Evidence that metapopulation dynamics contribute to the maintenance of a species range limit

Abstract

Species range limits may be enforced by various evolutionary and ecological processes. It is thought that many range limits are imposed by low fitness beyond a species’ range or the inability to disperse to suitable habitat beyond the range. However, transplant experiments and geographic surveys often do not support these hypotheses, requiring the use of an alternative framework. The metapopulation-hypothesis proposes that declines in habitat patch colonization rates or increasing patch extinction rates may cause the metapopulation of a species to collapse, thereby generating an abrupt range limit. Using coastal dune plant Camissoniopsis cheiranthifolia, I evaluated the metapopulation hypothesis by collecting data on the availability of suitable habitat, the abundance of C. cheiranthifolia and the plant community species composition at 7,031 randomly distributed 5m x 5m plots in two separate generations. Habitat structure changed towards the range limit, including a decline in the frequency of patches containing suitable habitat, the size of habitat patches, and the quality of patches, as well as an increase in patch isolation, but no clear trend in patch turnover. Occupancy by C. cheiranthifolia was more likely in larger patches that were less isolated from other suitable patches, as well as younger patches. Patches were more likely to be colonized by C. cheiranthifolia if they were larger and surrounded by a greater abundance of C. cheiranthifolia, while the species was most likely to go extinct from patches where the population was small. Patches were less likely to be colonized towards the range limit, but extinction did not vary geographically. Lastly, using a classic metapopulation model, measured rates of colonization and extinction closely predicted the decline in patch occupancy towards the range edge. This is the first study to estimate metapopulation parameters towards a species’ geographic range edge and demonstrate that metapopulation dynamics may contribute to the maintenance of species range limits.