THE STREAM AS A HABITAT TEMPLET FOR BRYOPHYTES - SPECIES DISTRIBUTIONS ALONG GRADIENTS IN DISTURBANCE AND SUBSTRATUM HETEROGENEITY

1. The structure of bryophyte communities in streams in relation to ha bitat characteristics, especially disturbance, is described. Disturban ce in rivers is quantified as movement of the stream bed, whereas in s mall streams water level fluctuation is used as an indicator of distur bance frequency. 2. Canonical correspondence analysis differentiated f requently disturbed sites from more stable ones. The existence of a di sturbance gradient was confirmed in a subset of the study sites using long-term records of discharge variation. A parallel change was detect ed in the species composition of bryophyte communities with low-statur ed, potentially fast colonizers dominating the disturbed end and large perennial species the stable end of the gradient. 3. A consistent pat tern of zonation of bryophyte species was found along the gradient fro m continually submersed to persistently dry conditions in small stream s and lake outlets. An abrupt increase in species richness occurred at or just above the water line, where facultatively aquatic species tol erant of both conditions formed the bulk of the community. 4. The rela tionship between species richness and standing crop in stream bryophyt e communities was consistent with the hump-backed model of Grime (1979 ), especially at the within-habitat scale. Quadrats of low and very hi gh standing crop were characterized by low species richness, while pea k richness was observed at intermediate standing crops. 5. A few peren nial species (e.g. Fontinalis spp. and Rhynchostegium riparioides) cap able of monopolizing space dominated the most stable habitats. Species composition in low biomass sites was more variable, yet only one basi c growth-form (small-statured species with high allocation to spore pr oduction) seemed possible in these highly disturbed streams. In habita ts of intermediate biomass, small-scale disturbances apparently allow the formation of a more varied bryophyte community. 6. A habitat templ et for stream bryophyte life strategies and community structure is pre sented. Disturbance is proposed to be the factor filtering out traits unsuitable for a given environment. The potential of stream bryophytes for testing and developing general ecological theory is emphasized.