Amphibians
Pool Frog
Pelophylax lessonae · also called pool frog, northern pool frog
A pool frog is a small, sun-loving green frog of European ponds — and the subject of one of conservation’s odder stories. For most of the last century Britain treated its pool frogs as foreign intruders and let the last colony die out, only to discover afterwards, from DNA and old bones, that they had been a native species all along. Britain then brought the frog back, from Sweden.
Overview
The pool frog, Pelophylax lessonae, is a green water frog of still, sunny, weedy pools across central and northern Europe. It hunts insects and other invertebrates at the water’s edge, males call together in a day-long crackling chorus, and it hibernates from about October to April in mud or soft ground.
The water frog puzzle
The pool frog is one of a strange trio. It hybridises with the marsh frog to make the edible frog, which is not a true species: edible frogs reproduce by hybridogenesis, discarding one parent’s genes each generation, so they can only persist where a pure pool frog or marsh frog is also breeding.
Lost and returned
Britain’s pool frogs were assumed non-native, given no protection, and the last Norfolk colony went extinct around 1995. Then genetic and subfossil evidence showed they were a distinct northern lineage that had lived in Britain since prehistory. Using the closest match — Swedish frogs — a reintroduction began at a secret Norfolk site in 2005, and the population established.
Status
Least Concern across Europe but declining at the edges, needing warm, sheltered, fast-warming pools to breed at the northern limit. In Britain it survives only at one or two protected Norfolk sites.
Pool Frog: common questions
What is the difference between a pool frog, a marsh frog, and an edible frog?
They are the three members of a peculiar European water frog complex. The pool frog and the marsh frog are true species, with the pool frog the smaller and the marsh frog the larger and more aquatic. The edible frog is the hybrid between them, and it is not a species in the normal sense. Edible frogs reproduce by a system called hybridogenesis, in which each generation throws away the genes it got from one parent species and must mate back with a pure pool frog or marsh frog to make the next generation of hybrids. So the edible frog cannot exist as a self-sustaining population without one of its parent species present.
Is the pool frog native to Britain?
Yes, though this was only accepted recently. For most of the 20th century the pool frog was treated as a non-native introduction and given no protection, and its last long-standing colony in Norfolk was allowed to fade out, becoming extinct in Britain around 1995. Afterwards, work on frog DNA, on subfossil bones from archaeological sites, and on old records showed that Britain's pool frogs were part of a distinct northern lineage, genetically close to Scandinavian frogs, that had colonised naturally after the last ice age. Britain had lost a native species by mistake.
How was the pool frog brought back?
Once the northern pool frog was recognised as a lost British native, a reintroduction was planned using the closest surviving stock. Because the extinct English frogs matched the northern group found in Sweden and Norway rather than the frogs of central Europe, pool frogs were collected from populations in Sweden and released at a carefully chosen, undisclosed site in the Norfolk fens from 2005 onwards. The frogs bred, and the population established, and a second reintroduction site has since been used. It remains one of the few deliberate reintroductions of a frog to a country it had gone extinct in.
Why does the pool frog need warm pools?
The water frogs are fundamentally a warm-climate group, and the northern pool frog lives close to the cold limit of what they can manage. It depends on small, sheltered, sun-trapping pools with plenty of vegetation that warm up fast in spring, giving the adults the heat they need to become active and breed and giving the tadpoles a long enough warm season to complete development before winter. The frog basks heavily to raise its body temperature. Shaded, deep, or cool pools, and cold late springs, all reduce breeding success, which is part of why the species is patchy and vulnerable at the northern edge of its range.