Pool frog released at Thompson Common this week, photo copyright Jacob King, PA

Historic reintroduction reverses extinction of England’s rarest frog

Tuesday 27 July, 2021

The northern pool frog, England’s rarest amphibian, has been successfully reintroduced to Thompson Common in Norfolk – reversing its disappearance from there in the 1990s. Thompson Common was the last site at which this species occurred prior to its extinction from England. 

Staff from Amphibian and Reptile Conservation (ARC) released more than 300 tadpoles into the reserve's pingos, post-glacial pools which offer the perfect habitat for the creatures. The tadpoles had been reared in captivity, away from predators, to increase the numbers of young frogs that survive through the tadpole stage.

This month’s release saw the final phase in a process which began in 2015 to re-establish a population of northern pool frogs at their Norfolk home. It is the fourth since 2015, bringing the total to more than 1,000 tadpoles released. This year’s release completes the reintroduction process. ARC’s experience of species reintroductions is that three or four years of releasing animals helps to replicate a natural population structure with a varied range of ages.

The pool frog Pelophylax lessonae is assessed as critically endangered in England. The rare species went extinct in Britain in the mid-1990s, following the deterioration of its wetland habitat in the Fenland and Breckland areas of East Anglia. Its recovery has been made possible due to carefully planned, innovative conservation action by ARC and partners, including Norfolk Wildlife Trust, Natural England and Forestry England, and with support from Anglian Water, National Lottery Heritage Fund, Anglian Water Flourishing Environment Fund, Amphibian Ark the Keith Ewart Charitable Fund and the British Herpetological Society.

ARC conservation director, Jim Foster, said: "It is not often that you can say that you brought an animal back from extinction, but that is exactly what we have achieved with our partners and with funding from the government’s Green Recovery Challenge Fund.

"It is only because of the dedicated work here over the last few years that we have been able to bring it back. The fantastic thing about Thompson Common is the sheer number and quality of the ponds here. Some of these glacial relic ponds, known as pingos, form ideal conditions for pool frogs. They are very open to the sun and they hold water all through the year, which is really important for the sun loving pool frog.”

Following the northern pool frog's demise, ARC and partners initially brought it back to a secret location in Norfolk in 2005, using frogs sourced from Sweden. The next step, to return them to Thompson Common, began in 2015, using tadpoles taken as spawn produced by the population which became established at the secret location.

The pool frog has not always been recognised as a native British species. Naturalists have long been aware of unusual frogs at Thompson Common and nearby. Within a few years, pool frogs jumped from being a non-native curiosity, to Britain’s rarest amphibian and a conservation priority, to extinction. Although they were present within living memory, by the late 1990s, only a few years before recognition of native status, pool frogs had disappeared from Thompson Common, their final home. Research has shown that the native UK pool frogs are closely related to Scandinavian frogs, and have been present before reintroductions began and formed part of a distinct norther clade. 

“The northern pool frog is very different from the common frog, which is the other native species of frog that we have in Britain. They look and behave differently – and they call loudly.  Also, they lay fewer eggs, so they're much more vulnerable. A common frog can produce around 2,000 eggs, whereas a pool frog produces only about 500 or so.

Early indications at Thompson Common are good and we’re confident that the frogs will form a self-sustaining population. The new population has begun moving around different areas of the common and has several hundred ponds to choose from,” said project manager, John Baker.

Jon Preston, conservation manager for Norfolk Wildlife Trust, which manages the reserve, said: "This is such an iconic species that fits in so well with the landscape we have got here. We are starting to see the sustainability of the population, they are breeding on site and we are seeing them spread further out from the release pools. It means the partnership, the head-starting and all the management work we do on site is all working."

Working with a range of partners, until March 2022, ARC aims to continue to restore pool frog breeding ponds, train volunteers to survey for pool frogs, undertake specialist monitoring, bring pool frogs to a wider audience through commissioned videos, and plan future pool frog reintroductions. 

This project is funded by the Government's Green Recovery Challenge Fund. The fund was developed by Defra and its Arm's-Length Bodies. It is being delivered by The National Lottery Heritage Fund in partnership with Natural England, the Environment Agency and Forestry Commission.
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