We begin this chapter with a review of the nominally abiotic factors that may contribute to the distribution of freshwater bivalves and gastropods on a regional scale. At least a dozen studies over the last 30 years have returned strong positive correlations between the abundance and/or diversity of freshwater molluscs and calcium concentration or related water quality variables. Laboratory experiments show both that calcium concentration in normal ranges can affect growth, survivorship, and fecundity, and that the various species offen differ strikingly in their calcium optima. Some workers have suggested that calcium concentration (and correlated variables) may not act on abundance in the field, however, but rather serve as a ‘filter for colonists’ only. Others have suggested that the effects of calcium on natural abundances of molluscs are real but indirect, calcium concentration influencing the abundance of food.

We next review a relationship with analogues throughout biology, the number of mollusc species as a power function of habitat area. About a dozen studies worldwide have returned species/area regression coefficients somewhat less than or equal to the ‘canonical’ value of 0.25. Species vary in the mean areas of the habitats they occupy, just as they vary in calcium range. And an interaction is sometimes noticeable between area and calcium in their effects on the freshwater mollusc community, such that sites with larger areas tend to have more species than might be predicted from their calcium concentrations.