Are African Black Oystercatchers good indicators of large-scale trends in intertidal communities? A stable isotope study

摘要

The African Black Oystercatcher Haematopus moquini is a resident shorebird confined to the coasts of Namibia and South Africa, where it feeds exclusively in intertidal zones. Globally, oystercatchers are known for their specialized feeding habits; however, their potential as bio-indicators has not yet been investigated. The southern African coastline is influenced by the Benguela Upwelling System in the west and the Agulhas Current in the south, resulting in major differences in ocean productivity. We investigated biogeo-graphic trends in stable isotope composition of the African Black Oystercatcher to test whether the observed patterns in isotopic signatures of the primary food of filter-feeders have deeply penetrating effects, transmitted to top-predators. Blood samples and feathers of oystercatchers were collected from breeding adults and chicks feeding at 13 rocky shore locations throughout the range of the species, between East London, on the south-east coast of South Africa and Halifax Island, off Laderitz, S Namibia. Main prey species (mussels and limpets) were collected at all sampling sites. Oystercatchers and their prey showed an overall westward enrichment in 615N values reflecting a shift at the base of the food web, from oligotrophic conditions on the east coast to eutrophic conditions on the west coast. The 613C values of oystercatcher tissues did not show clear geographic gradients but rather reflected variations in prey assemblages along the coastline. Blood and feathers of adults displayed strong geographic correlations based on their 613C and 615N composition, suggesting high fidelity to diet and/or feeding area throughout the year. We suggest that African Black Oystercatchers are excellent indicators of local and biogeographic trends on the southern African coastline, both in terms of intertidal community structures and oceanic conditions throughout the year.