Individuals within populations: No evidences of individual specialization in the trophic habits of an opportunistic predator

Abstract

Human activities can affect species in different ways, with some species being unable to cope with the human-driven changes, while others can persist and even benefit from these alterations. The main factors explaining the population dynamics of successful species include their use of food resources derived from human activities and the adaptability in their feeding behavior. Among marine predators, some gull species of the genus Larus, such as the yellow-ledged gull Larus michahellis, are particularly successful within the current context of human-induced global change. While the yellow-ledged gull is considered a generalist and opportunistic predator at the population level, some individuals may show specialisation in feeding habitat and diet. Here, we examined the degree of individual specialisation in the trophic habits and temporal variation in food resource utilisation within a population of this gull in the southwestern Iberian Peninsula. We analysed the isotopic composition (δ15N and δ13C) in blood and different feathers molted throughout the annual cycle for reconstructing the diet of individuals and quantifying the degree of individual specialisation. Individuals of this species in the studied population preferably fed on demersal fish throughout the entire annual cycle, with a low consumption of terrestrial prey and human-related resources. Isotopic values also revealed a generalist feeding behavior in this gull species with a very small proportion of individuals acting as specialists throughout the annual cycle. Taking into account the generalist strategy of this population, management measures should mainly be focused on controlling the availability of demersal sources from fishery discards. Future research should prioritise multispecific approaches to study how general or flexible the behavior is in this winning species.