A BROAD SCALE INVESTIGATION OF DISPERSAL CONSTRAINTS ON THE NORTHERN RANGE LIMIT OF A PACIFIC COASTAL DUNE PLANT
Species are expected to occur and persist where environments allow for population self-replacement, and be limited where biotic and abiotic conditions shift outside their recognized niche. However, many species lack the declines in fitness towards their range edges expected under niche limitation, and even show persistence when moved beyond them, which suggests a role of dispersal limitation in maintaining species' ranges. For species that exist within patchy environments, dispersal limitation can occur through increased heterogeneity of suitable habitat, rather than through an absolute barrier to dispersal. Through reductions in habitat patch frequency and size, as well as greater patch isolation, a species’ ability to successfully disperse to new habitat can decrease along a gradient, with a range limit forming where the recolonization of unoccupied patches can no longer match stochastic patch extinction. I examined the predictions of this hypothesis across the northern range of a Pacific coastal dune endemic, Camissoniopsis cheiranthifolia (Onagraceae). This species occurs in patchy dune habitat, and previous studies of beyond-range transplants strongly suggest that dispersal may limit its northern range. By quantifying geographic variation in the distribution of coastal dune habitat across the northern half of the species' range, and measuring habitat suitability and occupancy at > 7000 randomly distributed 5x5 m plots, I investigated the predictions that towards the species northern limit: (1) habitat availability decreases through reduced patch frequency and size, (2) patch isolation increases through greater inter-patch distances, and (3) occupancy and abundance within suitable habitat patches decreases in response. While coastal dune habitat increased and became more continuous towards and beyond the species northern limit, patch frequency decreased and inter-patch distance increased among suitable habitat patches, with no effect in patch size. This gradient of habitat availability resulted in a gradual reduction in patch occupancy towards the range limit, yet remained higher at the range edge than predicted under the hypothesis of constrained patch dispersal dynamics through habitat availability. Further study on the congruent effects of gradients in reduced patch recolonization and/or increased patch extinction towards the northern limit may further reveal whether dispersal is primarily limiting C. cheiranthifolia’s range.