Imported animals

"Humans are inveterate and incorrigible meddlers, never content to leave anything as they find it but always seeking to alter and – as they see it – to improve. In no field is this truer than in those of the animal and plant kingdoms," says Christopher Lever, author of a new edition of The Naturalized Animals of Britain and Ireland, which was originally published back in 1977.

In fact, as conservationist Lever's revised book asserts, mankind has treated the animal kingdom rather like a vast chessboard – moving one piece forward and another back, while not always being aware of what the consequences might be. Not that, some might argue, this has always resulted in a negative outcome. For example, the European rabbit (which, before the 12th century, was only found in the south of France and Iberian peninsula), after being brought over here as a food source, seems to have been completely integrated into the British environment. Perhaps a few gardeners might want to ship these furry grazers back home, but most of us are pretty tolerant of them.

However, more of the human population might wish that the common rat (which reached Europe in the 18th century or thereabouts, after stowing away on ships) had stayed where it was originally found – in central Asia.

These are only a couple of the many familiar furry faces in this comprehensive read, which outlines the 19 animals, 21 birds, six reptiles, 13 amphibians and 18 fish that have been released on to our island since Neolithic times.

The most familiar figurehead of these non-native species must be the much maligned grey squirrel. In three regional sections, including one on Scotland, Lever reveals this animal's speedy trajectory.

"In 1892, a pair of grey squirrels, imported from the United States by GS Page of New Jersey, was released at Finnart on the Dunbartonshire shore of Loch Long," he writes. "From there the species spread north to Arrochar and Tarbet by 1903, east to Luss (1904), Inverbeg (1906), south-west to Garelochhead and Rosneath, and as far south as Helensburgh (all Argyllshire), Alexandria and Culdross (west Dunbartonshire) by 1912, an overall distance of 30km. By 1915, grey squirrels had penetrated to the eastern side of Loch Lomond, Stirlingshire, where they established themselves at Drymen. An importation to Edinburgh Zoo in 1913 resulted in the establishment by the end of the decade of a number of escapees and their progeny outside the zoo grounds at Corstorphine."

Before we knew it, the familiar grey squirrel was everywhere. The book also outlines the environmental effect of the introduction of this species, which has included tree damage and the steady decline of the red squirrel population.

However, a few beasties don't really have much chance of their population skyrocketing. The Chinese grass carp, for example, has relatively recently been imported to UK rivers from the lowland streams of China and the Amure River Basin of East Siberia. It's been introduced in order to control aquatic vegetation and can live in British water quite happily, apart from its picky procreation habits.

"Its strict breeding requirements – especially the necessity for a water temperature of 27C to 29C over a period of some nine weeks and a current flow of 60cm to 150cm per second – preclude it from breeding in British and Irish waters, where acclimatised (but not naturalised) populations are maintained by restocking," explains Lever.

Other similarly abstinent beasts include the European pond terrapin and red-eared sliders, both of which are escapees from domestic ponds. There isn't, however, any evidence that they can reproduce in this climate – although the fact that they live to a great age means that they can still be spotted crawling around on rocks in certain public ponds in Cardiff and London. Once they die out, however, the semi-aquatic red-eared sliders will disappear. Perhaps they'll be excluded, then, from the next edition of this book – if another is ever required. Thanks to a law passed 29 years ago, it's unlikely that we'll ever see any other major introductions of non-native creatures to the UK.

As Lever says: "The Wildlife and Countryside Act in 1981 in Britain and similar legislation in the Republic of Ireland have ensured that no further alien species may be released or negligently allowed to escape into the wild in Britain or Ireland".

The Naturalized Animals of Britain and Ireland by Christopher Lever is out now, published by New Holland, 35.

This article was first published in The Scotsman on Saturday 09 January, 2010.