In 2005, at a top secret site somewhere in Norfolk, conservationists released the first small group of northern pool frogs in the hope of re-establishing the species in Britain, after it became locally extinct in the 1990s.
Groups of the distinctive striped frogs were flown in from Sweden until 2008, when they numbered 30, and were closely monitored at “Site X”, which has been kept confidential to prevent any interference.
Before it became extinct in Britain, apparently due to declining water levels, the frog had last been recorded at Norfolk Wildlife Trust’s Thompson Common, in the Fens.
Now, spawn from the growing population at Site X has been used to establish another population at Thompson Common, making it the second population of the frogs in England and a return to the site where they were last known to occur.
Jim Foster, conservation director at Amphibian and Reptile Conservation, said more than 1,000 tadpoles, reared in captivity from spawn, have been released at Thompson Common over the past four years and the project was completed this month.
He said the landscape at Thompson Common has been improved, with ponds holding water for longer and trees removed from some areas as “pool frogs are real sun lovers”.
"That means we can reverse extinction, and it’s not often you can say that for an animal," he said.
"After the species went extinct in England in the 1990s, a load of research was done into why they’d disappeared and what we could do to get them back.
"They should be sustainable on their own, that’s our aim," he said, but added that further top up releases would be considered if the population "needs a helping hand".
He said the frogs have moved "under their own steam" from their private release site "out into areas where the public can freely come and go, and now you can see pool frogs in public parts of Thompson Common".
The northern pool frog is "very different from the common frog, which is the other native species of frog that we have in Britain", said Mr Foster.
"They superficially look quite similar but if you look a bit closer, pool frogs always have this bright green or yellow stripe down the back.
"The males have a very loud mating call.
"Adult pool frogs are quite acrobatic.
"They leap out of the water on a sunny day and take dragonflies and damselflies on the wing, and it’s really impressive to see.
"Pool frogs are quite different from common frogs also in terms of their ecology.
"They lay fewer eggs, so they’re much more vulnerable.
"A common frog would lay around 2,000 eggs, each female, whereas a pool frog would only lay up to about 500 eggs."
He said the animals, which live for between three and five years, have "several hundred ponds... to choose from" at NWT Thompson Common.
Norfolk Wildlife Trust said tadpoles are exposed to high rates of predation in the wild and rearing northern pool frog tadpoles within a protected environment results in many more young frogs than would occur naturally.
Additional reporting by PA.
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