in affected areas in the city of Kassel," it said.

Raccoons kept as pets in England "periodically escape or are released" and have the potential to "affect biodiversity through predation and disease transmission", Dr Helen Roy's paper said.

Last year, the BBC reported two escaped raccoons on the loose in Britain at the same time - one in East Sussex, one in Perthshire.

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The Asian hornet is a major predator of insects including honeybees, which could affect pollination, the report said.

Tim Lovett, of the British Beekeepers Association, said the insect was introduced to France by accident in 2004 and "seems to like the climatic conditions" here.

Likely to travel in shipping containers from Europe, once the insects are here they will infiltrate beehives to get at the honey.

Like our native wasp, the Asian hornet can use its sting again and again, giving it the edge over the honeybee which dies after it uses its defence.

Wasps, however, only infrequently attack beehives.

The Asian hornet looks very similar to the UK's native hornet but the last section of its body is yellow rather than black.

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Top of the list is an innocent-looking shellfish. The quagga mussel is named after an extinct zebra-type creature and shows no signs of disappearing.

Its close relative, the zebra mussel, has been here for 200 years and is beginning to cause havoc with England's water supplies.

Dr Helen Roy said that, after looking at hundreds of species from all over the world, the quagga mussel was the most likely to arrive and establish itself in Britain and pose a danger to biodversity here.

It's danger is due to its "very efficient" filtering of water to get the food it needs, which has an impact on other things that live in the water, such as tiny animals and plants.

"It changes the water that it lives in - it can make the water a lot clearer for instance.

The mussel, a native of the Ponto-Caspian region of eastern Europe, can be tackled by cleaning the hulls of boats, yachts and even kayaks and fishing equipment that have been in the area, she said.