The origin of British mammals

THE ORIGIN OF BRITISH MAMMALS

P. A. MORRIS DĂźring the Pleistocene Epoch, popularly known as 'The Ice A g e \ there were at least four separate glacial periods. In between, there were warmer interglacials, when mammals such as the hippopotamus (Hippopotamus amphibiiis) and spotted hyaena (Crocuta crocuta) roamed widely in Britain, just as they do in Africa today. These became extinct as the climate cooled again in the last glaciation. This was not so severe as previous ones and some mammals managed to survive in Britain despite the harsh conditions.

When the last glaciation ended, about 10,000 years ago, species that were adapted to cold moved further north, to be replaced by animals from the south. These spread into the areas that are now England, Wales, Ireland and Scotland. They continued to do so as forests began to cover the land - first birch, pine and juniper and later oak and alder as the land became warmer and wetter. This was possible because Britain was still linked to the Continent by low-lying land at that time. Those species that arrived naturally, before Britain and Ireland were cut off from the rest of Europe, are called 'native' species.

The first animals to spread would have been those able to withstand cold, such as the fox (Vulpes vulpes), otter (Lutra lutra), stoat (Mustela erminea), mountain hare (Lepus timidus), badger (Meies meles) and red deer (Cervus elaphus). Animals such as bats, hedgehog (Erinaceus europaeus), and weasel (.Mustela nivalis) would have followed as the climate became warmer.

Among the last native species to arrive would have been the mole (Talpa europaea) and the red squirrel (Sciurus vulgaris).

By about 7,000 BC, the melting of the polar ice cap had liberated enough water to raise the sea level and flood the swampy land between England and the Continental mainland. Britain became an island, and the climate abruptly warmed encouraging the rapid spread of woodland communities here. The only way land animals could now reach Britain was to be brought in by man.

Several species, such as the garden dormouse (Eliomys quercinus) and white toothed shrew (Crocidura leucodon) reached northern France only after Britain had been cut off by the sea. Consequently, although they are still found only 35km away across the English Channel, they were never native to Britain.

The changing climate caused many species to die out. Later, others that were once common succumbed to competition from our own species and disappeared in comparatively recent times. With the development of farming and Settlements, there was simply no room for them to exist alongside the spreading human population. Bears (Ursus arctos) and wolves (Canis lupus) roamed free in Britain until only a few centuries ago. Wolves survived in England until about 1550, and small numbers were still to be found in Scotland and Ireland in the 1700s. Other large carnivores once native to Britain include the wolverine (Gulo gulo) and lynx (Lynx lynx). They probably became extinct in Britain during the Stone Age, some 5,000 years ago. There has been no evidence of wild beavers (Castor fiber) in Britain

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THE ORIGIN OF BRITISH MAMMALS

101 since the 12th Century, but they are recalled in place-names such as Beverley in Humberside and their remains are not uncommon in fenland peat deposits. The wild boar (Sus scrofa) was also widespread, but died out here in the 17th Century as forests were felled.

Even before Britain was cut off from the mainland of Europe, the rising sea had isolated Ireland from Britain. Consequently, there are fewer mammals in Ireland than in Britain, and probably only eight or nine species are native. Animals better adapted to withstand cold were most likely to have reached Ireland before it became surrounded by sea. They include the pine marten (Martes martes), stoat, mountain hare, otter, fox, badger, red deer and probably the pygmy shrew (Sorex minutus). The hedgehog may be native to Ireland, but could have been introduced as it was to many other islands.

Species that need a milder climate, such as the weasel, brown hare, common shrew (Sorex araneus) and water shrew (Neomys fodiens) spread westwards later and never reached Ireland. Ireland also has no moles nor, until recently, any voles. Bank voles (Clethrionomys glareolus) werefirstdiscovered in ä small area in south-western Ireland in 1964, and are still spreading.

The Scottish islands have even fewer native species than Ireland. This is because they are separated from the mainland by such deep water that they were probably isolated even before the sea level rose to cut Britain off from Europe. Otters might have swum to some islands, but red deer are more likely to have been introduced deliberately. The presence of small mammals such as mice and shrews is due to accidental importation, transported in prehistoric times with supplies brought from the mainland. The wood mouse (.Apodemus sylvaticus) was already present in Orkney some 5,000 years ago and was later distributed widely among the Scottish islands by the Vikings among their supplies. The house mouse (Mus musculus), black rat (Rattus rattus) and brown rat (R. norvegicus) also arrived in Britain by accident, imported in the course of trade.

A number of our more familiar mammals were introduced to Britain deliberately for farming, hunting or interest. Foremost among the useful species imported are the domesticated animals such as cattle sheep and horses. Rabbits (Oryctolagus cuniculus) were brought to Britain by the Normans in the 12th Century as a useful source of meat and skins. In later centuries many country estates had their own warrens set aside for rearing rabbits. The rabbit has more recently become a widespread and serious pest which has had a greater impact on the ecology of our country than perhaps any other wild mammal.

The roe deer (Capreolus capreolus), widespread in medieval times, gradually disappeared and was reintroduced about 100 years ago. Similarly, the reindeer (Rangifer tarandus), was originally a native species which died out as the Post Pleistocene climate ameliorated, and was reinstated to the Cairngorms in 1952. Most of our deer are non-native. The fallow deer (Dama dama) was brought to Britain for the chase and for venison. More recent introductions have been the muntjac (Muntiacus reevesi) and Chinese water deer (Hydropotes inermis). Both originate from Asia, and were released into Woburn Park in Bedfordshire. While the latter is still confined to a few wetland areas (in East Anglia and the east Midlands) the muntjac is still Trans. Suffolk Nat. Soc. 29 (1993)

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spreading rapidly. It has already reached the west country and has been established deep in the urban areas of London and the Midlands for many years. The sika deer (Cervus nippon) is another Asian import, now widespread in Dorset and also Scotland, where it poses a genetic threat to our native red deer through inter-breeding with it.

The grey squirrel (Sciurus carolinensis) from North America is undoubtedly one of the most successful introductions. It was deliberately released into the wild in the last Century, and has largely replaced our native red squirrel in the space of less than 100 years. The fat dormouse (Glis glis) another arboreal woodland mammal of similar size and appearance was introduced in 1902 from Hungary. It has not spread far, being restricted to areas in the Chilterns. However, it has become a nuisance indoors and well meaning householders have been trapping them and (illegally) releasing the animals at distant sites, thus circumventing the natural barriers that have constrained itsspread hitherto. Its numbers and impact are likely to increase.

D端ring this Century, several species have been brought in to be bred in captivity for their f端r. Some have escaped or been released and become established in the wild. The most successful has been the mink (Mustela vison), which came from North America in the late 1920s. The coypu (Muyocastor coypus) from South America escaped into the wild in the 1930s and became a nuisance because of its damage to crops and riparian habitats.

It has now been exterminated, like the introduced musk rat (Ondatra zibethecus) before it. Brought from North America in the 1920s, this aquatic f端r bearer had been eradicated by 1937.

White-toothed shrews are found in certain of the Channel Islands and Isles of Scilly, but nowhere eise in Britain. They are common in southern and eastern Europe, from whence they were introduced accidentally, perhaps among imported fodder for farm animals. Many island mammal populations have developed from a small number of founders, even perhaps a Single pregnant female. Because of their isolation island animals often gradually evolve unique into forms. There is less competition with no other similar mammals present, and in the absence of predators they could become larger and tamer without disadvantage. This occurred with the Skomer vole, a form of the bank vole living on Skomer island off the Welsh coast and the St Kilda mouse, found on a remote Hebridean island. Both are nearly twice the size of their mainland ancestors, with thicker f端r. Despite the differences, both will breed with mainland types in captivity, indicating that their present form has relatively recently evolved.

In summary, it appears that the major factors shaping our fauna in Pleistocene times were natural climatic changes. In Post Pleistocene times, the main factor has been human activity, resulting in the addition of some species and the loss (extinction) of others. Today, the British mammal fauna is unlikely to be further enhanced owing to the provisions of the Wildlife and Countryside Act, 1981. This makes it illegal to release into Britain any nonnative species (even those that are already established here). Other legislation reduces the probability that escapes will occur from captive collections.

At the same time, this legislation seeks to prevent further extinctions by offering protection to many of our declining species. This has not saved the

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mouse eared bat (Myotis myotis) as a British species, and several others are threatened by factors such as habitat fragmentation and loss which the Law is powerless to prevent. Thus the history of the British mammal fauna is one of continual change in response to environmental alteration, change which the Law now seeks to constrain, but cannot altogether prevent.

References Corbet, G . B. & Harris, S. (1991). The Handbook of British Mammals (3rd edn), Blackwell, Oxford.

Yalden, D . W. (1982). When did the mammal fauna of the British Isles arrive? Mammal Review, 12, 1-57.

Dr. P. A. Morris, Biology Department, Royal Holloway, University of London.

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