Carnivore/non-carnivore ratios in northeastern Pacific marine gastropods

For 2321 species of shelled gastropods of the northeastern Pacific, the ratio of carnivorous to non-carnivorous species (C/NC ratio), computed for each degree of latitude, reveals striking spatial changes, with tropical and arctic areas characterized by high values and with the mid-latitudes having the lowest ratios. This latitudinal trend is markedly different from trends for terrestrial clades. The zonal variation in C/NC ratios within bins is largely due to differences in geographic ranges of the groups; for example, tropical carnivorous species range farther than non-carnivorous ones, thus overlapping them in more latitudinal bins. Differences in the distribution and diversity of carnivorous and non-carnivorous species may arise from a number of sources, including variability of primary production in the tropical eastern Pacific, patchiness of substrates to which non-carnivores are adapted, narrow dietary specializations of tropical carnivores, and higher provinciality found in extratropical regions.