Global Gradients in Intertidal Species Richness and Functional Groups

Whether global latitudinal diversity gradients exist in rocky intertidal alpha-diversity and across functional groups remains unknown. Using literature data from 433 intertidal sites, we investigated alpha-diversity patterns across 155 degrees of latitude, and whether local-scale or global-scale structuring processes control alpha-diversity. We, furthermore, investigated how the relative composition of functional groups changes with latitude. alpha-Diversity differed among hemispheres with a mid-latitudinal peak in the north, and a non-significant unimodal pattern in the south, but there was no support for a tropical-to-polar decrease in alpha-diversity. Although global-scale drivers had no discernible effect, the local-scale drivers significantly affected alpha-diversity, and our results reveal that latitudinal diversity gradients are outweighed by local processes. In contrast to adiversity patterns, species richness of three functional groups (predators, grazers, and suspension feeders) declined with latitude, coinciding with an inverse gradient in algae. Polar and tropical intertidal data were sparse, and more sampling is required to improve knowledge of marine biodiversity.