Ocean currents and competitive strength interact to cluster benthic species range boundaries in the coastal ocean

Dispersal of many coastal marine species is mediated by flows with strong directionality; bathymetric and topographic effects lead to strong alongshore variability in this transport. Using a simple model of the population dynamics of competing benthic species in a coastal ocean, we found that alongshore variability in dispersal can lead to clustering of species range boundaries for species whose dispersal is dominated by coastal currents. Furthermore, species can be absent from areas where they would have a relative competitive advantage because the presence or absence of a species is determined not only by local conditions but also by propagule supply, which is often affected by larval transport from far upstream. Our model demonstrates the quanti tative linkages between alongshore variation in coastal currents, spatial gradients in competitive strength, and the geographic extent of a species. We show that the predictions of the model are consistent with observed species distributions in the Gulf of Maine and Mid-Atlantic Bight, USA. A mechanism for extensive coexistence of competing species where range boundaries cluster is described. The implication of the clustering highlighted by our model suggests that for species whose dispersal is dominated by long-distance planktonic periods, climate change induced changes in the relative competitiveness of species will lead to abrupt changes in species range boundaries and not gradual range extension.