Gains and losses: extinctions and colonisations in B ritain since 1900

Estimating extinction and colonisation rates for most species is difficult. Only small groups of species are sufficiently well known for us to tell when new species have arrived or when species have become extinct. Often we cannot distinguish between new natural arrivals, overlooked residents, and new introductions. Extinction levels since 1900 vary between different groups from 7.4% of species in bumblebees to 0.0% in O rthoptera and allies, but these extremes are from small samples. In most groups, between 0.5 and 3.0% of species have been lost since 1900. Among the 7420 species I have assessed, 1.70% have arrived as natives since 1900. Unsurprisingly, there is a big difference between groups that contain many species that are able to fly (2.64% species are colonists) and those groups that are less mobile (0.18%). Numbers of natural colonists and extinct species since 1900 are roughly equal, but we should not see the colonisers as balancing out or replacing the extinctions because the species in each set are from different groups and are likely to have different roles. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society , 2015, ●● , ●●–●●.