Bidirectional changes in seabird populations drive terrestrial ecosystem function and biodiversity

Seabird populations fluctuate dramatically in response to perturbations and human activity, such as the current highly pathogenic avian influenza (HPAI) panzootic and fishery practices. Seabirds vector marine nutrients into terrestrial and coastal environments causing cascading effects through ecosystems, but quantifying the inter-related mechanisms underpinning marine-terrestrial cascades in response to changes in seabird populations is challenging. Here we simulate decreases and increases in seabird density, as have occurred due to HPAI and fishing practices respectively, through turf transplant experiments and stratified surveys to quantify marine nutrient cascades and show how they regulate terrestrial ecosystem processes and biodiversity. We use structural equation modelling to quantify the relative strengths of each hypothesised mechanistic pathway in the ecological network. This analysis identified that seabirds shape terrestrial plant and invertebrate biodiversity primarily indirectly, through modifying soil nutrient availability. Our findings highlight the ecosystem changes that are likely occurring, and the pathways through which they operate, due to acute declines and chronic increases in seabirds caused by HPAI, fishing practices or other drivers.