Over the past 50 years, the population of Mauritius has nearly doubled, to 1.2 million. It currently has the highest population density of any African country, 652 people per km2. Mauritius has also seen a dramatic growth in its economy, which has increased demands on its environment. Coral reefs almost surround Mauritius. These are complex ecosystems, rich in biodiversity yet only able to survive in very clear, warm and nutrient-poor ocean waters. In these satellite images, coral reefs (yellow arrows) form a fringe along the island's shores and create shallow lagoons that are extremely important to the fishing and tourism industries. The island's population density as well as agricultural runoff, untreated sewage, changes in freshwater runoff, tourist activity, and global warming all threaten the health of the reefs.