Body size and abundance are decoupled from species richness in Australian marine bivalves

Abstract

The “more individuals hypothesis” suggests that increases in the total number of individuals per species leads to increases in community richness. Abundance, body size distributions and richness do vary with latitude in several taxonomic groups. However, support for this hypothesis has otherwise been mixed. In this paper, we investigate latitudinal changes in all three variables for marine bivalves along the eastern coastline of Australia. We utilise a large, uniformly sampled field dataset of 5670 shells representing 157 species that spans 20º of latitude and crosses a major biogeographic transition. For each of 15 field sites, 10 quadrats were randomly placed and completely sampled, making it possible to quantify total abundance. Species richness was calculated using a new estimator based on the geometric series distribution. Body size was computed as the geometric mean of length and width. Despite uncovering a strong latitudinal gradient in species richness, we found no significant gradient in total abundance and body size at any taxonomic level. Although previous work found family-level trends in bivalve size, it was done at a larger spatial scale and therefore did not pertain to individual communities. Environmental variables do correlate with both abundance and richness, but they are not directly related. Because we find no gradient in abundance and no relationship between abundance and richness, we reject the “more individuals hypothesis” for our system. Instead, latitudinal richness trends in coastal Australia may result from an environmental gradient in dispersal constraint.