Foraging in the Anthropocene: Feeding plasticity of an opportunistic predator revealed by long term monitoring

For centuries, human activities have altered the population dynamics of wildlife. New anthropogenic food sources provide a predictable and abundant food supply that often induces very significant changes in the size, distribution, and behaviour of many populations, with ultimate consequences on the structure and functioning of natural ecosystems. Here, we combine historical and contemporary feather samples of a population of a superabundant, opportunistic predator, the yellow-legged gull Larus michahellis, to assess its trophic ecology and relate it to human activities in the long term. Dietary assessments were based on stable isotope analysis of carbon, nitrogen, and sulphur (δ13C, δ15N and δ34S), and were conducted through three end-point (marine prey, waste from landfills-slaughterhouses, and terrestrial invertebrates) Bayesian mixing models. Our results suggest that gulls’ diet showed a progressive decrease in the consumption of marine prey throughout the most recent period (late 20th century onwards), linked to an increase in the consumption of meat waste and small terrestrial invertebrates. Reported dietary changes over the sampling period correlated positively with the availability of marine resources around the breeding area. We provide evidence suggesting that the ability of gulls to exploit efficiently diverse anthropogenic food subsidies likely resulted in the exponential demographic increase of this population throughout the 20th century. In addition, current regulations affecting the availability of these food resources (e.g., fishing discards and landfill waste) likely reversed this trend over the last decade. Long-term evidence of population trophic plasticity, like the one we present here, is essential to implement and support management and conservation actions that limit the availability of anthropogenic resources, especially when it comes to superabundant, problematic species.