Factors determining the distribution patterns of mud snails (Hydrobiidae).

The distribution patterns of four species of mud snails (Hydrobiidae) in a complex, estuarine environment are described. The species show habitat selection with respect to salinity but the tolerance ranges of the species overlap and it is shown that this factor alone cannot predict the distribution in the field. The found patterns show that the species are subdivided into many, more or less isolated sub-populations. It is conjectured that the distribution of the species results from the interaction of habitat selection, dispersal rates, colonizations, interspecific competition and extinctions. This interpretation explains features of the distribution patterns; e.g., the boundaries between two species in salinity gradients occur at different salinities in different areas, the species with an intermediate position with respect to salinity preference is the least frequent one, and that coexistence between two, and sometimes three species may occur under certain conditions. The possibility that the species may sustain populations within some areas as "fugitive" species is also discussed.The interpretations are compared with the theory of island biogeography and many parallels are found. The findings are also contrasted to the concept of communities as "natural units".Finally, in an appendix, a simple mathematical model is described. It treats the possibility of coexistence between competing species as a result of migrations between different habitats.