THE PEAK OF THE LATITUDINAL DIVERSITY GRADIENT IN LIVING AND FOSSIL MARINE BIOTAS

The latitudinal diversity gradient is the fundamental spatial distribution of living species and, although data are somewhat limited, a gradient seems to have been present continuously throughout the Phanerozoic Eon as well. Data for several higher taxa, including brachiopods, bivalves, crinoids, and corals, show that peak diversity tracked the region of greatest continental shelf area as it shifted through time. This may indicate either that the present-day gradient, which, in contrast, is commonly assumed to peak at the equator, is an ephemeral feature of the biota that does not represent normative diversity dynamics, or it may indicate a substantial geographic bias in the fossil data. If the fossil data are unbiased, then they predict that peak diversity of living marine taxa should now occur in Northern mid-latitudes, where shelf area is greatest, rather than at the equator. Unfortunately, there have been no studies that have explicitly determined this value for comparison. Here, we have attempted to close this gap by determining the latitude of peak diversity for living marine taxa, using a database of 149 present-day gradients. We find that peak biodiversity currently occurs between 10-20°N, even after correcting for a Northern hemisphere sampling bias. Moreover, this peak position is a global phenomenon: it is found across habitats, higher taxa, ecological groups, and evolutionary faunas, and within all sampled ocean basins and on both sides of the Atlantic and Pacific oceans. This peak does not coincide with the greatest continental shelf area, as seen in the fossil data, but instead seems to correspond to the position of the intertropical convergence zone, whose mean latitude (currently about 10°N) is displaced away from the geographic equator due to the distribution of continents and the presence of ice caps. It may be that the modern marine biota has been structured by the climate of the modern ice age, which has overwhelmed other factors such as shelf area that exerted control during more equable times. Whatever its ultimate cause, the position of peak marine diversity at 10-20°N should be considered a fundamental feature of the present-day latitudinal diversity gradient and factor into hypotheses of the gradient’s origin.