Corals are simple, clonal invertebrates that serve as ecosystem engineers, building living structures (reefs) so large that they can be seen from space. These structures, which rival the greatest feats of human engineering, are powered through symbiosis with single-celled algae that are housed within the coral animal. This coral-algal cooperation facilitates a productive ecosystem that can grow in the nutrient-poor 'desert' of isolated tropical seas. The rich structural complexity provided by the coral's hard bodies gives shelter to many other species of plants and animals making coral reefs among the Earth's most biologically diverse ecosystems, harboring hundreds of thousands to millions of species worldwide (Figure 1).

Coral reefs also support human societies by providing critical sources of protein, protecting coasts from damaging waves, attracting tourists, and serving as the backbone of the economies for many tropical islands. In addition, coral reefs are crucial in the fight against human diseases, as many of the plants and animals that live on coral reefs produce chemicals that are useful as pharmaceuticals. Reefs have also fascinated naturalists and scientists for centuries. Before publishing his

Figure 1 Coral reefs, like this one in the Indo-Pacific, harbor hundreds of thousands to millions of species worldwide. Photo credit M.E. Hay.

Landscape Ecology of Coral Reefs: Connections of Coral Reefs to Mangrove and Seagrass Systems Geographic Distribution of Coral Reefs Threats to Coral Reefs Summary Further Reading groundbreaking work on natural selection, Charles Darwin published a treatise on reefs in 1842 hypothesizing that coral atolls (rings of reefs in the deep tropical Pacific) were formed around mountain tops as these mountains sank back into the Earth's crust under their own weight. It was more than 100 years before drilling technologies developed to the point where this hypothesis was tested - like with so many other aspects of Darwin's writings, he proved to be correct. For modern ecologists, reefs serve as a model ecosystem for developing basic hypotheses about the ecology and evolution of population structure, of community organization, and about how species diversity evolves and is maintained. In addition, reefs give us a glimpse of the spectacular record of Earth's history because the hard skeletons of corals fossilize to provide a long record of changes in coral distribution and abundance and also record chemical signals of past climatic events, like temperature and sea-level changes. Thus, reefs not only feed and protect humans and other species, but also provide a valuable window into our past, including how our present activities may be changing our environment, and possibly our future.

In this article, we review the major ecological interactions that shape coral reef ecosystems. We pay particular attention to (1) the dynamic relationship between corals and the symbiotic algae living within their tissues, (2) the role of reef herbivores in protecting corals from being overgrown by seaweeds, (3) the numerous ecological processes such as predation, competition, recruitment of juvenile reef organisms, and disturbance that influence the structure of coral reefs, and (4) the dynamic ecological connections between reefs and nearby ecosystems such as seagrass beds and mangrove forests. Finally, we review the current dangers to coral reefs and how these threats undermine the ecological integrity of these diverse ecosystems.

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