1 We tested for a unimodal ('hump-backed') relationship between species ric
hness and standing crop at various spatial scales in stream bryophyte commu
nities. Bryophyte species and their biomasses were determined from 20 to 25
quadrats in eight river and six stream sites in northern Finland.
2 Regression analyses revealed a quadratic relationship between richness an
d biomass in only two of the river sites and a positive correlation in one
other. A quadratic relationship was detected in three stream sites and rich
ness increased linearly with biomass in another.
3 We also tested for the hump-shaped pattern across individual stream bould
ers, representing an elevational gradient from continuously submerged to pe
rmanently dry conditions. with an intermediate zone with fluctuating water
4 Species richness-standing crop relationship conformed to the hump-backed
model only when samples from all three microhabitats were included in the a
nalysis. A significant positive correlation occurred in the exposed low bio
mass end of the gradient which is characterized by semi-aquatic species, wh
ereas the relationship tended to be negative in permanently submerged areas
with high biomass of large canopy-forming species. Quadrats close to the w
ater level had intermediate standing crop and highest species richness.
5 Species dominant at either end of the gradient appeared unable to monopol
ize space in the intermediate zone where disturbances (e.g. scouring by ice
) detach mosses from the substratum, creating vacant gaps for colonization.
The unimodal relationship between richness and biomass is likely to occur
only in streams that contain large boulders protruding above the water line
, thus providing scope for community diversification along very short verti