Are bryophyte communities different from higher-plant communities? Abundance relations

The considerable differences in biology between bryophytes and higher plants have led to speculation that their community structure might be different. Ten bryophyte communities were sampled for species biomass composition, and for comparison ten higher-plant communities that were similar in physiognomy and in total community biomass. The rather insecure theory in the bryophyte literature was distilled into eight quantifiable predictions, which were tested. For seven, there was no sign of the predicted differences: i.e. no indication of the predicted low within-community heterogeneity, higher species richness, more variable species richness, lower rank consistency, a poor fit for the geometric model of RAD (relative abundance distribution), better fit for the broken-stick and general-lognormal RAD models with general-lognormal parameter gamma deviating further from 1.0, or of a good fit for the Zipf-Mandelbrot RAD model. However, evenness was, on average, significantly (p=0.005) less in the bryophyte communities, using any of four evenness indices. Two possible features of bryophytes are suggested that might cause this: (a) a smaller module (i.e. shoot, leaf) size, allowing species to be present with a lower threshold biomass, and (b) less efficient competitive exclusion among bryophytes because of weaker competition and a predominance of mutualism, as suggested in the literature. However, the striking conclusion from the results is that in spite of all the biological differences between the two groups of organisms, their community organisation is remarkably similar.