Macroecology of marine mammal species in the Mexican Pacific Ocean: diversity and distribution

The Mexican Pacific Ocean, defined as east of 122 degrees W and from 12 to 34 degrees N, encompasses an oceanographic transition zone with dynamic population and community processes. In order to gain insight into possible marine mammal ecological responses to the current rapid environmental changes, we compiled and analyzed data from 11 145 marine mammal sightings made between 1981 and 2008 by the National Oceanic and Atmospheric Administration, the Universidad Autonoma de Baja California Sur, and the Universidad Nacional Autonoma de Mexico. In 2 x 2 degrees cells, we computed alpha and beta diversities in terms of species richness, commonness as the occurrence of species among years, tropicality as the proportion of tropical species, and degree of threat according to the IUCN Red List. Current marine mammal distributions seem to derive from a postglacial northwards expansion of tropical pelagic species and a retention of upwelling-rich tropical and subtropical habitats nearshore by antitropical species. Analysis of commonness indicates that species distributions and thus biogeographic boundaries are dynamic, since marine mammals transit large distances, especially around southern Baja California. Analysis of beta diversity showed a considerable degree of spatial heterogeneity, but without well-defined regions. The species at risk are concentrated in the northern Gulf of California, but also occur off Pacific Baja California, Jalisco, and Guerrero coasts. The Gulf of California could be considered a refuge of global value for these animals.