The native common frog belongs to a group of frogs often referred to interchangeably as either the ‘brown frogs’ or the ‘grass frogs’, broadly reflecting both their colouration and their predominantly terrestrial habits. By contrast there is another group of frogs referred to as the ‘green frogs’, or ‘water frogs’, likewise reflecting their characteristics.
There are about 25 species of green, or water, frogs all belonging to the genus Pelophylax distributed across the European mainland, Asia and North Africa. One species is native to the UK, although it is only found in a very restricted area of Norfolk and all the water frogs found in the UK outside this area, including in Somerset, are introduced aliens. Identifying individual species of water frogs is notoriously difficult as, not only do many species appear very similar to one another but they often occur in mixed populations and many of the species can hybridise with one another.
Several species of water frogs are known to have been introduced into various parts of Britain, most commonly the pool frog, P. lessonae and the marsh frog, P. ridibundus. However, a further frog, the edible frog P. esculentus, is a hybrid of these two species and is commonly found amongst them.
Generally we define a species by its inability to breed with another species to produce fertile offspring. The water frogs, however, are a rare exception to this rule. The hybrids are not only fertile but they possess a curious genetic twist that results in the destruction of an entire set of chromosomes when they produce their own sex cells, or gametes, in preparation to breed.
To illustrate the effects of this, consider the example of the hybrid edible frog which contains a set of chromosomes from both a marsh frog and a pool frog. If, as is most frequently the case, the pool frog chromosomes are destroyed during the production of gametes, then the resulting gametes will contain only marsh frog chromosomes; thus if this animal were to mate with a pool frog, further edible frogs would be produced. If, however, it were to breed with a marsh frog, genetically pure marsh frogs would result. If, on the other hand, the marsh frog chromosomes were destroyed during the process of gamete production then the resulting gametes would contain only pool frog chromosomes. In this case breeding with a marsh frog would produce further edible frogs while breeding with a pool frog would produce genetically pure pool frogs. In most situations edible frogs appear to prefer to breed with pool frogs, but there are populations where this is not an option and edible frogs will then breed with marsh frogs. Alternatively, edible frogs may breed with other edible frogs producing either pool frogs, marsh frogs or further edible frogs, depending on which sets of chromosomes were destroyed during the production of gametes in the parent edible frogs. To compound the situation further, edible frogs are occasionally produced that have an additional set of chromosomes and so can produce gametes containing various combinations of pool or marsh frog chromosomes.
Considering that comparable genetic complexity also exists among other species of the genus, it is not difficult to see why identification may be tricky and analysis of DNA is the most reliable method.
Two quite separate populations of water frogs exist in Somerset, one on the Avalon Marshes and another 30 kilometres to the north in the Timsbury and Paulton basins of the old Somerset coal canal in Paulton. DNA samples have been collected and analysed from both populations showing that the large population on the Avalon marshes is comprised of southern, or Iberian, marsh frogs, P. perezi, normally distributed across Southern France, Spain and Portugal. To date no evidence has been found of any hybrids amongst this population, although the Iberian marsh frog is known to be able to hybridise with the northern marsh frog, P. ridibundus, to produce the fertile hybrid known as Graf’s frog P. grafi. The apparent absence of this, or any other, hybrid on the Avalon Marshes suggests that only one species is present on the site.
The frogs in the old coal canal at Paulton have been identified as pool frogs, P. lessonae, but not those of the native race, which are restricted to Norfolk. The Paulton pool frogs are normally distributed throughout eastern France, eastwards through central Europe and northern Italy to western Russia and north as far as the Baltic.
Both of these introduced species are easily distinguishable from the native common frog.
Southern Marsh Frog
Marsh frogs are large, reaching up to 10cm. The usual ground colour is a shade of green, sometimes bright green, contrasting with the darker browns of the common frog. The two dorso-lateral folds tend to be brown and the back may either be uniform or patterned with darker brown or black spots or blotches. There is usually a pale green or white stripe reaching from the snout, along the top lip, to the shoulder while the lower lips, flanks and underside are white with an overlaid pattern of black marbling. Most conspicuously, they frequently have a clear green or yellow vertebral stripe running the length of the body. A similar vertebral stripe is also found on natterjack toads, a feature that may have influenced Bernard Storer to erroneously claim, in his book on Sedgemoor, that natterjacks exist on the Somerset levels.
Similar in colouration to marsh frogs but generally a smaller animal, rarely exceeding 7cm, with proportionately shorter back legs.
The behaviour of both these species differs greatly from that of the native frog. Unlike the native frog, water frogs are highly aquatic throughout the year, most often encountered either in the water or basking immediately adjacent to it, from where they leap with a loud plop back into the water when disturbed. They are also raucously vocal and often the first indication of the presence of water frogs is the loud cacophony of chorusing which can occur throughout the summer months, but usually peaks in May.
Spawn is produced in several small clumps hidden amongst submerged vegetation and the resulting tadpoles can grow very large, reaching up to 6cms immediately prior to metamorphosis.
No record exists of when or how either of these frogs arrived in Somerset. Anecdotal evidence regarding the marsh frogs on the Avalon Marshes is conflicting, some suggesting the frogs arrived in the 1960s while others suggest much later. What is clear however is that this species was well established on the Somerset levels by the 1990s. The pool frogs at Paulton probably arrived during the first decade of the 21st century.
Water frogs have long been popular in the pet trade, and it is perfectly possible that either of these populations may have originated from a deliberate release. Equally however, established populations of water frogs in some other parts of the country are believed to have been inadvertently introduced, probably as eggs or small tadpoles, during the translocation of fishes between fisheries. This appears to be the most probable explanation for the introduction of water frogs at Paulton, and a possible explanation for those on the levels, as several introductions of fishes occurred soon after the Avalon Marshes reserves were established.
To date there is no obvious evidence of any detrimental ecological impact from water frogs at either of the Somerset sites. They occupy a slightly different ecological niche than the common frog, reducing competition between the two species. Water frogs predominantly occupy, and breed in, large water bodies, such as the coal canal basins or the lakes created by peat diggings on the levels, where the mild toxins in their skin protect them from predation by fish, while the common frogs tend to breed in ditches and smaller water bodies.
In fact, on the Somerset levels it could be argued that the water frogs are making a positive ecological contribution. In their relatively restricted range within the Avalon Marshes of the Brue Valley, water frogs exist in high density, and the recent welcome increase in rare wildlife such as Bitterns, Great white egrets and otters in the area may, in part, be fuelled by water frogs, on which they will undoubtedly be feeding. Additionally, the water frogs probably contribute to the disproportionately large number of grass snakes in the area. While healthy populations of waders, otters and grass snakes are to be welcomed, the potential downside is that an increase in amphibian predators will increase predatory pressure on the native amphibians, particularly as those predators move away into other areas where water frogs do not exist. However there is little evidence of this so far and native amphibians are easy to find alongside the water frogs.
Southern marsh frogs in Somerset are predominantly confined to the Avalon Marshes in the Brue Valley, reaching their highest densities in the Ham Wall, Shapwick Heath and Canada Farm reserves. Despite a few scattered, and possibly erroneous, records from other parts of the levels, they have not dispersed significantly beyond this reserve complex and are not currently established either south of the Polden Hills or north of the Wedmore Ridge. Under normal conditions, however, this species is found in Southern France, Spain and Portugal.
Pool frogs in Somerset are confined to the Timsbury and Paulton basins of the old Somerset Coal Canal in Paulton. These basins are currently isolated from other water bodies, but on-going work to reconnect them to the canal network may provide opportunities for pool frogs to increase their range across the county.