The majority of coral reefs are marine ecosystems that are distributed in the tropics and low-latitude subtropics, although they can also survive at higher latitudes if warm currents are present. 20°C is considered the minimum temperature for sustained coral reef development. Optimal coral reef development is usually found at sites with clear water with low nutrient concentrations and at sites with relatively shallow depths. Coral reefs consist of individual coral animals or polyps that usually occur in colonies and have a calcareous skeleton. For their nutrition they depend heavily on microscopic algae (zooxanthellae) that live within their tissues and produce carbohydrates (saccharides) on which the coral feeds, although most corals can switch to an active feeding mode under certain conditions. It is this relationship that drives the necessity for light in corals and the energy generated allows for the accretion of the structural framework that makes a coral reef. There are well over 500 species of hard corals in the Indo-Pacific alone, 400 being the magic number that qualifies a country to be part of the “Coral Triangle”.
The ecosystem services of coral reefs are well documented, and include production services and coastal protection from wave attack. The concept of pioneer and climax species originated in plant literature and has been successfully used in other Building Blocks (e.g mangroves, shellfish and seagrasses), but is poorly applicable to corals. Instead, a classification is made based on the behaviour of groups of species to disturbance, competition and stress. These are the three major factors that limit diversity and abundances in natural reefs. The species are categorized in ruderals (initial colonizers), competitors and stress-tolerators. The relative abundance of each of these species can be indicative of succession state of a reef and the disturbance regime. Species within this classification can therefore be selected and added to the coral reef to optimize the chance of survival of the reef under a range of environmental conditions.