Sea-level rise causes feeding habitat loss for migratory shorebirds in remote coastal wetlands of Brazilian Amazon

Sea-level rise (SLR) can cause significant changes in coastal wetlands, such as the retreat of coastlines and sedimentary shifts in tidal flats. In areas lacking coastal defenses, rising sea levels are expected to drive the inland migration of coastal wetlands, generally maintaining the extent of tidal flat habitats but also triggering important ecosystem changes. Migratory shorebirds are apex predators in coastal wetlands, thus being highly sensitive to such changes. Despite the worldwide decline of this group of birds, the impacts of SLR on their habitats have not been readily evaluated. In this study, we investigated how migratory shorebirds are responding to the gradual occupation of tidal flats by areas originating from marine transgression of terrestrial habitats, which is a consequence of inland migration of coastal wetlands. We conducted aerial surveys to assess the distribution of shorebirds along 630 km of tidal flats in coastal wetlands of the Brazilian Amazon. We then mapped the distribution of tidal flats in the late 1980s and for the survey period using satellite imagery to identify the tidal areas created by marine transgression over the past four decades. Finally, we sampled these areas and nearby tidal flats to assess shorebird prey abundance and sediment characteristics. We found that shorebirds avoid transgressed areas as feeding grounds, with their numbers sharply declining with the increasing occupancy of this habitat. The dominant shorebird species, the semipalmated sandpiper (Calidris pusilla), presented densities one order of magnitude lower in transgressed areas than in other tidal flats, indicating a clear response to the reduced availability of its main prey, the crustacean Discapseudes surinamensis. We conclude that, although inland migration of coastal wetlands may preserve the extent of tidal flats over time, their increased occupation by transgressed areas can lead to significant losses in feeding habitat for migratory shorebirds.