Coral Reef Refugees

April 2010
By:
Peter Glynn, Ph.D., professor, University of Miami Rosenstiel School of Marine and Atmospheric Science
This is the third of three essays on coral reefs and climate change.

Coral reefs in all tropical seas have experienced unprecedented mortality and devastation during the past few decades. This is a result of several factors, all largely precipitated by humankind's mismanagement of Earth's resources. Global warming and ocean acidification are a result of accelerated increases in greenhouse gas emissions (especially CO2). These conditions result in coral bleaching, death and lowered rates of calcification in surviving corals. Less than one-half of global reefs (46%) are considered to be in a stable or recovering state.

When corals, the building blocks of reef formations, die, a multitude of organisms will directly or indirectly contribute to the erosion of their calcareous skeletons (bioerosion). The loss of reef structures results in the loss of critical habitats for the numerous species associated with reefs - fishes, crustaceans, mollusks. As with the numerous species extinctions and biodiversity declines documented for rain forests, coral reef ecologists are projecting similar declines in coral reefs.

A Hopeful Note

The discovery that certain special habitats can act as refugia, an area providing suitable conditions for coral survival even after wide scale changes have occurred in surrounding areas. Some of these refugia occur in deeper water where temperatures are lower, solar radiant energy less strong, and water exchange (current velocity) more vigorous. Such coral refugia have been found in the Galapagos Islands, especially in the central and northern islands of the archipelago. Coral communities in relatively deep refugia (Marchena Island, Darwin Island) have survived two recent extraordinarily strong El Nino Southern Oscillation events (1982-83, 1997-98), and are recovering well. It is possible that such refugia, formerly considered as marginal environments for reef corals, may actually serve as repositories of reef biodiversity and centers of replenishment should humankind manage to reverse global warming.

There is a spatial component to the response of ecological communities to climate change that must be considered when trying to understand and predict future community states. In the oceans, processes that reduce or intensify the potential stress caused by altered temperatures, chemistries, and flow regimes vary greatly in space, creating a seascape of risk and refuge. However, refuges with reduced risk may not provide quality habitat for part or all of the community, and species within these refuges may contribute less to regional resilience. Thus, understanding and predicting how ecological systems, and their component species, will respond to future warming, acidification, and flow changes, requires an understanding of where potential refugia form in space and how organisms perform within these refugia.

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