If you like amphibians and non-avian reptiles, Britain is a crappy place to live: we have just three native lizard species, three snakes, three newts, two toads and two frogs. But do we have a few more: are various ‘neglected natives’ lurking in our midst?
This depauperate herpetofauna mostly owes itself to the fact that Britain was glaciated for most of the time that it was connected to the European mainland, and by the time conditions were more equable there was a window of just a few thousand years before (at about 7000 years ago) the English Channel formed. As a result of all this the only herps we have are those cold-tolerant ones that already inhabited northern Europe prior to the formation of the English Channel.
Within recent years it’s been discovered that more species than conventionally thought were able to colonise Britain before the cut-off date. Two additional frogs, the Agile frog Rana dalmatina and Moor frog R. arvalis, are now known to have inhabited England until as recently as the 1st century AD and apparently became extinct thanks to anthropogenic habitat change (Gleed-Owen 2000). A colony of Pool frogs R. lessonae from Norfolk – traditionally dismissed as an introduction from Italy – are now generally accepted as having been native (Beebee & Griffiths 2000, Buckley & Foster 2005, Snell et al. 2005), though this was only appreciated in 1999, the year in which the last individual in the colony died. Archaeological samples show that several additional species, including the European pond terrapin Emys orbicularis and Aesculapian snake Elaphe longissima, also lived in Britain during the recent geological past (Gleed-Owen 1999) [adjacent Pool frog image from here. Common tree frog image at top from here].
You might know that Britain is full of aliens. We have, or have had, deer from China, ducks from the Americas and elsewhere, rodents from South America, wallabies from Australia, and all manner of other bizarre introductions [the standard reference work on introduced British animals is Christopher Lever's The Naturalized Animals of the British Isles (Lever 1977)]. And among herps, we have a pretty impressive list: Marsh frog R. ridibunda (and its hybridogenic hybrid with the Pool frog, the Edible frog R. kl. esculenta), Midwife toad Alytes obstetricans, African clawed toad Xenopus laevis, Alpine newt Mesotriton alpestris, Italian crested newt Triturus carnifex and Wall lizard Podarcis muralis are all well established and apparently here to stay (Beebee & Griffiths 2000). Multiple other species are also present but less well established.
Given what happened with the Norfolk Pool frog colony, it is now worth asking, however, whether all of these aliens really are aliens. Might some of them actually be overlooked natives? European pond terrapins and Aesculapian snakes both live wild in Britain today and are regarded as aliens, but given the fact that they once occurred here naturally it’s worth considering that they might really be neglected natives. In July 2002 herpetologist Chris Gleed-Owen of the Herpetological Conservation Trust was on his way to work when he accidentally discovered a colony of Western green lizards Lacerta bilineata at Southbourne and Boscombe Cliffs, Bournemouth (Gleed-Owen 2004). These animals, which are breeding and expanding their range along the coastline, are most likely introductions, but their presence has resulted in widespread speculation about the possibility of native status [adjacent Xenopus image from here. Yes, we have Xenopus].
Britain also has the Common tree frog Hyla arborea and, like the Pool frogs, these have generally been regarded as introduced aliens. We know without doubt that some colonies, such as those in Devon, London and the Isle of Wight, really were introduced, but this is not so certain for a breeding colony that existed at Beaulieu in the New Forest, Hampshire. No record exists of when and how this colony was introduced*, if indeed it was, and in fact a remarkably good case has recently been made for native status for these frogs. This idea that this charismatic little green frog might be a British native is pretty exciting to those of us living in such a herp-poor country, and in the interests of promoting the idea I’m going to discuss it further here.
* There is a FOAF tale (= Friend Of A Friend) ‘explaining’ how a Mr Turner Turner, a rich casino gambler, brought the frogs back from either Africa or Monte Carlo (Snell 2006). Of course Common tree frogs don’t occur in either Africa or Monte Carlo.
The main champion of the possible native status of the Common tree frog is Charles Snell, a freelance naturalist/photographer and researcher based at Greenwich University. Note that he was one of the first researchers to suggest that the Norfolk Pool frogs were neglected natives (Snell 1994), a contention since supported by morphological, genetic and archaeological evidence (Beebee & Griffiths 2000, Buckley & Foster 2005, Snell et al. 2005). Several British herp species were only recognized as members of the British fauna comparatively recently. The Sand lizard Lacerta agilis wasn’t officially noted until 1804, the Natterjack toad Bufo calamita not until 1835 and the Smooth snake Coronella austriaca not until 1859. In view of this it’s remarkable that Common tree frogs were recorded as part of the British fauna as early as 1646. Sir Thomas Browne (1605-1682), who wrote about British tree frogs in his 1646 text, implied that tree frogs were familiar and common-place, and other 17th century writers described the tree frog as a native British animal (Snell 2006). We also know from fossils that tree frogs were present in England during the Pleistocene (Gleed-Owen 1999) [image above from here].
Snell (2006) also noted that, like definite British natives, Common tree frogs occur as far north as southern Sweden, and – as a rough rule of thumb – those herp species that occur in Scandinavia also occur in Britain. Surprisingly perhaps, it turns out that Common tree frogs are extremely cold-tolerant and can withstand freezing temperatures down to -10 degrees C (the frogs actually survive being frozen solid). They are also highly resistant to dehydration.
The case for possible native status for the Common tree frog is therefore pretty good; the species was present in England during the Pleistocene, was apparently present during the 1600s at least, is cold-tolerant and has a continental distribution similar to that of other definite British natives, and – in the case of the New Forest colony at least – lacks a definite ‘trail of introduction’. Like the Common tree frogs of France and Sweden, the New Forest tree frogs did not inhabit closed-canopy woodland, but were instead denizens of open scrub dominated by bramble and gorse. In fact the Common tree frogs of northern Europe seem not to be woodland animals (Snell 2006).
If the Common tree frog was native to Britain – and thus able to survive in the British climate – why have those colonies known, without doubt, to have been introduced not persisted and bred as did the New Forest colony? Snell (2006) made several suggestions. Firstly, some introduced tree frog populations seemed not to consist of the cold-tolerant Common tree frog, but instead of the closely related, yet highly cold-sensitive Italian tree frog H. intermedia and/or Stripeless tree frog H. meridionalis. Secondly, some introduced colonies may have consisted only of males, as these are far easier to find and catch than females because of their loud calling. It’s not exactly surprising that these colonies failed to persist. Thirdly, introductions of continental Common tree frog to England may have failed as the animals were released into woodland environments, and not into the scrublands that the species in fact seems to favour [adjacent image from here].
At present Snell’s idea for native status is a hypothesis and requires confirmation. Part of the reason for concluding that the Norfolk Pool frog colony was native came from DNA work showing that the Norfolk animals were not like the pool frogs of southern Europe, but instead closely related to Swedish populations. A similar result for English tree frogs would also help confirm their native status but, unfortunately, work like this cannot be carried out as there are no soft tissue specimens.
The end to this story is a sad one. Assumed to be introduced, the New Forest tree frogs have never been awarded any sort of protection, and both habitat degradation caused by an introduced plant from New Zealand, and capture of the frogs by interested individuals, resulted in their decline. Tadpoles were last seen at the site in 1975 and adults were still being seen and/or heard in 1986, with one adult reported away from the main site in 1988. I visited the pond twice, once in the late 1980s and again in the mid 1990s, but never saw any frogs there, and Snell (2006) reports that no frogs have been seen in the area since 1988. It therefore appears that the colony has become extinct. Given that unnamed amateur naturalists were in the habit of capturing these frogs (and presumably keeping them in captivity), the remote chance remains that individuals survive somewhere in captivity, but I’m not holding my breath. History has repeated itself, as native status for the animals was never taken seriously until it was too late, exactly as occurred with the Norfolk Pool frogs. For a country with such a low diversity of amphibian species, this potential loss is a big deal.
This post has been promised for a long long time. See, for example, The deer-pig, the Raksasa, the only living anthracothere… welcome to the world of babirusas. For previous blog posts of mine on the British herpetofauna see In quest of anguids and Hunting Green lizards in Dorset: new aliens or old natives?
Tomorrow I’m attending a lecture by Mongolian palaeontologist Altangerel Perle and hope to talk with him about theropods. Stay tuned.
Refs – -
Beebee, T. & Griffiths, R. 2000. Amphibians and Reptiles. HarperCollins, London.
Buckley, J. & Foster, J. 2005. Reintroduction strategy for the pool frog Rana lessonae in England. English Nature Research Reports 642, 1-53.
Gleed-Owen, C. P. 1999. The palaeoclimatic and biostratigraphic significance of herpetofaunal remains from the British Quaternary. In Andrews, P. & Banham, P. (eds) Late Cenozoic Environments and Hominid Evolution: a Tribute to Bill Bishop. Geological Society (London), pp. 201-215.
- . 2000. Subfossil records of Rana cf. lessonae, Rana arvalis and Rana cf. dalmatina from Middle Saxon (c. 600-950 AD) deposits in eastern England: evidence for native status. Amphibia-Reptilia 21, 57-65.
- . 2004. Green lizards and Wall lizards on Bournemouth cliffs. Herpetological Bulletin 88, 3-7.
Lever, C. 1977. The Naturalized Animals of the British Isles. Hutchinson & Co, London.
Snell, C. 1994. The pool frog: a neglected native? British Wildlife 5 (1), 1-4.
- . 2006. Status of the Common tree frog in Britain. British Wildlife 17 (3), 153-160.
- ., Tetteh, J. & Evans, I. H. 2005. Phylogeography of the Pool frog (Rana lessonae Camerano) in Europe: evidence for native status in Great Britain and for an unusual postglacial colonization route. Biological Journal of the Linnean Society 85, 41-51.