Short-range dispersal maintains a volatile marine metapopulation: the brown alga Postelsia palmaeformis.

The annual brown alga Postelsia palmaeformis is dependent for its survival on short-distance dispersal (SDD) where it is already established, as well as occasional long-distance colonization of novel sites. To quantify SDD, we transplanted Postelsia to sites lacking established plants within ≥10 m. The spatial distribution of the first naturally produced sporophyte generation was used to fit dispersal kernels in a hierarchical Bayesian framework. Mean dispersal distance within a year ranged from 0.16 to 0.50 m across sites; 95% of the recruits were within 0.38-1.32 m of the transplant. The fat-tailed exponential square root kernel was the best among the candidate models at describing offspring density and dispersal. Independent measurements of patch size over two to five generations permitted an evaluation of whether models parameterized by individual-level data could adequately predict longer-term persistence and spread at the patch scale. The observed spread rates generally fell within the 95% predictive intervals. Finally, Postelsia was eliminated from 14 occupied sites that were then followed for ≥27 yr. The probability of invasion when unoccupied declined and the probability of extinction when occupied increased with distance from the nearest propagule source. Sites >10 m from a source were rarely invaded, and one initially densely populated site isolated by 39 m has remained Postelsia-free since 1981. In spite of dispersal that is almost entirely within 2 m of the parent, the ability of our models to capture the observed dynamics of Postelsia indicates that short-range dispersal adequately explains the persistent and thriving Postelsia metapopulation on Tatoosh Island. However, the presence of Postelsia over a 2000-km coastal range with many gaps >1 km makes it clear that rare long-distance dispersal must be required to explain the geographic range of the species.