Bryophytes form one-layered communities, which provide good opportunities to test predictions of the species pool hypothesis. The prediction that competitive exclusion occurs rarely, if at all, in established communities seems to hold for many types of bryophyte vegetation, but the drastic effects on local diversity of some invading exotic species suggests, that this may be partly due to the "ghost of competition past". In a survey of bryophyte communities 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 respect. Small-scale repeated chartings and permanent grid studies suggest, that competitive exclusion hardly ever occurs in a range of terrestrial bryophyte communities. Evidence for interspecific competition (as derived from small-scale association between behaviour -increase or decrease in grid cells - of species pairs) was equally rare, but this may have been due to interaction with positively density-dependent facilitation effects. However, a rigorous proof of the hypothesis that facilitation may prevent competitive exclusion is still lacking.
- Invading species