During the night, the dayshift sleeps. This sharing of quarters allows a reef to shelter two separate populations of marine organisms. Coral reefs are tightly woven ecosystems, with incredibly complex interlinkages. Remove, or disturb one key component of the reef's fabric, and it may trigger a nasty chain reaction of events. Parrot fish and urchins are the sanitary engineers of the reefs, cleaning the coral of excess algae.
Sponges are also keystone species.
They act as the lungs of the reef, cleaning and recycling the water. Remove them and a crucial ecosystem function is lost, with consequences for the entire community. In a clear coral lagoon, north of Mombasa, Kenya, Rodney Salm, a marine scientist working for the World Conservation Union, takes time out from his reef survey to explain why coral reefs are so diverse and productive. Salm, lean and tall, his skin burnished bronze from years of work in the tropics, has studied reefs for two decades. In a sense, they act like a very efficient production system.