Other combinations of distance-to-coast and elevation may be used as definition of coastal zones. Another approach is to measure the population living in river delta areas, which are important areas at the land-ocean interface. The indicator quantifies an important driver of coastal ecosystem pressure, and it also quantifies an important component of vulnerability to sea-level rise and other coastal hazards. Among the most important pressures are habitat conversion, land cover change, pollutant loads, and introduction of invasive species.