Pool frog  (Rana lessonae)

The native form is currently extinct in the UK. This species is listed on Annex IV of the EC Habitats and Species Directive.

The pool frog was only recognised as a distinct species as recently as 1973. It is usually olive-brown above, with dark-brown blotches, and a prominent yellow line running down the centre of the back. Pool frogs from central and southern Europe are often green in colour. Pool frogs differ in shape from the common frog, having a more pointed snout, and they lack the dark patch behind the eye around the eardrum.

Pool frogs can be easily confused with two other species of frogs: the marsh frog and the edible frog (both introduced). They are best distinguished by a combination of characters, mainly geographic location, size, call, and to some extent coloration. Pool frogs have a loud call, somewhat similar to a duck quacking.

The pool frog is found across much of central Europe, as far north as Sweden and Norway (isolated populations only), and east to Russia. In the UK, they have only been recorded as possibly native from one site in Norfolk in recent years, with the last wild sighting in 1994. There has been much speculation as to whether the pool frog was native to the UK or an introduced species. Whilst there have been documented introductions in the past, recent evidence strongly indicates the species was in fact native to some areas. Evidence comes from genetic, vocalisation and fossil studies; taken together the information suggests that British pool frogs are most closely related to the Scandinavian pool frog populations. It was likely to have occurred in certain habitats in the east of England. Introduced pool frogs from central/southern Europe do occur at a few sites in England.

Pool frogs tend to prefer permanent, un-shaded ponds which can warm up quickly. The pool frogs at the last known possibly native site were found in pingos, which are a special type of pond formed by depressions in the ground as relics of the last ice age. The species seems to thrive where there are many ponds in close proximity. Pool frogs also spend part of the year away from the breeding pond, dispersing across grassland and woodland.

Pool frogs emerge from hibernation in spring, at a date depending on the weather conditions. They spend the first few weeks basking in the sun. Around mid-May, breeding begins, with the males croaking loudly by day and night. Females lay 2 or 3 clumps of spawn in the male chorusing area. The tadpoles feed on algae and detritus, and metamorphosis (when the tadpoles emerge as froglets) occurs from August-September. Pool frogs mature after 2-3 years. The adults feed on invertebrates, and take some flying insects by snapping at them as they fly past.

It is not fully understood exactly why the pool frog has disappeared from Britain, but it is likely that draining of wetlands, loss of suitable pools, successional changes to ponds, and growth of over-shading scrub have all contributed. 'Lucky', the last captive specimen from the Norfolk site, died in 1999.

The pool frog is a priority species under the UK Biodiversity Action Plan (UKBAP), and has been included in English Nature’s Species Recovery Programme (SRP). An action plan was produced following surveys that concluded that the frog was probably extinct in the wild, and evidence increasingly indicated that native status was possible.

If the case for the native status of the pool frog is confirmed, re-introduction of the species to carefully selected sites will be attempted, subject to the necessary permissions. It is most likely that frogs from Sweden would be used for such a reintroduction. Measures to rectify the factors that led to the decline of the species have begun, and would continue alongside the reintroduction efforts.

• Hibernation: a winter survival strategy characteristic of some mammals in which an animal's metabolic rate slows down and a state of deep sleep is attained. Whilst hibernating, animals survive on stored reserves of fat that they have accumulated in summer. In insects, the correct term for hibernation is 'diapause', a temporary pause in development and growth. Any stage of the lifecycle (eggs, larvae, pupae or adults) may enter diapause, which is typically associated with winter.
• Metamorphosis: an abrupt physical change from the larval to the adult form.