The Role of Disturbance in the Pattern of a Riparian Bryophyte Community

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Abstract
A riparian-cliff bryophyte community subject to frequent flooding was investigated to determine the effect of natural disturbance on community structure and dynamics. The contributions of microclimate, competition and tolerance of submergence were examined. Analysis of species composition, abundance and diversity shows spatial pattern at 2 scales: large-scale elevational zonation of the dominant species, upon which is superimposed a smaller-scale patchy distribution of species and open substrate. The effects of both flood frequency and disturbance magnitude in producing this pattern were evaluated. A gradient of species composition and abundance was identified which corresponded to the coincident gradients of elevation and flood frequency. The effects of flood frequency were reflected in the elevational zonation. Small-scale pattern in the form of discrete patches was significantly influenced by the magnitude of individual disturbance events. Disturbance produced gaps in the vegetation through periodic removal of the dominant species. Diversity was increased by colonization of the gaps by opportunistic species which are competitively excluded in the absence of disturbance. Frequent disturbance thus facilitates multispecies coexistence. A combination of equilibrial and nonequilibrial coexistence is postulated.

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