Factors determining bryophyte species richness and community composition on insular siliceous erratic boulders in calcareous landscapes

Aim Pleistocene erratic boulders are rocks that were relocated by glaciers during the Ice Ages. When their geology differs from the geology of the landscape that surrounds them, erratic boulders form habitat islands for regionally rare, edaphically specialised, rock-dwelling cryptogams (bryophytes, ferns and lichens). Such boulders constitute terrestrial model systems for exploring island biogeographic predictions and the effect of environmental variables on species diversity and community composition, which we studied in order to provide basic knowledge of the ecology, with relevance for the conservation, of these unusual island systems. Location Siliceous erratic boulders in the calcareous Swiss Plateau and Jura Mountains. Methods For 160 erratic boulders we recorded all bryophyte species and a diverse set of environmental variables. For all species and for specialist species (acidophile rock-dwellers) separately, we analysed species-area relationships and nestedness, and explored relationships between environmental variables, species diversity and community composition. Results We found 138 bryophyte species, 19 of which were specialists of erratic boulders. A steeper species-area curve for boulder specialists than for total species richness underlined the island properties of boulders for specialist species. Large boulders were more likely to harbour numerous boulder specialists and communities on small boulders were nested within the communities present on large boulders. However, at the landscape level small boulders contributed more specialist species than a few large boulders of the same surface area. Erratic boulders near settlements were less likely to harbour boulder specialists. Boulders in open land harboured different and more specialist species than boulders in forests. Conclusions Large undisturbed erratic boulders in open land harbour rare bryophyte communities with a large number of specialist species. Conservation should thus prioritise this type of boulders. Furthermore, conserving large boulders is logistically easier, and they may function as flagships for small boulders that also contribute to the biodiversity within landscapes.