Plastic biovectoring by two closely-related gull species to coastal marshes

Plastic pollution is a global threat to biodiversity. While physical factors are well known, the role of animals in the spread of plastic is less understood. Here, we compared the role of two closely related gull species with different migratory behaviour, dispersive patterns and habitat use-the yellow-legged gull (Larus michahellis) and the lesser black-backed gull (Larus fuscus)- as biovectors of plastics to a coastal Ramsar site in southwestern Spain. We quantified and characterized plastic present in pellets of both species. We combined pellet analyses with individual movements tracked by GPS and census data to estimate the amount plastic imported, in terms of mass and number of items. In total, 89.9% of pellets contained plastics, mostly as microplastics (<5 mm), or other anthropogenic debris, with significant differences in plastic polymers and morphotypes between species. Differences in habitat selection between both species explained much of this variation: yellow-legged gulls were associated to urban habitats whereas lesser black-backed gulls relied heavily on landfills. We estimated that during winter ∼270 kg of plastics (over 8.1 million plastic particles) were transported by both species into the protected area. Lesser black backed gulls transported plastics from further away and a greater quantity than yellow-legged gull, and plastic load was spatially heterogeneous. These findings show that the two gull species act differently as biovectors of plastic pollution, linking human-modified environments with ecologically important wetlands and introducing anthropogenic plastic to these ecosystems. Understanding these pathways of plastic transport is essential for developing management strategies that address waste control.