Jersey provides a natural habitat for a few species of amphibian but one of the most rare is the Rana dalmatina – or the agile frog. While the agile frog is widespread throughout Europe, Jersey is the only place in the British Isles where this long-legged species can be found. The agile frog is protected by the Wildlife (Jersey) Law 2021.
What does the agile frog look like?
The agile frog is a european brown frog, growing up to 90mm. The hind limbs are long, the snout is rather pointed, and the ear drum is large and close to the eye.
An adult agile frog’s diet is made up of earthworms, and small insects (such as flies and beetles) and their larvae. Tadpoles feed on aquatic plant and animal matter and detritus.
Wild agile frogs in Jersey have been recorded to spawn any time from the end of February to the beginning of April. Adults start to congregate at their traditional breeding site in February and March. The timing of breeding varies both between years and at different sites, depending on the temperature.
The number of eggs in a Jersey ball of spawn is significantly smaller than that recorded in Europe – 70 - 250 eggs as opposed to 450 - 1750 eggs. It is not clear why there is this difference in the number of eggs produced, although it may be because in Jersey the species is at the northernmost limit of its range.
Eggs are deposited in shallow pond margins (as little as 4cm deep), where the water is warmest, usually in the vegetative margins, attached to a twig or plant stem. The speed of development and time taken for tadpoles to hatch seems to be largely dependent on temperature – the time that the eggs take to hatch is significantly greater in cold water.
Tadpoles usually take two - four months to metamorphose into baby frogs, although they will sometimes overwinter. Again, water temperature plays an important role in determining how quickly the tadpoles develop. In Jersey it has been estimated that 2.3% of agile frog tadpoles survive to the froglet stage, which is comparable to the result of studies in Denmark (1.8 - 2.3%). However, these figures will vary immensely between seasons and ponds.
If they have a plentiful food supply, newly emergent froglets will grow quickly throughout their first summer and may reach sexual maturity in their second year. The minimum size of a mature frog is not yet known.
Its population in Jersey has been declining in both range and numbers since the early 1900s. By the 1970s it could be found at only 7 sites, and by the mid 1980s this had fallen to only 2 sites. In 1987, one of the remaining 2 breeding populations was lost as a result of a catastrophic spill of agricultural pesticide into a pond. The species is now believed to be confined to a single fragile population in the south-west of the island.
Reasons for the population decline
There are several factors causing the decline of the agile frog. These include:
- habitat loss due to development
- pollution of groundwater
- water shortages and the loss of ponds due to abstraction for domestic and agricultural purposes
- predator pressure from cats, birds and ferrets
The agile frog is not currently entered on the World Conservation Union’s Red Data List (IUCN, 2000). However, as a result of its desperate situation in Jersey, it is classified as locally Critically Endangered. The species is also thought to be declining throughout its relatively large european range, but further studies are required to determine how serious this trend is.
Amphibians are especially susceptible to environmental degradation, which they face both in water and on land. Their breathable skin makes them most sensitive to air and water borne toxins and climate change – they are an early warning system for the health of our environment. Unfortunately, all over the world, this warning is being largely ignored. The agile frog is among thousands of species whose populations are rapidly declining because of water pollution, acid rain, global warming, depletion of the ozone layer, disease and of course habitat loss.
Agile Frog Group
In an attempt to prevent the extinction of the agile frog in Jersey, the Agile Frog Group, a group effort of local environmental and conservation organisations and individuals, has launched a comprehensive action plan. This lays out the various threats faced by the species and details what action is needed to save it.
Agile Frog Species Action Plan