Causality of the relationship between geographic distribution and species abundance

The positive relationship between a species’ geographic distribution and its abundance is one of ecology’s most well-documented patterns, yet the causes behind this relationship remain unclear. Although many hypotheses have been proposed to account for distribution–abundance relationships, none have attained unequivocal support. Accordingly, the positive association in distribution–abundance relationships is generally considered to be due to a combination of these proposed mechanisms acting in concert. In this review, we suggest that much of the disparity between these hypotheses is due to differences in terminology and ecological viewpoint. Realizing and accounting for these differences facilitates integration, so that the relative contributions of each mechanism may be evaluated. Here, we review all the mechanisms proposed to account for distribution–abundance relationships, in a framework that facilitates a comparison between them. We identify and discuss the central factors governing the individual mechanisms, and elucidate their effect on empirical patterns.