The relationship of environment size and population parameters Oxytrema proxima (say) (Gastropoda: Pleuroceridae)

Environmental size, as measured by cross-sectional area of the inhabited streams, and population density of Oxytrema proxima are related by the power function Y=8.074X ^-0.584, where Y=population density and X=cross-sectional area of the stream. Relationships were also found between maximum size and biomass and cross-sectional area, but maximum size and biomass were more closely related to population density by the equations Y=8.832X ^-0.064 (Y=maximum individual size, X=density) and Y=6.730X ^0.459 (Y=biomass (total live weight), X=density). This is interpreted as evidence that environment size acts directly on density, which in turn produces effects on maximum size, total live weight, and to a lesser extent mean size. The inverse relationship between population size and environment has not been previously reported. It is clear that in this case this distribution and abundance of O. proxima are closely related, and that this can be so when populations disperse extremely poorly as well as extremely well. This must mean that dispersal is secondary in importance to population stability as an influence on distribution and abundance.