The United Kingdom of Great Britain and Northern Ireland is a sovereign country in north-western Europe, off the north-western coast of the European mainland. The United Kingdom includes the island of Great Britain, the north-eastern part of the island of Ireland, and many smaller islands within the British Isles. Northern Ireland shares a land border with the Republic of Ireland. Otherwise, the United Kingdom is surrounded by the Atlantic Ocean, with the North Sea to the east, the English Channel to the south and the Celtic Sea to the south-west, giving it the 12th-longest coastline in the world. The Irish Sea separates Great Britain and Ireland. The total area of the United Kingdom is 93,628 square miles, with an estimated population in 2020 of over 67 million.
The island of Great Britain, along with the rest of the archipelago known as the British Isles, has a largely temperate climate. It contains a relatively small fraction of the world's wildlife. The biota was severely diminished in the last ice age, and shortly (in geological terms) thereafter was separated from the continent by the English Channel's formation. Since then, humans have hunted the most dangerous forms (the wolf, the brown bear and the wild boar) to extinction, though domesticated forms such as the dog and the pig remain. The wild boar has subsequently been reintroduced as a meat animal.
Large mammals are not particularly numerous in Great Britain. Many of the bigger species, such as the grey wolf and the brown bear, were hunted to extinction many centuries ago. However, in recent times some of these large mammals have been tentatively reintroduced to some areas of mainland Britain. The largest wild mammals that remain in Britain today are predominantly members of the deer family. The red deer is the largest native mammal species, and is common throughout England, Scotland and Wales.
The other indigenous species is the roe deer. The common fallow deer is in fact not native to Britain, having been brought over from France by the Normans in the late 11th century. It has become well established. The sika deer is another small species of deer which is not indigenous, originating from Japan. It is widespread and expanding in Scotland from west to east, with a strong population in Peeblesshire. Bands of sika exist across the north and south of England though the species is absent in Wales.
There are also several species of insectivore found in Britain. The hedgehog is probably the most widely known as it is a regular visitor to urban gardens. The mole is also widely recognised and its subterranean lifestyle causes much damage to garden lawns. Shrews are also fairly common, and the smallest, the pygmy shrew, is one of the smallest mammals in the world. There are also seventeen species of bat found in Britain: the pipistrelle is the smallest and the most common.
Rodents are also numerous across Britain, particularly the brown rat which is by far the most abundant urban mammal after humans. Some however, are becoming increasingly rare. Habitat destruction has led to a decrease in the population of dormice and bank voles found in Britain. Due to the introduction of the North American grey squirrel, the red squirrel had become largely extinct in England and Wales, with the last populations existing in parts of North West England and on the Isle of Wight. European rabbit and European hare were introduced in Roman times, while the indigenous mountain hare remains only in Scotland and a small re-introduced population in Derbyshire.
There are a variety of carnivores, especially from the weasel family (ranging in size from the weasel, stoat and European polecat to the European badger, pine marten, recently introduced mink and semiaquatic otter). In the absence of the locally extinct grey wolf and brown bear the largest carnivores are the badger, red fox, the adaptability and opportunism of which has allowed it to proliferate in the urban environment, and the European wildcat whose elusiveness has caused some confusion over population numbers, and is believed to be highly endangered, partly by hybridisation with the domestic cat.
Various species of seal and dolphin are found seasonally on British shores and coastlines, along with harbour porpoises, orcas, and many other sea mammals.
In general the avifauna of Britain is similar to that of Europe, consisting largely of Palaearctic species. As an island, it has fewer breeding species than continental Europe, with some species, like crested lark, breeding as close as northern France, yet unable to colonise Britain. The mild winters mean that many species that cannot cope with harsher conditions can winter in Britain, and also that there is a large influx of wintering birds from the continent or beyond. There are about 250 species regularly recorded in Great Britain, and another 350 that occur with varying degrees of rarity.
There are 220 species of non-marine molluscs that have been recorded as living in the wild in Britain. Two of them (Fruticicola fruticum and Cernuella neglecta) are locally extinct. In addition there are 14 gastropod species that live only in greenhouses.
The species of amphibian native to Britain are the great crested newt, smooth newt, palmate newt, common toad, natterjack toad, common frog and the pool frog. Several other species have become naturalised.
Like many temperate areas, Great Britain exhibits a relative lack of snakes, with the European adder being the only venomous snake to be found there. The other notable snakes found in Great Britain are the grass snake and the smooth snake. Great Britain has three native breeds of lizard: slowworms, sand lizards and viviparous lizards. There are also turtles, such as leatherback turtles to be found in the Irish Sea, although these are rarely seen by the public. Other reptile species exist but are not native: aesculapian snake, wall lizard and the green lizard.