Stopping Ecosystem Collapse: Caribbean Coral Reefs

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What is natural

Guest post by Jim Steele

Media headlines have fueled unrealistic fears of the collapse of the ecosystem due to climate change. Such fears are supported when the International Union for Conservation of Nature (IUCN) identifies some ecosystems as endangered, such as Caribbean coral reefs. But reefs are resilient, and the human factors that threaten individual reefs can be remedied.

The Caribbean reef ecosystem is made up of thousands of individual reefs that stretch from the east coast of Mexico and Central America to Florida and the Bahamas, and south to the coast of Venezuela. Fifteen thousand years ago, these reefs did not exist because the sea level had dropped 400 feet during the Ice Age. Modern reefs were established 8,000 years ago by colonizing newly flooded coasts.

Based on an IUCN criterion, the Caribbean reef ecosystem was classified as “least of all concern” due to the widespread occurrence of individual reefs. In contrast, the loss of 59% of the total coral cover between 1971 and 2006 caused the IUCN to label the reef system as "endangered".

The coral cover naturally varies with algae (macroalgae). Corals are killed by hurricanes, diseases or bleaching, which means that algae can also colonize the space that has become free. The algae are gradually reduced by algae-eating animals, allowing the corals to return to their former dominance. Corals usually recover within 15 to 20 years, but recently their recovery has been extremely limited, reducing coral cover. Unlike the demonized sea urchins that threaten Alaska's kelp forest, algae-eating sea urchins are vital to maintaining the balance between algae and Caribbean coral. The recent shortage of coral restoration is largely due to a new disease that devastated sea urchin populations and minimized sea urchin consumption by sea urchins in the 1980s.

Caribbean corals had been decimated by the novel White Band Disease in the 1980s. However, this disease only affected two species of coral in the Acropora genus – staghorn and elkhorn corals. These species are now considered endangered. Acropora's evolutionary strategy was to rapidly colonize freed coastlines caused by natural disturbances such as hurricanes. These coral species therefore devoted their energy to rapid growth in order to compete with the algae. This adaptation allowed staghorn and elkhorn corals to colonize flooded coasts quickly as sea levels recovered from the last Ice Age and dominated modern Caribbean reefs. However, this strategy required the derivation of energy to build stronger reefs or withstand disease.

Because Acropora require a shallow habitat, they are prone to storm damage. So they devised a reproductive strategy that spawned new colonies by cloning new corals from storm-damaged fragments. However, cloning reduces genetic diversity, making them more susceptible to new diseases as well.

Mortality from bleaching also decreased coral cover. The bleaching of unusually warm temperatures in summer 2005 and in El Nino 1998 is often highlighted. Surprisingly, deadly cold weather bleaching is rarely mentioned. In January 2010, cold weather killed 11.5 percent of coral along the Florida Keys, 20 times worse than the 2005 warm weather mortality rate. Understanding why both warm and cold weather cause bleaching explains how Corals have successfully adapted to constantly changing climates over the past 220 million years.

Shallow water corals rely on photosynthesis from symbiotic algae (also known as symbionts), which provide over 90% of the coral's energy. However, these corals remove one species of symbiote and acquire a new symbiote that is better adapted to changing weather conditions. In winter, colder temperatures and less light reduce photosynthesis. Corals increase their density of symbiotic algae to counteract decreased productivity. But when it is too cold, the symbionts keep their energy supply to themselves. As a result, corals remove the "freeloaders" that cause bleaching. Then a more productive cold tolerant symbiote has to be acquired or the coral dies.

In contrast, in summer more light and higher temperatures produce so much energy that corals reduce their number of symbionts. Since photosynthesis also creates potentially harmful chemical by-products, corals remove symbionts to reduce the production of harmful chemicals. This, too, causes corals to bleach, and if a better adapted symbiote is not acquired, the coral will die. Despite this mortality risk, corals can quickly adapt to warmer or cooler climates after changing their symbionts and improve the survival of the species.

When investigating fossil reefs, scientists found that Caribbean corals had declined decades before widespread bleaching and disease outbreaks. Growing human populations cleared the land for farms, sugar cane and banana plantations. The resulting soil runoff reduced the clarity of the water, which is required for efficient photosynthesis. Increased wastewater also decreased clarity and introduced pathogens. These stressors made corals more susceptible to subsequent bleaching and disease. Soil runoff also added nutrients that favored the ecological balance to encourage algae growth, while overfishing removed algae-eating fish that once limited algae dominance.

We can and control the soil runoff and wastewater treatment. The fishing regulations restore the ecosystem in which algae and corals were balanced. And with these protective measures, of course, resilient corals steadily recover.

Jim Steele is the retired director of the Sierra Nevada Field Campus for the state of San Francisco.

composed landscapes and cycles: The journey of an environmentalist to climate skepticism,

and a member of the CO2 coalition

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