Seagrass ecosystems are essential components of coastal zones, due to their extremely high productivity and the high biodiversity they support. Humans depend on seagrass meadows because they serve as nutrient and carbon sinks, fishing grounds and storm buffers. However, seagrass communities are increasingly subject to multiple human-induced global changes, including on the one hand defaunation (i.e. removal of large animals from ecosystems), and on the other hand invasive species expansion. An urgent and open question is how these alterations interactively affect the functioning and services of ecosystems. I aim to clarify how large herbivores and exotic plant species affect the multiple ecosystem services of seagrass beds. To achieve this, I will compare important seagrass ecosystem services under grazed and ungrazed conditions (with Green sea turtles as large herbivores), with and without invasive seagrass, at different spatial scales. I will use a multi-disciplinary approach combining: 1) large-herbivore exclosure experiments (on Bonaire) with manipulated densities of invasive seagrass, and simultaneous measurements of 6 key ecosystem services, 2) a regional upscaling (Caribbean Sea) to examine how spatial grazing patterns of large herbivores and prevalence of invasive seagrass change underwater landscapes and seagrass bed services, 3) a literature review and 4) a global initiative to develop tools for mapping (invasive) seagrass biomass by using green turtles foraging movements. Together, this research will demonstrate the consequences of defaunation and plant invasion for seagrass bed ecosystem services. This will facilitate predicting the adaptive capacity of seagrass ecosystems, and large herbivores that rely on them, under different scenarios of human-induced global change. Furthermore, the proposed research will identify pathways for effective management of critical seagrass habitat through management of its herbivores and their predators. The knowledge gained in this research is urgently needed to safeguard human wellbeing, which depends on the provided seagrass ecosystem services.