INCUMBENCY AND LATITUDINAL DIVERSITY GRADIENTS

In the sea, the latitudinal diversity gradient (LDG) in bivalve genera and subgenera (hereafter, genera) largely arises through a high lineage origination rate in the tropics and a poleward spread of the lineages, which retain a tropical presence; this dynamic creates a rich tropical diversity. While the continuing tropical originations impart a relatively young average age to genera there, average generic age increases towards the poles. The low standing diversity and old genus ages at high latitudes might result from (a) the time required for taxa originating in the tropics to expand their ranges to the poles, (b) exclusion of taxa by abiotic aspects of environmental harshness; or (c) resistance by incumbent taxa to invading clades. We propose that a gradient in some diversity-dependent factor(s) creates a gradient in carrying capacity, filtering the poleward lineage spread by inhibiting invasion of the younger taxa. We eliminated factors whose effects do not correlate with the LDG. The narrowing of peak primary productivity with increasing seasonality at higher latitudes provides a challenge, to which lineages appear to respond by evolving more generalized adaptations to feeding strategies and habitat requirements, two key diversity-dependent factors that may set carrying capacities. On average, higher-latitude taxa thus require larger shares of these resources than do lower-latitude forms, and communities of incumbents would be resistant to invasion by lineages adapted to conditions at lower latitudes. We found for the late Cenozoic that bivalve generic local and global extinction rates are similar in tropical and polar latitudes, but apparently somewhat higher in mid-latitudes, so extinctions are not a source of the LDG. In this hypothesis, the role of extinction in the LDG is to push diversity below the carrying capacities and thus create opportunities for poleward spreading of younger lineages.