animal diversity is modest, as a result of factors including the island's small land area, the relatively recent age of the habitats developed since the last glacial period and the island's physical separation from continental europe, and the effects of seasonal variability. great britain also experienced early industrialisation and is subject to continuing urbanisation, which have contributed towards the overall loss of species. a defra (department for environment, food and rural affairs) study from 2006 suggested that 100 species have become extinct in the uk during the 20th century, about 100 times the background extinction rate. however, some species, such as the brown rat, red fox, and introduced grey squirrel, are well adapted to urban areas.