The Frog With An Accent

The origins and native status of the pool frog (Pelophylax lessonae, formally Rana lessonae) had left British ecologists and herpetologists stumped for many years. For a long time, the species was considered an "alien" species imported from mainland Europe. Introductions of foreign close relatives of the pool frog further complicated the origins of the pool frog. However, in the late 20th century, the pool frog's native status was re-evaluated. Historical records of pool frog bones dating back to the mid-Saxon times indicated that pool frogs were present in England before any known introductions of the species. Additionally, research into their genetics and mating calls further solidified the native status of the pool frogs. Research found that pool frogs native to England have an infliction, an "accent" if you will, in their mating call, distinguishing them from other populations. Unfortunately for the pool frog, these discoveries were made too late and the last known population, at Thompson Common, became locally extinct in 1995 before the native status of the pool frog could be confirmed. But there is a happy ending to this story; after years of research and habitat restoration, a population of pool frogs was re-introduced to Norfolk from 2005 to 2008, making it one of England's rarest amphibian species.

A pool frog simply vibin' on some vegetation. Photo by: Jim Foster

Distribution & Habitat

The pool frog is confined to Norfolk and is very patchily distributed. Populations were reintroduced to two locations in Norfolk from 2005 to 2008. Found in damp densely vegetated areas, pool frogs prefer slow flowing waterways such as calm rivers, ponds or marshes, and fenland and breckland habitats. In 2015, the pool frog was re-introduced to Thompson Common, the last known location of pool frogs before they were locally extinct. Thompson Common has the perfect habitat for pool frogs, such as clusters of large, sunny ponds set in open woodland.

Identification

Colouration of the pool frog varies, however the pool frogs introduced to England are predominantly brown in colour with darker blotches. Pool frogs are further characterised by a light yellow or green dorsal stripe, ridges that run from the eyes down the back, and horizontal oval shaped pupils. Male pool frogs have a loud mating call that has been likened to that of a duck. When a male pool frog makes its mating call a pair of white inflatable pouches (vocal sacs) each side of the head are visible. Britain's other native frog species, the common frog, lack this feature. The white vocal sacs of the pool frogs distinguish it from its similar looking relatives, the marsh frog and the edible frog, which have grey vocal sacs. Pool frogs are a sizable frog, similar in size to common frogs, with male pool frogs being slightly smaller than females. Adult females can grow up to 9 cm in length.

A male pool frog sings for his lover. Photo by: Jim Foster

Diet

Adult pool frogs have a varied diet of invertebrates including flying insects, which they jump up to catch. Pool frog tadpoles eat algae, tiny invertebrates and decaying material in the pond. Tasty.

Reproduction & Lifecycle

The pool frog breeds later in the year than the common frog to coincide with the onset of warm nights in May/June. The spawn forms rafts that are smaller than those of the common frog. The eggs are brown on top and yellowish on the bottom. After five days the eggs hatch and the tadpoles begin to grow rapidly. In July and September, the tadpoles will metamorphose into froglets and emerge from the ponds. From October to April, pool frogs will hibernate. Adult pool frogs typically live from 3 to 6 years.

Predation & Threats

The pool frog is predated upon by grass snakes, otters, and birds such as herons, owls and some waterfowl. Pool frogspawn and tadpoles are at risk to predation by aquatic invertebrates, such as water beetles, dragonflies, and damselflies. During the 19th and 20th centuries, pool frog populations in the UK faced major declines due degradation of fenland and breckland habitats caused by drainage, agricultural intensification and water pollution. This led to pool frogs becoming locally extinct by the mid-1990s. Pool frogs have since been reintroduced into specially managed habitats to help the population of this species. Nevertheless, their small populations, although stable, remain vulnerable to severe meteorological events and amphibian diseases.

A pool frog enjoying the sunshine. Photo by: Jim Foster

Conservation Efforts

A reintroduction project was orchestrated by Natural England and Amphibian and Reptile Conservation Trust (ARC) to re-establish pool frog populations in England. Genetic analysis indicated that English pool frogs were closely related to Scandinavian populations. From 2005 to 2008, pool frogs were imported from Sweden and reintroduced to two sites in Norfolk, where habitat had been specially enhanced. This population has since established and expanded its range. In 2015, ARC Trust began reintroducing pool frogs to Thompson Common. Thompson Common is considered a Site of Special Scientific Interest (SSSI) and is managed by the Norfolk Wildlife Trust. Before reintroductions could begin, the previous reasons for extinction at this site needed to be addressed. Major efforts were made to address the water level and restoring the habitat to fit the needs of the pool frog, whilst benefiting other biodiversity. Frogs were taken from other sites in Norfolk and reared in captivity before being released at Thompson Common.

ZSL's Institute of Zoology assisted in the pool frog reintroduction programme, conducting a detailed Disease Risk Analysis in 2003 to 2005 to provide valuable information for the best practice of disease risk management for amphibian translocations. On a global scale, the Disease Risk Analysis conducted by ZSL provides an insight into which hazards pose the greatest risk to amphibians and how these risks can be mitigated for future amphibian conservation. Natural England also provided support during the pool frog reintroduction programme, ensuring the presence of biosecurity protocols during head-starting, and carrying out clinical examinations of native amphibians and pool frogs at the release sites. Natural England also performed post-mortems on any dead amphibians found at the sites to ensure disease risk management.

The pool frog is considered "Least Concern" on the IUCN Red List of Endangered Species and is fully protected under UK law. The Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017 make it an offense to kill, injure, capture or disturb the, and to damage or destroy pool frog breeding or resting habitat. Additionally, it is an offense to sell or trade pool frogs. Pool frogs are internationally listed on Appendix III of the Bern Convention and on Annex IV of the EU Natural Habitats Directive

A very handsome chappy sits on his leaf. Photo by: Jim Foster

How You Can Help

If you are passionate about pool frogs and conservation, and wish to get involved, you can volunteer with ARC, ARG and the Norfolk Wildlife Trust to assist with their conservation projects. Additionally, you can donate to ARC and sponsor a pool frog! Check out the links below to learn more:

Sponser a pool frog: https://www.arc-trust.org/Appeal/sponsor-pool-frogs

Volunteer with ARC: https://www.arc-trust.org/Listing/Category/volunteer

Volunteer with Norfolk Wildlife Trust: https://www.norfolkwildlifetrust.org.uk/support-us/volunteering/vacancies

Find your local ARG to volunteer with: https://www.arguk.org/get-involved/local-groups