To most people, Pt. Conception is the place where California's coastline bends sharply to the north, creating a boundary between southern and central California. To marine ecologists, Pt. Conception is famous as a different kind of boundary -- a biogeographic boundary. Biogeography is the study of where species occur and why. No species occurs everywhere. Rather, all species have borders that enclose the range of habitats and locations where they occur. Most species have unique distributions, but in a few situations, a large number of species share common borders. Such locations create sharp boundaries between areas dominated by distinctly different species. Pt. Conception is just such a biogeographic boundary, which separates very different groups of marine species.

Although marine ecologists have known about biogeographic boundaries like Pt. Conception for over a century, the cause of such boundaries remains largely unknown. Most speculation has centered on gradients in water temperature. All of the major marine biogeographic boundaries are at points or headlands where ocean currents converge in such a way that water temperature changes abruptly along the coast. Pt. Conception typically separates the colder waters of the California Current, which flow southward along the California coast, from the relatively warmer waters of a gyre circulating in the Santa Barbara Channel. The presumption has been that species borders cluster at Pt. Conception and other marine biogeographic boundaries because some species do better in warm water, whereas others do better in cold water.

Our research, however, suggests a different cause for marine biogeographic boundaries -- the effect of currents on the movement of larvae. When an invertebrate releases her young, they begin a journey. The distance and direction they travel on this journey is largely determined by the pattern of ocean currents. Our data suggest that some destinations are essentially inaccessible. The collision of currents that creates the temperature gradient at Pt. Conception also creates a barrier to larval movement -- essentially forming a wall in the ocean. Larvae released in the Santa Barbara Channel that travel toward Pt. Conception have a difficult time rounding the point. The strong southward flows of the California Current preclude the northward movement of even the most athletic larvae. Some scenarios of future climate change predict that ocean circulation in this region may change. If such changes remove the wall at Pt. Conception, the changes in marine communities along our coast could be catastrophic.