Birds influence vegetation coverage and structure on sandy biogeomorphic islands in the Dutch Wadden Sea

Small uninhabited islands form important roosting and breeding habitats for many coastal birds. Previous studies have demonstrated that guano can promote ecosystem productivity and functionality on island ecosystems. Here, we assess the role of external nutrient input by coastal birds on the vegetation structure and coverage on sandy biogeomorphic islands, where island-forming processes depend on vegetation-sedimentation feedbacks. As a first step, we investigated whether breeding birds affect vegetation productivity on sandy back-barrier islands in the Wadden Sea. Using a combination of bird observations and plant stable isotope (δ¹⁵N) analyses, we demonstrate that (i) breeding birds transport large quantities of nutrients via their faecal outputs to these islands annually and that (ii) this external nitrogen source influences vegetation development on these sandy, nutrient-limited, islands. Based on these results we discuss how this avian nutrient pump could impact island development and habitat suitability for coastal birds and discuss future directions for research. In general, we conclude that avian subsidies have the potential to affect both the ecological and biogeomorphic functioning of coastal soft-sediment systems. However, the strength and scale of especially these biogeomorphic interactions are not fully understood. For the conservation of both threatened coastal birds and sandy back-barrier islands and the design of appropriate management strategies, we argue that three-way interactions between birds, vegetation and sandy island morphodynamics need to be further elucidated.