Migration models for coastal transition communities subject to sea level rise

Coastal landscapes are naturally dynamic, in a state of flux with changing environmental conditions. Rapid sea level rise is affecting coastal ecosystems along low-lying coastlines around the world. Sea level rise is projected to increase over the upcoming decades, threatening coastal landscapes of the southeastern United States, where the aquatic-terrestrial transition occurs over small and gradual elevations.  Coastal species may cope with environmental changes over the short term, but eventually they will be eliminated unless migration occurs inland. Migration depends on the ability to colonize environmentally suitable areas.  We have developed and are testing conceptual models for responses of coastal assemblages to chronic sea level rise coupled with natural disturbances. Our empirical field studies at Weeks Bay, Alabama, a site disturbed by Hurricanes Ivan and Katrina, are the first to describe the coastal transitions and attempt to elucidate controls on migration in these systems.  Our results indicate that hurricanes disassemble coastal communities in different ways along the aquatic to terrestrial transition, and thus provide different backdrops for reassembly. We have found widespread disassembly and species losses, but also additions of species from newly uncovered seedbanks and the "wrack bank" deposited by Katrina's storm surge.  Some species distributions have contracted, others have stayed the same, while others have expanded.  Some salt-tolerant species have shown inland shifts consistent with disturbance-catalyzed release of biotic constraints to migration.  We suggest that natural disturbances can play an important role in facilitating migration of species responding to changing sea level.