Differential responses of three sympatric seabirds to spatio-temporal variability in shared resources

Abstract

Environmental change linked to climate and human activities may affect top predators via the food chain in marine systems. Understanding the functional link between resources and predator responses (e.g. foraging effort, reproductive success, population growth) is of crucial importance in the context of an ecosystem-based approach to fisheries. In the Benguela upwelling ecosystem located off the South African coast, both climate and overfishing may have modified the spatio-temporal availability of sardines and anchovies over the past 2 decades. These fish are prevalent prey items for 3 seabirds: the African penguin Spheniscus demersus, Cape gannet Morus capensis and Cape cormorant Phalacrocorax capensis. Species-specific responses of these 3 sympatric seabirds to the fluctuations in their shared prey are poorly understood. Here we used generalized additive modelling to examine the ‘local population response’ (i.e. number of breeders) of the 3 seabirds to spatio-temporal variations in prey availability from 1987 to 2007. We showed that prey fluctuations influence seabird local population responses, presumably by modulating recruitment, breeding decision and breeding performance. We also identified speciesspecific population responses to prey availability: positive in African penguins and Cape gannets, and null in Cape cormorants. These patterns could be explained by seabirds partitioning prey in time via separated breeding periods, and space via accessing different foraging grounds due to specific feeding techniques. These responses also suggest different degrees of dependency upon pelagic resources among predators, which are likely due to contrasting foraging ecologies (e.g. dietary plasticity, foraging techniques, foraging ranges).