Human activities have opposing impacts on Mediterranean Yellow-Legged Gull (Larus michahellis) breeding populations

Abstract

The islands of the Aegean Sea (Greece, NE Mediterranean Basin) are areas of high biodiversity and endemism, and harbor globally important seabird communities. Resident seabirds breed on offshore islands where they often form strong nesting colonies. Breeding seabirds are important determinants of island ecosystem function while also being subject to a plethora of human activities. Understanding how anthropogenic activities impact such colonies is not just essential for seabird conservation but is also critically important for the management of small insular ecosystems and the native species communities they support. This study aims to quantify the effect of relevant human activities on the size and locations of Yellow-legged Gull (Larus michahellis) colonies, a generalist gull species native to the western Palearctic that is the most abundant among resident seabirds. We censused gull colonies from 152 islands located in the Cyclades and Sporades archipelagos. We also gathered data on variables suspected to influence seabird colonies, including physical islet characteristics, resource availability (e.g., open-air landfills and fisheries activity), and type and extent of human disturbance. Analyses were conducted on the local (islet) and on the regional (island cluster) levels to identify proximate and ultimate factors shaping the density and breeding population sizes of resident gull colonies. Our results reveal divergent impacts of human activities in resident gull populations. On the local level we identify a clear negative effect of the presence of invasive rats (Rattus sp.) on gull nesting density. Similarly, presence of feral grazing mammals such as goats (Capra hircus) and rabbits (Oryctolagus cuniculus) had negative impacts on gull populations, an effect that appears to be primarily mediated through nest disturbance rather than through vegetation degradation. Access to landfills and fishing vessels both had positive impacts on gull nesting density. Presence of olive groves was also positively associated with the size of resident Yellowlegged Gull populations, highlighting the role of these anthropogenic food resources in local gull diets. Our results suggest approaches to manage Yellow-legged Gull populations in the Mediterranean Basin by taking into consideration the roles of introduced mammals and fishing activities on seabirds in the region.