Bryophyte Biogeography

Bryophytes include about 20,000 species characterized by their poikilohydric condition, high long-distance dispersal capacities, and cold tolerance. Despite these specific life-history traits, large-scale biogeographic patterns in bryophytes are consistent with those observed in other groups, wherein they have been interpreted in terms of historical factors. Comparative phylogeographic analyses in bryophytes and angiosperms suggest, however, that spatially congruent patterns may not necessarily arise from common processes. This is best illustrated by the strikingly lower rates of endemism at all taxonomic and spatial scales in bryophytes due to their failure to diversify in-situ and the rapidity at which they enlarge their distribution range. In particular, the striking transoceanic disjunctions that are typical for many bryophyte species and their low community turnover at broad geographic scales both point to the higher capacities of bryophytes for long-distance dispersal than angiosperms. Such high long-distance dispersal capacities are reflected in the lower spatial genetic structure of bryophytes as compared to angiosperms at large geographic scales. This explains why, as opposed to the expectations of MacArthur & Wilson's model, bryophyte species richness is not necessarily lower on islands than on continents, suggesting that community assembly is more constrained by ecological filtering than dispersal limitations in bryophytes. The low relevance of historical factors for global patterns of bryophyte species richness has contributed to the idea that, as opposed to the predictions of one of the most general rules in ecology, bryophyte species richness does not decrease with latitude due to a strong tropical niche conservatism. Recent evidence for the existence of a latitudinal species richness gradient in bryophytes raises, however, the question of why bryophytes diversified faster in the tropics. This and other avenues of research in bryophyte biogeography are discussed.