Age-specific density-dependent survival in Mediterranean Gulls Larus melanocephalus

Survival and reproductive rates often decrease with increasing population density. Such negative density dependence reflects a changing net balance between the benefits and costs of presence of others with increasing density. When densities are low, however, survival and reproductive rates might increase rather than decrease with increasing density (Allee effect), for example in colonial species. Relationships between fitness and density are therefore expected to change with population density. We studied the relationship between apparent annual survival and population density in a population of Mediterranean Gulls Larus melanocephalus from establishment onwards for a period of 13 years, using capture–recapture techniques. The results confirmed our expectation: apparent survival increased with density at low densities (i.e. an Allee effect), but survival decreased with density at high densities. Post-hoc analyses revealed that the Allee effect could only be shown for juveniles, not for adults, suggesting that the Allee effect might have been age-specific. These patterns in apparent survival reflect the combined effects of density on true survival and on permanent emigration, and both mechanisms are discussed