37 RUNNING HEAD

Trans. Suffolk Nat. Soc. 42 (2006)

GREEN/WATER FROGS IN SUFFOLK

JOHN BAKER AND ROSIE NORTON

Green, or water, frogs are a complex group of congenerics, which naturally occur throughout continental Europe. However, there has been a long history of introductions of these frogs into England – the earliest documented case occurring in 1837 (Smith, 1964). One of the most notable introductions was that of marsh frogs Rana ridibunda to Romney Marsh, Kent, in 1935. These frogs have thrived and become firmly established. There have been other introductions from a variety of sources, and more recently populations have been reported in many counties, but primarily in south-east England (Wycherley, 2003).

In Suffolk, Rope (1934) noted introductions of the edible frog Rana esculenta at three locations. The edible frog is a fertile hybrid within the green frog complex. In Western Europe it normally co-exists with the pool frog Rana lessonae (Beebee and Griffiths, 2000). In fact, releases of ‘edible’ frogs may have included pool frogs. Edible frogs were released at Brandon in about 1884, by G. E. Mason. Frogs from Normandy were released at Blaxhall in 1882 and 1892. A later release was carried out by E. H. Buxton at Snape in about 1930. None of these populations persisted. Buxton attributed the failure of these frogs to thrive at Snape to mating male common frogs killing still dormant female edible frogs early in the spring.

In the last review of Suffolk’s herpetofauna, Jones (1989) does not mention any green frog populations. However, since 1999 (Jim Foster, personal communication) there have been credible, or verified, reports of green frogs within a relatively small, rural, area within the Suffolk Plain, measuring approximately 3�75 × 1�25 km. At the request of one property owner, on whose land one of the green frog sites occurs, the locations are not being published. One of the sites is a cluster of four closely-spaced ponds, the other sites comprise single ponds. The ponds are not linked by a major watercourse, so the frogs must have crossed arable land, possibly using drainage ditches for part of their migration, to move between water bodies.

Green frogs can be difficult to identify, particularly if their origin is unknown. However, appearance (see Plate 4) and vocalisations suggest that these frogs are marsh frogs. The adults are large, much bigger than our native common frog. They have rough skins, typical of marsh frogs. The coloration is variable. Some frogs are dark brown, others are bright green at the head end, fading to brown or grey towards the rear. None of the frogs observed had vertebral stripes, as usually seen in edible frogs. The paired vocal sacs are dark grey in colour, as opposed to the paler sacs in other green frogs. The calls of a limited number of frogs have been identified as marsh frogs, by Julia Wycherley (pers. comm.) who has used green frog calls as a means of species identification (Wycherley, 2003).

Green frogs seem likely to spread further in Britain (Wycherley, 2003). Although the current Suffolk range is relatively small and well-defined, green frogs may appear elsewhere in the county in future. Green frogs may expand their range by natural dispersion, or they may, as elsewhere, be translocated by

Trans. Suffolk Nat. Soc. 42 (2006)

Suffolk Natural History, Vol. 42 38

humans (even though releasing such non-native species is illegal under the Wildlife and Countryside Act 1981). There are populations of green frogs in both Norfolk and Essex. In Essex marsh frogs are particularly well-established in coastal marshes, which could eventually lead to range expansion into Suffolk. They are present in the Colne estuary, below Colchester, and an individual frog was reported from near Manningtree in 2004.

The impact of marsh frogs on native fauna has not been investigated. However, marsh frogs are not only predators, but also prey. If established in coastal marshes in Suffolk, they would most probably become a food item for species such as grass snake, bittern, grey heron and little egret.

Green frogs are readily distinguishable from native common frogs Rana temporaria. They are more aquatic, and bask around the edges of water bodies. They breed later in the year, calling loudly towards the end of May and, sporadically, throughout the summer. Any further records of green frogs would be welcomed by Suffolk’s Amphibian and Reptile Recorder, Rosie Norton (Rosie@brundish.demon.co.uk).

Acknowledgements We would like to thank the landowners concerned for allowing access to their ponds. We are grateful to Dr. Julia Wycherley for analysis of frog calls and Jim Foster and Juliet Hawkins for information on site locations.

References Beebee, T. J. C. and Griffiths, R. A. (2000). Amphibians and Reptiles. A

Natural History of the British Herpetofauna. The New Naturalist Library, HarperCollins.

Jones, M. L. (1989). A survey of the reptilia and amphibia of Suffolk. Trans. Suffolk Nat. Soc. 24: 9–24.

Rope, E. J. (1934). The Reptiles of Suffolk. Trans. Suffolk Nat. Soc. 2: 209–224.

Smith, M. (1964). The British Amphibians and Reptiles (3rd Edition). The New Naturalist, Collins.

Wycherley, J. (2003). Water frogs in Britain. British Wildlife, April 2003, 14 (4): 260–269.

John Baker and Rosie Norton Suffolk Amphibian and Reptile Group

Plate 4: Marsh Frog, Rana ridibunda from Suffolk (p. 37).

J.

Ba

ker