Britain's lost tree frogs: sigh, not another 'neglected native'


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.

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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.

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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.


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Very interesting post, Darren (not that that is anything new).

Treefrogs going extinct, sadly, isn't limited to the UK. Over here, in the Netherlands, they seem to be gradually disappearing as well. Though, of course, there might be more females there than that there are males and as you said, females are much more difficult to locate. One can only hope.

A minor correction to your post, though. Presently the Alpine Newt is placed in the monotypic genus Mesotriton, rather then Triturus. Likewise, the smaller newts formerly in Triturus are now in Lessotriton. Only Triturus cristatus, carnifex, dobrochicus,marmoratus and pygmaeus remain in Triturus.

Has it occurred to you that "depauperate herpetofauna" would be a good name for a rock band?

> ... wallabies from Australia ...

Eh? Do we have wild wallabies in Britain? Where?

> Yes, we have Xenopus

Good thing you didn't finish that sentence.

By Mike Taylor (not verified) on 20 Feb 2007 #permalink

As a native of the U.S. West Coast I'm first struck that the basic color pattern for tree frogs must be quite conservative.

Our West Coast frog, the Pacific tree frog Pseudacris (=Hyla) regilla, comes in two morphs, one all green, like its British cousin, and the other a mottled brown. You may have never seen the Pacific tree frog, but you've probably heard them. When Hollywood needs some frog sounds for a night scene they use sound obtained locally. I've come across ponds and marshes where there are hundreds of these guys singing at once. When they're really thick, one or two cryptically colored frogs, invisible until they move, will scatter with every step.

Our West Coast frog, the Pacific tree frog Pseudacris (=Hyla) regilla, comes in two morphs, one all green, like its British cousin, and the other a mottled brown.

One in 1000 (or whatever) male Hyla arborea is bright blue instead of bright green, making it one of the two European blue frog species.

By David Marjanović (not verified) on 22 Feb 2007 #permalink

I sympathize... here in Michigan even the warmest parts of the state have occasional winter lows of -25°C, yet we have 12 native frogs and toads (in 3 families), 10 salamanders and newts (5 families), 2 lizards (2 families), 10 turtles (4 families) and 18 snakes (2 families), for a total of at least 52 native amphibians and reptiles. Only one additional species of salamander is considered an introduction, though most of our populations of red-eared sliders are also introduced. Considering Michigan was also fully glaciated, it's impressive what a difference a lack of major barriers to recolonization makes. I assume Britain's mild summer weather didn't help, either.

You think YOU have it bad in the UK?? Here in Ireland, we have three species of native amphibian (Smooth Newt, Common Frog and Natterjack Toad, though questions have even been raised about whether or not Common Frog was introduced), ONE species of native lizard (Vipivarious), one probably introduced lizard (Slow-worm, though, given the fact that a few butterfly species common in the UK are also confined to the Burren in Ireland, perhaps these are actually natives?), no snakes, one regularly visiting marine turtle (Leatherback) and a few vagrant ones...and THAT'S THAT!

By Harry Hussey (not verified) on 23 Feb 2007 #permalink

I gave information to Christopher Lever for his chapter on Tree Frogs in his book as well as Charles Snell for his paper. I had always thought that the story of the original animals being brought back from N.Africa or Monte Carlo was untrue. I would have thought that the animals would have been purchased from the pet trade, which if you look at G.Bateman 1897 The Vivarium Upcott Gill, was large and varied in this country at the turn of the century, Hyla arborea costing 6d each. I did speak to the landlord of the pub next to the Tree Frog pond in the 1970's and he told me that as a child he was paid money to catch the Tree Frogs from the present pond and return them to a pond further down the hill towards Bealieu and in a cooler wooded area where the owner and person who released them originally, presumably the Turner-Turner mentioned by C.Snell at Monks well (Abbey Springs)lived. I did see several specimens of adult frog plus tadpoles in the early 1970's and also tape recorded the calling male frogs one evening when I camped next to the pond one evening. Every time I visited the pond at a weekend there were local children present with nets looking for newts, all 3 species present, who knew other children who had taken the odd tree frog,caught at the pond, to school

David Bird

By David Bird (not verified) on 22 Mar 2007 #permalink

please help, i found 6 frogs in my back yard and we put them in a terrarium 1 of them got out i can't find it we feed them too where might the frohg be pleases help i love my frogs

Hey Darren, you actually missed to mention Anguis fragilis in your list of Britains herpetofauna.
But I really understand your frustration about the local herpetofauna. Germany is really not much better situated, there are just a handfull more species of amphibians and reptiles. At least we have a few nice species like one of my favourite Tetrapods Salamandra salamandra and several years ago I was happy to see a specimen of Emys orbicularis in the wild.
When I was many years ago in Greece I was really amazed of their rich reptile-fauna. I saw several harduns and skinks, a chameleon, many lizards, snakes and even a dead Sheltopusik.