A: Corals are called ecosystem engineers because they build the three-dimensional structure known to us as a coral reef. Other organisms also contribute to this structure, such as stony (calcareous) seaweeds and sponges, but corals are by far the most important.
You can think of a coral reef as being like a city. At any point in time, part of the city is going up (new construction) and part of it is going down (demolition). Corals provide most of the new construction, and many organisms tear down coral skeletons by chewing on them, either to eat the coral tissues or else to eat other organisms growing on dead corals. These “bioeroders” – fishes, urchins and sponges – would slowly but surely turn a coral reef into a big pile of sand if no new coral growth occurred.
What would happen then? Coral reefs are the most diverse marine ecosystems on the planet. We don’t know how many species live on reefs, but estimates range from about one to nine million. Some of these species must live on reefs; others prefer reefs but can probably live in other places as well. The problem is that we don’t really know how many species fall into each of these two categories. It is safe to assume that if corals vanished, then coral reefs could not persist and many organisms would go extinct.
As to the health of the oceans more broadly, other habitats depend on coral reefs. For example, both mangroves and sea grass beds depend on the physical protection provided by coral reefs, which absorb the energy of large storm waves and even tsunamis. These habitats would thus also suffer.
And people would be huge losers! We depend on the food and other services that reefs provide, such as shoreline protection, free of charge. Also, the extraordinary beauty of coral reefs would be lost to the detriment of the tourism industry and everyone who appreciates the exquisite complexity and indescribable beauty of these habitats.
— Nancy Knowlton, professor of marine biology and director of the Center for Marine Biodiversity and Conservation.