The pool frog is a member of the 'green frog complex`. It often occurs together with other green frogs, notably the edible frog (Rana kl. esculenta), which is a hybrid of the pool frog and marsh frog (R. ridibunda). Pool frogs usually exist as metapopulations dependent on multiple ponds quite close together and interconnected by suitable terrestrial habitat. Pool frog tadpoles can overwinter but survival is often low. The species relies heavily on the best summers for recruitment of new juveniles.
The status of the pool frog in Britain is uncertain, but a series of factors indicate that it may be native and not simply a recent introduction. The possibly native populations of this species may now be extinct in the wild in Britain. At potentially the last remaining native UK site for the species, in Norfolk, there have been no confirmed sightings since 1993 and only a single male animal from that population exists in captivity. Pool frog is found across much of central Europe, as far north as south-west Sweden and Norway and as far east as Asia.
The pool frog is listed on Annex IV of the EC Habitats and Species Directive.
Current factors causing loss or decline
The reasons for the decline of this species are largely unknown, but several factors are suspected and these may now be providing constraints on its recovery. These factors include:
Reduction in the number and quality of suitable ponds in close proximity to each other. This can be caused by encroachment of scrub, presence of fish, atmospheric pollution (eg nitrogen pollution) leading to reduced breeding success, drought and/or lowered water tables.
Reduction in quality of terrestrial habitat due to succession to woodland.
Increased numbers of water fowl, notably geese (which will overgraze grasses, foul water and eat frogs and tadpoles).
Small populations are vulnerable to grass snake predation and collecting.
This species is the subject of a Species Recovery Programme project coordinated by English Nature, which involves work on a number of areas including:
Habitat management. Small areas around the ponds that yielded the last sightings are being opened up with a view to creating suitable habitat for re-establishment of the species.
Survey of the last known site in Norfolk and other possible sites (in Cambridgeshire and Northamptonshire).
Captive breeding. The one surviving `native` male has bred with pool frogs of different origins. Attempts to find other native origin animals have been initiated and a captive breeding programme using Swedish origin animals has begun.
Investigation of native status using a variety of approaches including bioacoustics, genetics, palaeoarchaeology and literature review.
Action plan objectives and targets
Recover the population at the Norfolk site (if not already extinct).
Restore viable populations to three suitable sites in the likely former range of the species through (re)introduction by 2003. This should only be undertaken if the native status of the pool frog is confirmed.
Proposed actions with lead agencies
Policy and legislation
If the native status of this species is confirmed, and if extant populations are found or reintroductions undertaken, then add the species to Schedule 5 of the Wildlife and Countryside Act 1981. (ACTION: DETR, JNCC)
Review legislative mechanisms for restricting release to, and movement in, the wild of pool frogs of non-native origin (eg through Wildlife and Countryside Act 1981 Section 9 or 14, and Schedule 9 review). (ACTION: NE, JNCC)
Determine the most suitable approach to reintroduction of the species in the light of studies on its native status. (ACTION: NE, JNCC)
Site safeguard and management
Ensure appropriate management at a core area of the Norfolk site to allow the persistence of any remaining animals and prepare for possible reintroduction. (ACTION: NE)
Review protected status and security of other sites with potential for reintroduction; assess need for consents, etc., and potential impacts on other features of conservation interest (Action: NE, site owners [sites to be identified by Pool Frog SAP SG])
Species management and protection
Continue to develop a captive breeding programme to maintain native origin animals (or at least to preserve some genetic material) and provide animals that could be used for release into the wild. This action should follow the proposals outlined in the Species Recovery Programme and, if necessary, captive breeding work should be extended to other suitable institutions. (ACTION: NE)
Following management to re-create suitable habitat, reintroduce the pool frog at three suitable former sites (subject to outcome of research on native status). (ACTION: NE)
Produce guidance on suitable management for pool frog habitats and distribute to landowners and managers at extant and restored sites, and sites where the species has been recorded in the recent past. (ACTION: NE)
Provide guidance on the management of non-native populations of pool frogs. This action should be informed by the work outlined under 5.5.4. (ACTION: NE)
Ensure that relevant agri-environment scheme Project Officers and members of regional agri-environment consultation groups are advised of actual or potential sites for this species, its importance and management needed for its conservation. (ACTION: NE)
Future Research and Monitoring
Continue with research to determine whether the pool frog is native to Britain. A number of lines of investigation should be pursued including genetic studies, literature research, archaeological study and archiving of reference bone material, study of calls, behaviour and morphology. (ACTION: NE)
Ensure the completion of the current initiative to survey all known former sites and to determine whether an extant population remains in Norfolk. (ACTION: NE)
Undertake a thorough survey of green frog populations with a view to identifying colonies of pool frogs that have so far gone undetected. (ACTION: NE)
Review studies of habitat requirements and management needs, looking particularly at effects of numbers and sizes of ponds, shading, fish and pollution. (ACTION: NE)
Undertake monitoring of water quality and water levels at the Norfolk site. This should be carried out with a view to maintaining suitable habitat. If necessary, action should be taken to curtail pollution and excessive water abstraction. (ACTION: EA, NE)
If non-native populations of the pool frog are discovered, assess the threat that they may pose to the genetic identity of any reintroduced pool frog colonies. (ACTION: NE, JNCC)
Undertake an assessment of the reasons for the decline (and possible extinction) of the pool frog in Britain and specifically at the last known ‘British’ site. (ACTION: NE/ Pool Frog SAP Steering Group)
Communications and Publicity
Continue to develop links with relevant European experts with a view to exchanging information on conservation management and ex-situ breeding techniques. (ACTION: NE)
Depending on the outcome of the studies on native status and subsequent conservation actions/decisions, publicise the presence and conservation importance of a 'new' British species. (ACTION: NE)
Links with other action plans
Originally published in: UK Biodiversity Group Tranche 2 Action Plans - Volume I: Vertebrates and vascular plants (June 1998, Tranche 2, Vol I, p115)
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