RAUK - Archived Forum - Posts by John A Burton:

I am one of those who are highly sceptical about the native status of the pool frog in Britain.

One area I am seeking clarification is the evidence for native status of the species in Norway, Sweden and Denmark. Together with any archeo/palaeontological evidence in those countries. 

The arguments put foward so far for native status, as summarised in the English Nature bibliography (report 480) are far from conclusive, and  seem to be based on a tiny number of bones (notoriously difficult to identify), and the fact that there are genetic similarities betwen British and Scaninavian populations. If the latter are non-native, where does it leave the English populations?

I am aware of the work quoted by Charles Snell, but none of it addresses some of the key issues. One is what is the origin of the numerous esculenta populations in Sweden? Another is why is it that lessonae does not occur in Denmark or on Bornholm, but apparently does occur in Sweden and Norway?  My original question is also unresolved, viz.: what is the archeological evidence for green frogs in Sweden? I am having difficulty trying to find anything even pre 1940.

The problem with some of the conclusions based purely on genetics (such as an ancient origin for Swedish populations), is that they fly in the face of common sense zoogeography. If we accept the genetic evidence at face value, then the zoogographical explantions need to be overhauled. We can't have it both ways.

This is very useful indeed, and certainly helps. Is there any published references for all this data?

I am still very confused though. If there are populations of esculenta in Swecden, and esculenta is a hybrid, what is it a hybrid between, if the only other species present is lessonae.

And why are there no lessonae in Denmark? 

Do I understand from this that there are no Rana remains from archeological sites in Sweden, or just no green frog remains? One of the problems in the UK seems to be that although thousands of frog remains are known from archeological sites, only a handful are of alleged green frogs.

Incidentally, although there is a supposition that the English Green frogs are derived from the Scaninavian populations (based on genetic evidence), there is no empirical evidence to support this theory, to my knowledge. All that the genetic evidence shows, is that the two populations may be closely related. As yet nothing conclusive has been produced that would disprove that the Scandinavian frogs were derived from the English populations.

If we postulate colonisation of England from Scandinavia we need to look at other species that share similar ranges and habitats, and explain why so few others, if any took this route.  To my knowledge, one of the few vertebrate species that has been shown to have genetic similarities to Scandinavia populations is Apodemus sylvaticus, and this is generally assumed to have been a Medieval Norse introduction in the Scottish islands.

One way of backing up the idea of an East-west colonisation would be to find other species that might share habitats and see if they show genetic similarities.  Rana temporaria, Emys, Castor, and Arvicola are all obvious candidates. While Vipera and Natrix are also possibilities. Meanwhile it might also be productive to identify the continental European populations that have the closest genetic profiles to those of the northern Clade.

The problem at present is that all the evidence that has been proposed in support of Rana lessonae being native, has an alternative, and equally plausible explanation. None of the evidence on its own is conclusive, and even taken together, it does not add up to anything like enough to justify introducing Swedish animals into England, if  internationally accepted guidelines on reintroductions are adopted.

Many thanks for this. Yes your clarification is quite right. The Scandinavian and English Pool frogs are derived from a common ancestor. But this fact  does not make either population native to either or both countries. In fact, I would argue strongly that, for zoogeographical reasons, it supports the hypothesis that the species postulated distribution (together with that of ridibunda and kl.esculenta)  in England and Scandinavia is the result of human introductions. Unless, as I suggested in my earlier posting, some of the other species found in England and Scandinavia have the same genetic linkages.

The issue that remains though, is that all the specimens of English Pool frogs are all post-introduction material. Since green frogs are known to hybridise freely, there can be little doubt that any frogs collected by Thurnall et al are more than likely to have been involved in some sort of hybridisation, (assuming there were any pool frogs present prior to the releases) -- there were after all several thousand released, plus spawn, close to where the frogs were collected.

So then the question is: If the Swedish frogs show genetic similarities to the English frogs, and the English frogs are almost certainly 'contaminated' with introduced genes, how do we explain that?

Or if, as Boulenger believed, the specimens collected by Thurnall et al were all decended from the known introductions, how are the Swedish populations explained? Are they all the result of introductions from a common source? If so where was that source? St Omer? Paris? northern Italy, Vienna? All four are possibilities, with documentary support, that seems to have been largely overlooked. Zeisset & Beebee postulate an ancient origin from northern Germany, but this raises almost as many questions as it answers viz. the distribution of other green frogs in the region, as well as the lack of historical data.

This may help clarify, but I am not clear about these 'old records'. Where are they summarised? Did Linnaeus or any of his pupils record them. I have not  located any of the primary sources. Could these be posted (in orginal Swedish if possible)?

Many thanks

I am not  sure I quite follow the argument over the genetics of hybrid pool/esculenta. I presume Charles Snell is  saying  that only pure edible or pure pool can ever exist? If that is the case, then clearly no other genetic material will 'contaminate' populations - but this seems improbable to me -- but as I stated, I do not fully understand the genetics so will take that on trust.

Thank you Janne for clarifying the historical records. What is still curious, is that the dates for the first observations of green frogs in Sweden were remarkably close to those of the first English frogs. I still have the question, as to why Linne and his pupils did not observe them? Linne knew the species, and his numerous pupils included astute observers. Linne records Rana temporaria as in Sweden, but not esculenta (vide: Fauna Svec.).

Charles Snell's comments on Boulenger are not really very relevant, as they refer to a paper published in 1884. I think his Tailless Batrachians , 14 years later, gives his views more clearly.

The suggestion by Charles Snell that Berney imported only amounted to about 1500 individual frogs is quite erroneous. The original account, I think, is that published in the Zoologist p6539 (1859). This mentions '200 frogs and  a great quantity of spawn' in 1837, followed by 'another lot' in 1841, and 1300 in 1842.  And as any competent herpetologist knows, spawn is a very good way of colonising a pond. Alfred Newton also wrote in his article 'upwards of 1500, besides spawn, had been imported.. by him [Berney] alone -- implying that others were imported. In fact, on the  balance of probabilities,  it is very  unlikely that Berney was the only person importing frogs -- since there is evidence of Buckland at least, releasing some. And when one realises the size and scope of the trade in frogs in the 19th century [huge numbers were involved], it is very probable that plenty more were introduced. Just as in the 1950s, irresponsible (or rather ignorant) schoolboys such as myself introduced and released exotic species, such as green frogs.

Green frogs are known to be translocated extensively by humans, often in very large numbers. And this does not have to be for purely economic reasons. I have references to pet-keeping in Paleolithic cultures, and throughout history, a huge range of animals has been moved for a very wide variety of reasons. Children [and adults] have been taking jars or even buckets, of  frog spawn all over the place, and probably at no time more than in the 19^th century, when natural history studies were the height of fashion. A quick look at any of the numerous books designed for children shows just that. 

As an aside, if the origins of the English Marsh frogs were not documented in detail, the genetic evidence would now be postulating a common origin of Hungarian and English frogs -- probably somewhere in northern Germany, close to the postulated ancestral home of the northern clade pool frogs. This deonstrates the problems of relying on such circumstantial evidence.

I would summarise the information available as follows:

• Green frogs are known from archaeological sites in England. These sites are mostly [all?] associated with humans. The identification of green frogs to species level remains controversial.
• Specimens of lessonae from England have been shown to have genetic similarities with lessonae from Sweden; all the specimens used for the comparison were collected after known extensive introductions in the UK, and are recent material from Sweden.
• No specimens or reliable observations of green frogs are known from the 18th century from either Sweden or England
•  The present distribution of lessonae, ridibunda and esculenta, in the Baltic region, in common with many other parts of Europe, shows indications of being modified as a result of human translocations.

Conclusion:

The obvious and rational conclusion, based on the available evidence available at present, is that the Swedish and English populations both derive from human introductions. Many zoologists underestimate the impact of human introductions on the present day distribution of animals, but it is increasingly apparent that the anomalous distributions of many species are most readily explained by human introductions. None of the empirical evidence published so far would refute this hypothesis. In fact, I believe it supports it.