A history of exposure to high temperatures can influence the thermal tolerance of corals and their resistance to bleaching.

Parts of reefs that regularly experience heat stress conditions, such as reef flats and crests, may be populated by corals that are more resistant to bleaching.

For example, corals on reef flats are exposed to air during the lowest tides. These corals may be accustomed to extreme stress from heat, desiccation, high salinity, or tropical downpours. This might be why corals in some inner reefs appear less susceptible to bleaching than the same species growing in deeper waters. Check for living corals on reef flats and reef crests. Important areas to protect are those with multiple species and a wide range of sizes. Size often relates directly to the age of the coral, and thus a wide range of sizes may indicate a wide range of ages and serve as a proxy for survival prospects through bleaching events.

Coral species that tend to be more resistant can still bleach, but surveying species composition of a reef may indicate past history of exposure to bleaching, possible resistance, and provide indications of future bleaching risk.

• Reefs dominated by stands of a single species (e.g., staghorn coral in the Indo-Pacific) may be most susceptible to bleaching.

• Some corals may be particularly vulnerable to bleaching. These include Acropora, Millepora, and Pocillopora.

• Others, such as the massive coral Porites, Faviids, and Montipora, might be more resistant under specific conditions.

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Rodney Salm, Director of Transforming Coral Reef Conservation, The Nature Conservancy. Rod has launched a global program to build resilience in the face of change into marine protected areas and has developed the Reef Resilience toolkit concept.