Habitat vs. dispersal constraints on bryophyte diversity in the Mojave Desert, USA

• We studied spatial patterning of desert bryophyte diversity within a 1000-cell grid.
• The community exhibited a nearly equal balance of habitat and dispersal limitations.
• Habitat limitations were scale-dependent and related to shrub islands and topography.
• Dispersal limitations were scale-independent and specific to individual species.
• An ideal strategy for conserving bryophyte habitats is to retain native shrubs.

Preserving desert plant diversity requires an understanding of the degree to which plant communities are constrained either by suitable habitat or by dispersal limitations. Using a spatially explicit approach, we surveyed diversity in an undisturbed xeric bryophyte community within a regular sampling grid of 1000 adjacent cells. Eigenvector mapping and variation partitioning revealed that spatially independent habitat processes accounted for little of the variation in community composition (2.0%); most variation was attributed either to habitats that were spatially structured (25.6%) or to dispersal independent of habitat (28.0%). At three nested scales, the most influential habitat factors were topography (broad-scale >5 m2), shrub “islands” (meso-scale 1−5 m2), and physical substrate characteristics (fine-scale < 1 m2). Dispersal limitation was less obviously scale-dependent at the community level, yet was important when considering individual species, which showed a wide range of spatial autocorrelation (Moran's I = −0.003–0.601). Our findings suggest that bryophyte diversity is the joint result of dispersal limitations and the distinctive spatially patterned environment of desert shrublands. Therefore, conservation efforts must not only account for dispersal limitations of individual species of concern, but should also seek to retain native shrubs and consequent landscape physiognomy that create bryophyte habitat.