LATITUDINAL VARIATION IN RANGE SIZE OF MARINE BIVALVES: NESTEDNESS AND RANGE EXPANSION IN THE TROPICS

Although geographic range size commonly varies non-randomly with latitude, the relation between the dynamics of range size variation and latitudinal diversity gradients (LDGs) remains unclear. In extant marine bivalves (1) both the maximum and median latitudinal range of species within a genus, and the median species range per 5° grid cell, all peak in the tropics; and (2) exclusively tropical genera have larger ranges than exclusively extratropical genera. Both patterns contrast with expectations that range size correlates inversely with diversity (Rapoport’s Rule). However, tropical range-size distributions also include significantly more genera with very small ranges compared to extra-tropical genera. Given that proportions of young genera are higher at low latitudes, and that genus age and range is positively correlated, these patterns suggest that higher origination rates are associated with narrow ranges. The co-occurrence of wide- and narrow-ranging taxa in the tropics causes significant nesting of marine bivalve genus and species ranges. Higher latitudes tend to contain subsets of the faunas of more diverse bands, strong support for the role of differential range expansion in forming the LDG. Large geographic ranges at low latitudes are evidently related to low variation in sea-surface temperature over wide areas, rather than wide environmental tolerances—range size is inversely related to temperature range, and genera experiencing narrow temperature ranges (“temperature-specialized”) are generally limited to the most common temperature class (28°). However, the widest-ranging genera tend to range from the tropics into higher latitudes and are relatively old. Older genera are consistently more species-rich and less temperature-specialized than young genera for their geographic range size, but their constituent species are not less specialized than species of young or narrow-ranging genera. All these factors probably reflect long-term range expansion via speciation into novel thermal environments. The dynamics of geographic ranges along LDGs are thus influenced by counteracting processes of range expansion (enhanced by low spatial and temporal variation in environment) and the formation of new taxa (originating at small range size), both occurring at higher rates at low latitudes.