Human disturbance and the chick-rearing ability of African black oystercatchers (Haematopus moquini): a geographical perspective Original Research Article

African black oystercatchers (Haematopus moquini) breed on the open coast of the southwestern Afrotropics at the height of the summer tourist season. They are thus vulnerable to effects of human disturbance, including destruction of nests by people and vehicles and predation of eggs and chicks by domestic dogs. Because they forage exclusively intertidally, disturbance may further result in adults having insufficient time to satisfy both their own energy demands and those of their young. To test whether vulnerability to this form of disturbance varies regionally, in response to regional variation in foraging conditions, foraging times of adult oystercatchers were recorded on rocky shores at four sites around the South African coast that experience little or no human pressures. At the same time, a measure of prey abundance was obtained at each site for the dominant prey types — mussels and prosobranch limpets. Time spent foraging per day increased on a west–east axis. This pattern could not be explained from simple measures of food abundance, but does parallel a west–east decrease in intertidal primary productivity and hence in the biomass of grazing invertebrates, such as limpets, as well as a west–east decrease in oystercatcher density. A simple time-energy model was constructed to estimate the additional foraging time that adults would need to meet the energy demands of chick-rearing. At south coast sites, where adult intake rates are relatively low, they regularly experience difficulties in rearing two chicks under undisturbed conditions, and their ability to rear even a single chick could be compromised by fairly low levels of human disturbance. Both tourism and coastal development are growing rapidly in South Africa, and tourist activity on the south coast is greater than on the west coast. Oystercatchers have many attributes favouring their use as bio-indicators of coastal disturbance, but high adult survivorship makes census data alone a poor predictor of future population trends.