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 plants, liverworts and mosses recorded as natives of Britain and Ireland, and to assess the proportion and distribution of species in each group which belong 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 centre of numerical diversity of the genera worldwide. Methods A recent classification of the pteridophytes, flowering plants, liverworts and mosses into floristic elements is used in a quantitative study of the distribution of species in the four groups, the patterns of occurrence being compared by standardized residuals. Results Mosses and liverworts have more northerly distributions than the flowering plants in the Northern Hemisphere; pteridophytes show an intermediate pattern. There are marked differences in the longitudinal ranges of the groups south of the Boreal biome, with vascular plants tending to be confined to Europe or western Eurasia whereas cryptogams are more likely to occur additionally in North America or have a circumpolar distribution. The proportion of predominantly extra-European genera is particularly high in the pteridophytes and the liverworts and in these groups the rich hyperoceanic flora 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 patterns, reflecting their rates of evolution and powers of dispersal. The flowering plants, as the least mobile and most rapidly evolving group, have a greater proportion of their species in the European and Eurosiberian elements and a higher proportion of species from extra-European genera in the more northerly elements.