The species-pool hypothesis from a bryological perspective

Bryophytes form one-layered communities, which provide good opportunities t o test predictions of the species pool hypothesis. The prediction that comp etitive exclusion occurs rarely, if at all, in established communities seem s to hold for many types of bryophyte vegetation, but the drastic effects o n local diversity of some invading exotic species suggests, that this may b e partly due to the "ghost of competition past". In a survey of bryophyte c ommunities on earth banks, variation in species number per stand was rather larger than predicted by the species-pool hypothesis. Large differences in accessibility of the stands may have played an important role in this resp ect. Small-scale repeated chartings and permanent grid studies suggest, that com petitive exclusion hardly ever occurs in a range of terrestrial bryophyte c ommunities. Evidence for interspecific competition (as derived from small-s cale association between behaviour - increase or decrease in grid cells - o f species pairs) was equally rare, but this may have been due to interactio n with positively density-dependent facilitation effects. However, a rigoro us proof of the hypothesis that facilitation may prevent competitive exclus ion is still lacking.