Introduction

           Herpetofauna, i.e. reptiles and amphibians, as ectotherms, are understandably more sensitive to changes in temperature because they require external heat for their metabolisms to function. Turtles and crocodilians are additionally susceptible as the sex of an individual is determined by the temperature at which eggs are incubated. While temperate-zone Europe still hosts a respectable number of reptiles and amphibian taxa, several factors have, in combination, subtracted from this diversity over the past three million years. When the Pliocene ended and the cycle of Ice Ages that characterized the Pleistocene began, Northern Europe’s biodiversity was hit much harder than that of comparable climates in Eastern Asia and Eastern North America. This is because, while in the latter two continents taxa were merely pushed to southern refugia from which they could later recolonize, this was prevented in Europe by the presence of the Mediterranean Sea. As a result, taxa that could not survive in the (still rather cold) glacial refugia available in Southern Europe, would either need to make the longer journey southeast to West Asia or, in many cases, go extinct. Reptiles and amphibians, being more vulnerable to cooling temperatures and less able to migrate than birds or mammals, would have been affected particularly hard. Every time the glaciers retreated, the reptiles and amphibians that remained would recolonize the rest of Europe from refugia in Iberia, the Balkans, or elsewhere, only to again die out when the glaciers returned. Most of the herpetofauna from these periods represent taxa which still exist in Europe, often the same species going right back to the Pliocene, and so their past patterns of distribution, as revealed by the historical and palaeontological record, can tell us where some of these taxa might be able to live in the future, as Northern European environments grow warmer. Amphibians and reptiles, while often inconspicuous, are nevertheless vital ecosystem components, providing prey for many birds, mammals, and fish, in addition to acting as important predators themselves. In this article I will be discussing specific groups of reptiles and amphibians from Northern Europe, their past diversity, and their potential for expansion and reintroduction in the context of anthropogenic climate change. 

Anura (Frogs and Toads)

           The Anura, i.e. frogs and toads, are one of the largest groups represented in the temperate lowlands of Northern Europe. A huge proportion of the biomass in a wetland ecosystem is dominated by this group, and their presence is necessary to support populations of numerous predators, including storks, snakes, and otters. While many species of frogs and toads have distributions that extend quite far north, others are more central or southern in their super-Alpine range. The family Alytidae, represented in Northern Europe today by the midwife toad (Alytes obstetricans), and in the pre-glacial period by painted frogs (Latonia) as well, is a good example of the latter category. The current distribution of the midwife toad covers most of Iberia, France, and parts of Southern Belgium, the Southern Netherlands, and West-Central Germany. There are also several introduced populations in England. Another family, Bombinatoridae includes the fire-bellied (Bombina bombina) and yellow-bellied toads (Bombina variegata), which can be found in Eastern/Northern Europe and Southern/Western Europe respectively, with a significant area of overlap and, like the midwife toad, several introduced populations of both species in England. The Bufonidae, or true toads, are diverse and widespread, with the appropriately named common toad (Bufo bufo) being found across the entire continent. Natterjack toads (Epidalea calamita) are similarly cosmopolitan, at least for Northern and Western Europe. Comparatively, the green toad (Bufotes viridis) is restricted more to the north/east, though its Pleistocene/Holocene range extended much farther into Western Europe. Tree frogs (Hylidae) are represented by a single, cold-tolerant species (Hyla arborea) with a wide extent of occurrence across most of Northern and Western Europe, including several populations in England, where it is quite possibly a reintroduced native. Spadefoot toads (Pelobates fuscus) of the family Pelobatidae are found primarily in North-Eastern Europe but, much like the green toad, they had a much wider prehistoric distribution that covered Western Europe as well. The single-genus family Pelodytidae were also much more widespread, with the parsley frog (Pelodytes punctatus), today restricted to France and Eastern Iberia, occurring much more widely during other interglacial periods in areas as far north as England and Poland. The Ranidae are the most diverse of the remaining families, represented in Northern Europe by three species of brown frogs (Rana) and two green frogs (Pelophylax). The common frog (Rana temporaria) is the most widespread, though moor frogs (Rana arvalis) and agile frogs (Rana dalmatina) have broad ranges as well, despite both experiencing extirpation events, notably from England. The pool frog (Pelophylax lessonae) and marsh frog (Pelophylax ridibundus) also have large distributions, and though native populations of the former have seemingly been eliminated from England, both species (and their natural hybrid Pelophylax kl. esculentus) have been widely introduced there, further widening their distribution in Northern Europe. As a result, temperate-zone Europe has plenty of Anuran taxa to work with in the event of climate-driven range shifts. Most species are rather wide-ranging and should be rather flexible in their climatic requirements. Others may be less so but will have southerly, congeneric relatives that can migrate or be released farther north to compensate. Some species, like the parsley frog, may be able to thrive in areas not occupied for over a hundred millennia. Anuran diversity, despite periodic range shifts caused by glacial cycles, seems to have remained pretty constant since the Late Pliocene, just prior to the beginning of said cycles, but there are some notable exceptions. The aforementioned loss of the painted frog genus Latonia, today represented only by a relict and critically endangered population in the Levant, is one such exception. Another is the loss of the Palaeobatrachidae, a family of wholly aquatic frogs that disappeared sometime during the early-mid Pleistocene. Interestingly, a distant, yet morphologically similar relative, the African clawed frog (Xenopus laevis) exists in Europe as an introduced species in England and France. It is possible that this species, which is similarly aquatic and part of the same clade (Superfamily?: Pipimorpha) may be functionally similar, though this would be difficult to determine.

Urodela (Newts and Salamanders)

           Compared to the Anurans, the temperate European Urodela are less diverse, with all current species belonging to a single family, Salamandridae, represented by four genera and six species. The first, the alpine newt (Ichthyosaura alpestris), is curiously named for it is not found exclusively or even primarily in the Alps. The range of this species covers much of Central and North-Western Europe, including yet another introduced population in England, where it understandably does quite well, seeing as it managed to colonize the island naturally during at least one previous interglacial. While the palmate (Lissotriton helveticus) and smooth newts (Lissotriton vulgaris) both managed to do the same during the current interglacial, and they do overlap extensively, their mainland distributions differ significantly, with the former more southern/western and the latter more northern/eastern. Fire salamanders (Salamandra salamandra) have a more central distribution, though they ranged much further north during pre-glacial times. Great crested (Triturus cristatus) and marbled newts (Triturus marmoratus) both range into Northern Europe, though the former is much more widespread and the latter is confined to far Western Europe, though like the fire salamander it also had a wider distribution in prehistory. In pre-glacial times, another species, apparently conspecific with the living Caucasian salamander (Mertensiella caucasica) was also present until, presumably, the Ice Ages restricted it to its current refuge. Aside from the Salamandridae, two other families lived in Northern Europe in the late Tertiary/early Quaternary. The Proteidae, a family of fully aquatic and water-breathing salamanders, still live in the Balkans in the form of the olm (Proteus anguinus), a blind, translucent, long-bodied cave-salamander. A related taxon, Mioproteus wezei, was widespread in Northern Europe until the Middle Pleistocene. This species appears not to have been subterranean like its living relatives, instead living in above-ground streams and ponds like its North American relatives, the mudpuppies (Necturus). Interestingly, one subspecies of the olm, the black olm (Proteus anguinus parkelj) of Slovenia, appears to have become subterranean much more recently than the nominate subspecies, retaining pigmented skin, functioning eyes, and a more robust morphology. The other family to disappear from Northern Europe was the Cryptobranchidae, which were represented in Europe by the European giant salamander (Andrias scheuchzeri), a species very similar to and quite possibly synonymous with the living Chinese giant salamander (Andrias davidianus). This species persisted only to the late Pliocene, likely unable to find suitable refugia after the onset of the glacial cycle, but survived in Eastern Asia. This species, which typically weighs around 25 kg and is the largest living amphibian, is now Critically Endangered due to pollution, poaching, and habitat loss. It lives in lakes and streams and ranges into temperate latitudes in Asia comparable in climate to Central/Western Europe today. Salamanders and newts are, in general, quite well-adapted to cool, temperate climates compared to other herpetofauna. Though their familial diversity is less than that of the frogs and toads, their specific and subspecific diversity is high and forms can be found that are adapted to most European climates, providing ample stock for migration and translocation.

Squamata (Lizards and Snakes)

        Lizards and snakes constitute the bulk of the world’s reptiles, and even more so that of Europe. Since the end of the Miocene, when the geckos, chameleons, and agamas were extirpated from Northern Europe, only three families of lizards have managed to survive there. The Anguidae, or legless lizards, are represented today by the widespread and climatically flexible slowworm (Anguis fragilis), and in prehistory by the larger Pseudopus pannonicus, a relative of the living European glass lizard (Pseudopus apodus), which is still found in much of South-Eastern Europe. The Lacertidae have many European taxa, but only three genera and five species are found north of the Alps. The sand lizards (Lacerta agilis) and the green lizards (Lacerta bilineata and viridis) have patchy distributions, implying much greater and more continuous ranges in the past, and this is also reflected in the fossil record. While currently restricted to France, Italy, and Northern Spain, western green lizards (bilineata) have proven that they can survive and reproduce further north if given the chance, as evidenced by an introduced population in Southern England. This population is only a little further south than a relict population of the eastern green lizard (viridis), which otherwise is restricted to South-Eastern Europe, on the border between Germany and Poland. The English population of the western species is also sympatric with an introduced population of the European wall lizard (Podarcis muralis) which is otherwise found only in Southern Europe and on the margins of Western and Central Europe. This species, like many others, was much more widespread in the past, with pre-glacial fossils being relatively common in Poland. The last Lacertid, the viviparous lizard (Zootoca vivipara) is far more widespread than the other species, due to its namesake ability to give birth to live young, giving it a much greater potential range. This species occurs across Northern Europe, even as far as Northern Scandinavia. Records exist for two other Lacertid genera in Northern Europe (Dalmatolacerta and Ophisops), but the accuracy of these identifications has been questioned. The Scincidae, common in Northern Europe until the late Miocene, only barely enter Central Europe today. Oddly, the genus that was all over Northern Europe until the Pliocene, Chalcides, is restricted to Southern Europe today, while the one taxon that is still found north of the Alps, the juniper skink (Ablepharus kitaibelli) has hardly any fossil record at all, and then only from Southern Europe. Whether or not this species, which is found in a disjunct population in Southern Slovakia and Northern Hungary aside from its main range in South-Eastern Europe, or any other skink ever ranged farther north or west in either the Pliocene or Pleistocene, is unknown.

        The story of snakes in Northern Europe is less complicated. Two families, one more diverse than the other, are known: the Colubridae and the Viperidae. Colubrids are still common in Central/Northern Europe, represented by four genera and seven species. The smooth snake (Coronella austriaca) has a large distribution covering most of the European continent, while the whipsnake (Hierophis viridiflavus) and the Aesculapian snake (Zamenis longissimus) are restricted to the southern end of Central Europe. The whipsnake currently ranges well into Northern France and parts of Belgium, but had a much larger distribution in previous interglacial and pre-glacial periods, with records from Germany and Poland. The retraction of the Aesculapian snake has been much more recent, with a relict population in Denmark surviving into the last century. Aesculapian snakes have several successful introduced populations in England and Wales, where they were present in previous interglacials, and several relict populations in Germany and Czechia, suggesting that their current absence from most of Northern Europe is not solely due to climatic factors. As the only arboreal snake native to Northern Europe, they potentially have a very important role to play as predators. The last colubrid genus to be discussed, Natrix, has four Northern European members. The two species of grass snakes are collectively found across most of Northern Europe but are split in their distributions between western (Natrix helvetica) and eastern (Natrix natrix). A similar west-east split is seen in the ranges of the water snakes (Natrix maura and Natrix tesselata respectively), but these are not found further north than Central France and Southern Germany. As with other herpetofauna, water snakes enjoyed a more northerly distribution in previous interglacial periods, with Middle and Late Pleistocene records in England and Poland. Fossil remains of one other extant Colubrid, the four-lined snake (Elaphe quatuorlineata), also extends into Northern Europe, despite the species having a Mediterranean distribution today. Aside from the Colubrids, the only native family of snakes in Northern Europe is the Viperidae, represented by two species. The European adder (Vipera berus) is widely distributed across Northern Europe, except for the area of Central France, where it is replaced by the slightly smaller and slightly more venomous European asp (Vipera aspis), with an area of overlap and occasional hybridization in Northern France.

Testudines (Turtles)

           The last group to be discussed is the turtles. Currently, only one species is found in Northern Europe, the European pond turtle (Emys orbicularis). While previously widespread across the continent, covering all of Southern Europe and Northern Europe from England to Sweden to Russia, the range of this species has contracted significantly, such that its northern range is now confined to the east. The cause of this range decrease is debated but appears to have been due either to a shift in climate towards milder summers (possibly impacting incubation success) in North-Western Europe or human exploitation and habitat destruction, or some combination of the two. Either way, the potential for reintroduction to rewilded areas is significant, with one or two populations of released pets possibly reproducing already. While the pond turtle is the only representative of the Emydidae in Europe, and apparently the only one ever to live there, other groups of turtles were previously found in Northern Europe as well. There were snapping turtles (Chelydridae) of the genus Chelydropsis present until the early Pleistocene, and related North American snapping turtles (Chelydra serpentina) are now occasionally reported in Europe as an introduced species. Geomydid turtles of three genera, Mauremys, Melanochelys, and an extinct taxon of uncertain affinities dubbed Clemmydopsis, were also present in pre-glacial Northern Europe. Mauremys is still found in southern Europe where two species, the Spanish pond turtle (M. leprosa) and Balkan pond turtle (M. rivulata), are found in the Western and Eastern Mediterranean respectively. While these two species are confined to the warmer southern end of the continent in their natural range, Asian representatives of the genus, such as the Chinese pond turtle (M. reevesi) range farther north into more temperate latitudes, and introduced turtles of several Mauremys species have managed to overwinter in northern Europe more than once. Whether these turtles would be capable of reproducing there is unclear, but the published incubation temperatures for the genus are consistently similar to that of the European pond turtle. Both the Spanish and Balkan species, as well as the Caspian turtle (M. caspica), appear to be descendants or close relatives of the more widespread M. gaudryi, the species previously found across Europe. The assignment of some European turtle fossils to Melanochelys is not supported by some authors, but the point is likely moot since the two living species are both confined to tropical environments and unlikely to thrive in Europe today. Tortoises (Testudinidae) are more easily identified, with only one genus (Testudo) known from Northern Europe. Most post-Miocene remains north of the alps seem to be referable to the living Hermann’s tortoise (Testudo (Eurotestudo?) hermanni), one of the more cold-tolerant European tortoises. While restricted to Southern and South-Eastern Europe today, Pliocene remains are known from Poland and there is at least one Pleistocene (not clear if Early, Middle, or Late) record from Germany. Hermann’s tortoise can reportedly be kept outside in quite cool climates and will hibernate in the winter, but sex-determination in this species is different from that of Emys or Mauremys, with the male-female split at approximately 31.5 degrees incubation instead of 29.5, which would likely affect their ability to reproduce in higher European latitudes.

Discussion & Implications

           The reason I have outlined all of this is that, considering that another Ice Age is unlikely to happen anytime soon, and indeed we seem to be rapidly approaching conditions similar to those before the glacial cycle began, we should be planning our rewilding projects accordingly. Whether or not a species lived in a particular area in the past few hundred years will become increasingly unimportant, as species known from adjacent regions or past warm periods may become better suited to said area in the future. While many have addressed this issue when discussing palaeo-native trees and other economically vital taxa, other groups have been given less attention. Purposeful translocations of amphibians and reptiles to areas outside of their historic ranges and within their prehistoric ones is a logical strategy to ensure these species will retain healthy populations into the future. The more widespread they are, the less vulnerable they are to climate-induced extinction. Amphibians especially would benefit from more populations, as local outbreaks of diseases like Chytridiomycosis, implicated in the reduction or even extinction of amphibian species worldwide, would have less of an effect on the species as a whole. Some herp taxa are already widespread and will probably remain so, but others have the potential to inhabit regions unoccupied by their lineages for hundreds of thousands of years or more. The scope of these strategies could range from more tame, i.e. the acceptance of alpine newts and Aesculapian snakes as reintroduced natives in England, to more extreme measures like experimenting with backup populations of giant salamanders in Central Europe. The reintroduction of pond turtles to Western Europe, aided by warmer summers and the return of wetland-creating beavers, is an obvious early step. It is quite possible that we may see some taxa relocate naturally, especially those with more terrestrial habits, but others may be more effectively spread out through purposeful translocations. It would be interesting, for example, to trial introductions of whipsnakes or parsley frogs in Belgium or the Netherlands, just a bit north of their current range limits, to confirm that conditions there have become suitable. Introduced herpetofauna in various parts of Europe should be re-evaluated and reprioritized in the context of their natural and past ranges. There is little sense in considering something alien in Denmark when it is native in Germany, for example, especially in the context of a warming climate. Ultimately, rewilding will be most successful the less desperately it clings to black and white ideas of which species do or do not “belong”. Our potential to restore nature is not synonymous with our ability to return it to any previous state, but rather with our ability to restore biodiversity, functionality, and resilience.