Nutrient flows and biogeomorphic feedbacks: linking seabird guano to plant traits and morphological change on sandy islands

Abstract. Vegetated coastal landscapes are crucial for carbon storage, shoreline protection, and biodiversity. Their structure emerges from biogeomorphic feedbacks between vegetation growth and sedimentation, shaped by environmental conditions. Allochthonous nutrient inputs, particularly seabird guano, can significantly influence plant growth and distribution, potentially altering these feedbacks. This suggests that coastal birds may actively shape their own habitat by modifying plant-sediment dynamics. Yet, as sea-level rise and coastal squeeze reduce available habitat for already declining bird populations, understanding these interactions becomes increasingly urgent, particularly on small, uninhabited islands that are geomorphically dynamic and whose nutrient budgets are dominated by allochthonous rather than locally produced nutrients. Despite this, spatially explicit studies on bird–plant–sediment interactions remain lacking. This study addresses that gap by examining how guano deposition influences plant traits, community composition, and landscape morphology. We combined fine-scale field data with remote sensing and spatial modelling to assess guano effects on vegetation and sedimentation. Field measurements included plant traits, community composition, environmental variables, and δ15N to trace guano uptake. A guano dispersion model was linked to PlanetScope and LiDAR data, and Bayesian models (INLA) revealed spatial links between guano, vegetation change, and sediment accretion. Results show that guano-derived nitrogen promotes shifts in species composition toward later-successional, sediment-stabilizing species, particularly on sandy soils with low baseline nutrient levels. Guano enhanced early-season vegetation productivity, increasing sediment retention, but seasonal differences and local environmental context modulated these effects. We propose that seabirds act as indirect ecosystem engineers by fuelling vegetation–sediment feedbacks. Changes in breeding pair numbers may therefore influence coastal landscape evolution, and ultimately, shape the very habitats these birds depend on.