Preliminary investigation of a one-hundred-year-old population of introduced water frogs in Britain

We investigated the taxonomic composition and habitat selection of the longest surviving introduced water frog population in Britain. Water-frogs were first introduced to Beam Brook (Surrey, southeast England) around 1905. Using RAPD analysis of embryos and larvae, we identified three taxa (Rana esculenta, R. lessonae and R. ridibunda) and detected a fourth for which we had no RAPD reference material. We consider this was likely to be R. perezi, since male advertisement calls of this species as well as those of R. bergeri (closely related to R. lessonae) were recorded recently at Beam Brook. We have no evidence of adult male R. ridibunda at Beam Brook, and the embryos of this species were probably produced by matings of R. esculenta. The water frogs bred in a subset of the 44 ponds present at the site. There were no sharp distinctions among the different taxa with respect to ponds used for breeding. However, stepwise multiple regression analysis indicated that Rana lessonae reproduction was preferentially associated with the warmest ponds, R. esculenta spawn and larvae were most abundant in densely vegetated pools, and R. ridibunda was associated with relatively low oxygen concentrations. THE British Isles have an impoverished herpetofauna relative to mainland Europe (Gasc et al., 1997). This is largely due to relatively cool summer temperatures in Britain, but is also in part an accidental consequence of post-glacial colonisation processes. Biogeographical patterns in the recolonisation of Europe are discussed by Taberlet et al. (1998) and Hewitt, (1999). Many species spread north as the climate ameliorated during the postglacial warming, but there was only a limited window of time during which Britain could be colonised. Initially joined to mainland Europe by a land bridge, Britain finally became separated by sea around 7500-8000 years ago (Lambeck, (1995). Evidence suggests that at least nine species of amphibians reached Britain naturally before that time. Six species (Triturus helveticus, T vulgaris, T cristatus, Bufo be B. calamita and Rana temporaria) are still extant, while three (R. dalmatina, R. arvalis and R. lessonae) have subsequently become extinct (Gleed-Owen, (1998). A 'northern Glade' of R. lessonae, found also in Norway and Sweden (but morphologically distinct from pool frogs in mainland Europe), survived in England as a single isolated population until the 1990s (Zeisset & Beebee, 2001; Wycherley et al., 2002). By contrast there have been many introductions of amphibians into Britain, both accidental and deliberate, over the past 200 years (Lever, 1980). Water frogs have been among the most widespread of these introductions, with the first (probably mixed R. lessonae and R. esculenta) documented in 1837 (Smith, 1951). Most populations of these frogs became extinct within a few years, but there have been notable exceptions. The most successful amphibian invader in Britain is R. ridibunda, which was introduced to south-east England in 1935 and is now widespread in large wetland areas (Menzies, 1962; Beebee & Griffiths, 2000). However, the longest surviving introduced water frog population in Britain exists at Beam Brook in Number 84 Herpetological Bulletin [2003] 21 Water frogs in Britain Surrey. Frogs have been present at this site for almost 100 years (Gillett, 1988) and continue to thrive there. Repeated introductions, into artificial ponds created at a commercial nursery, occurred over many decades until the mid-1960s, when introductions ceased. Although the origins and species were not documented, the water frogs at Beam Brook were certainly of mixed taxa and were probably imported mainly from Belgium, France, the Netherlands, Italy and Spain. Since the late 1980s the frogs have extended their range in the Beam Brook area, coinciding with a general climatic warming (Beebee, 1995; Walther et al., 2002), and now occur over some 150 square kilometres (Wycherley & Anstis, 2001). The Beam Brook water-frogs have not been studied previously and are of interest for several reasons. Firstly, their taxonomic composition is unusual and, because of their mixed origins, may be more complex than can be found at any natural site in Europe. Analysis of male advertisement calls at Beam Brook has indicated the presence of R. bergeri, R. esculenta, R. lessonae and R. perezi (Wycherley et al., 2003). Because of their complex hybridogenetic reproduction mechanisms (Tunner, 1974; Graf & Pols-Pelaz, 1989; Roesli & Reyer, 2000), other forms — such as female R. ridibunda and cryptic hybrids — might also occur at the site. In addition, the spread of water frogs associated with climate change might pose threats to native communities and it will be important to evaluate the impact of this highly unusual water frog complex in future years. In this paper we report a preliminary investigation of the water frog community at Beam Brook and its association with pond habitat features.