A broad-scale test of dispersal constraints on the northern range limit of a Pacific coastal dune plant
Presented by: Mike Dungey from Queen’s University
October 29th from 12:30pm – 1:30pm EST
Species are expected to occur where environments allow populations to achieve self-replacement and be limited where biotic and abiotic conditions shift outside their recognized niche. However, many systems lack the declines in fitness towards their range edges expected under niche limitation and even show persistence beyond them, suggesting a role of dispersal limitation in maintaining species’ ranges. For species that exist within patchy environments, dispersal limitation can occur through an increase in the heterogeneity of suitable habitat, as opposed to absolute dispersal barriers. Reductions in habitat patch abundance and size, and higher isolation between patches can increase the cost of dispersal along a gradient, with a range limit forming where colonization can no longer match stochastic patch extinction. We tested 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 constraints could limit its northern range. By surveying all coastal dune habitat in the northern half of the species’ range, and quantifying habitat suitability and occupancy and at > 7000 randomly distributed 5×5 m plots, we tested the predictions that towards the northern range limit: (1) availability of suitable habitat decreases, (2) distances between habitat patches increases, and (3) occupancy of suitable habitat patches decreases. Our results suggest that available, suitable habitat for C. cheiranthifolia declines with increasing proximity to the range limit, despite an increase in coastal dune systems across the species northern range. These results suggest that dispersal constraints through reduced dispersal success may play a role in maintaining this species stable range limit.