Going against the flow: retention, range limits and invasions in advective environments

Increasing globalization has spread invasive marine organisms, but it is not well understood why some species invade more readily than others. It is also poorly understood how species' range limits are set generally, let alone how anthropogenic climate change may disrupt existing species boundaries. We find a quantitative relationship that determines if a coastal species with a benthic adult stage and planktonic larvae can be retained within its range and invade in the direction opposite that of the mean current experienced by the larvae (i.e. upstream). The derivation of the retention criterion extends prior riparian results to the coastal ocean by formulating the criterion as a function of observable oceanic parameters, focusing on species with obligate benthic adults and planktonic larvae, and quantifying the effects of iteroparity and longevity. By placing the solutions in a coastal context, the retention criterion isolates the role of 3 interacting factors that counteract downstream drift and set or advance the upstream edge of an oceanic species' distribution. First, spawning over several seasons or years enhances retention by increasing the variation in the currents encountered by the larvae. Second, for a given population growth rate, species with a shorter pelagic period are better retained and more able to spread upstream. And third, prodigious larval production improves retention. Long distance downstream dispersal may thus be a byproduct of the many propagules often necessary to ensure local recruitment and persistence of a population in an advective environment.