The 300 species of British and Irish liverworts and hornworts are classified into ten groups, based on their recorded presence in 10 × 10 km grid squares (hectads). A two-stage classification initially amalgamated single species into clusters and then systematically removed the smallest clusters, redistributing their species until only ten clusters remained; in the second stage, species were reassigned to the cluster to which they showed the greatest similarity until they all remained stationary. The groups were named after the species with distributions which were most similar to that of the cluster as a whole (Pellia epiphylla, Phaeoceros laevis sens. lat., Cladopodiella fluitans, Lophocolea heterophylla, Scapania undulata, Anastrepta orcadensis, Harpalejeunea molleri, Moerckia blyttii, Scapania degenii, Marsupella condensata). Most groups are nested within at least one other group but the south-eastern L. heterophylla cluster is distinctive. Four groups, Phaeoceros laevis, Cladopodiella fluitans, Scapania degenii and (especially) Marsupella condensata contain a high percentage (30-73) of species which are classed by IUCN criteria as Critically Endangered, Endangered or Vulnerable
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