The geographical relationships of the British and Irish flora: a comparison of pteridophytes, flowering plants, liverworts and mosses

Aim To compare the wider distribution of the pteridophytes, flowering plant s, liverworts and mosses recorded as natives of Britain and Ireland, and to assess the proportion and distribution of species in each group which belo ng to predominantly extra-European genera. Location The species native to the British Isles are considered in relation to their distribution in the Northern Hemisphere, and predominantly extra- European genera are identified by consideration of the distribution and cen tre of numerical diversity of the genera worldwide. Methods A recent classification of the pteridophytes, flowering plants, liv erworts and mosses into floristic elements is used in a quantitative study of the distribution of species in the four groups, the patterns of occurren ce being compared by standardized residuals. Results Mosses and liverworts have more northerly distributions than the fl owering plants in the Northern Hemisphere; pteridophytes show an intermedia te pattern. There are marked differences in the longitudinal ranges of the groups south of the Boreal biome, with vascular plants tending to be confin ed to Europe or western Eurasia whereas cryptogams are more likely to occur additionally in North America or have a circumpolar distribution. The prop ortion of predominantly extra-European genera is particularly high in the p teridophytes and the liverworts and in these groups the rich hyperoceanic f lora of the British Isles is almost entirely composed of species in genera that attain their centre of diversity in the tropics. Main conclusions Major taxonomic groups show different distribution pattern s, reflecting their rates of evolution and powers of dispersal. The floweri ng plants, as the least mobile and most rapidly evolving group, have a grea ter proportion of their species in the European and Eurosiberian elements a nd a higher proportion of species from extra-European genera in the more no rtherly elements.