VALENTINE, James W., Museum of Paleontology, University of California, Berkeley, CA 94720-4780; ROY, Kaustuv, Dept. of Biology, University of California, San Diego, La Jolla, CA 92093; JABLONSKI, David, Dept. of Geophysical Sci., University of Chicago, Chicago, IL 60537; and ROSENBERG, Gary, Academy of Natural Sciences, Philadelphia, PA 19103
Although the question of the regulation of species diversity is central to much of ecological and macroevolutionary theory, the nature of the regulatory processes remains in dispute. Latitudinal diversity gradients are first-order expressions of diversity patterns. The several current hypotheses that seek to explain them are based chiefly on terrestrial data. We have amassed a database of the geographic ranges of 3,916 species of marine prosobranch gastropods living on the shelves of the western Atlantic (WA) and eastern Pacific (EP) Oceans, from the tropics to the Arctic Ocean. WA and EP diversities are similar, and indeed the diversity gradients are highly correlated despite many important physical and historical differences between the oceans. This shared diversity pattern cannot be explained by: (a) latitudinal differences in species range-length (Rapoport's rule), for the gradient is present whether species ranges average longer or shorter when proceeding to lower latitudes; (b) species-area effects, for the areas of the diverse low-latitude shelves are smaller than those of the depauperate high-latitude shelves, and provincial areas and diversities are not significantly correlated; or (c) recent geologic histories, for late Neogene extinctions and recoveries have been perhaps twice as high in the WA, including tropical latitudes, than in the EP.
One parameter that does correlate significantly with diversity in both oceans is average sea-surface temperature (SST). We infer that SST is an energy parameter; if this correlation is causal, SST is probably linked to diversity through some aspect of productivity. In this case, diversity is an evolutionary outcome of trophodynamic processes inherent in ecosystems, and not just a byproduct of physical geographies.