Effects of anthropogenic food resources on yellow-legged gull colony size on Mediterranean islands

Yellow-legged gull Larus michahellis populations have been studied on three archipelagos consisting of 20 islands distributed along 80 km of the French Mediterranean coastline. Population changes were analyzed between 1920 and 2006. In the first decades following their settlement on these islands, the yellow-legged gull populations showed a continuous exponential growth in the three archipelagos, in agreement with an annual geometric growth rate lambda above 1. The population growth ceased to fit this model during the 1980s for the older colonies (Riou and Hyeres Islands archipelagos). Thus, we focused on population changes occurring during the period 1982-2000, a pivotal period for which we have both precise census and anthropogenic food resource data, in order to determine environmental factors influencing these population changes using multiple linear regression models. An average annual growth rate of colony size was 1.02 for the last two decades. The changes in landfill availability, the gull density in 1982, and the nesting area in 1982 explained 84.4% of variation in colony size changes between 1982 and 2000. The yellow-legged gull changes on the islands in the last two decades increased as availability in anthropogenic food resources increased near the colony (positive Delta K). As a consequence, given no reduction in landfill activity or in accessibility for gulls, we expect this region to sustain continuous species expansion in the future.