Coral reefs are the most diverse of all marine ecosystems. They are home to almost one quarter of all ocean species. Coral reefs are also important to people, providing food, protection of shorelines, tourism jobs, and even medicines. However, this is a fragile ecosystem, threatened by human activity and environmental changes. Which of these threats is not caused by abiotic factors? A) Coral reefs are affected by overfishing. B) Coral reefs are affected by changing ocean chemistry. C) Coral reefs are affected by global warming due to increases in greenhouse gases. D) Coral reefs are affected by pollution due to runoff of chemicals into the ocean.