Environmental Problems, Their Causes, and Sustainability 1

7Aquatic BiodiversityBiodiversityWaterWasteTreatmentCASE STUDYWhy Should We Careabout Coral Reefs?In the shallow coastal zones of warm tropical and subtropicaloceans we often find coral reefs (Figure 7-1,left). These stunningly beautiful natural wonders areamong the world’s oldest, most diverse, and mostproductive ecosystems.Coral reefs are formed by massive coloniesof tiny animals called polyps. They slowly build reefsby secreting a protective crust of limestone (calciumcarbonate) around their soft bodies.When the polyps die, their empty crusts remainas a platform for more reef growth. The result is anelaborate network of crevices, ledges, and holes thatserve as calcium carbonate “condominiums” for avariety of marine animals.Coral reefs involve a mutually beneficial relationshipbetween the polyps and tiny single-celled algaecalled zooxanthellae (“zoh-ZAN-thel-ee”) that live inthe tissues of the polyps. The algae provide thepolyps with color, food, and oxygen through photosynthesisand help produce calcium carbonate, whichforms the coral skeleton. The polyps in turn providethe algae with a well-protected home and some oftheir nutrients. This is a win-win deal for bothspecies.Coral reefs provide a number of important ecologicaland economic services. When coral polypsform their limestone shells they remove carbon dioxidefrom the atmosphere as part of the carbon cycle.Coral reefs act as natural barriers that help protectabout 15% of the world’s coastlines from erosion bybattering waves and storms. They also support at leastone-fourth of all identified marine species and abouttwo-thirds of marine fish species and produce about atenth of the global fish catch.The reefs provide jobs and building materials(which can help destroy the reefs) for some of theworld’s poorest countries and support fishingand tourism industries worth billions of dollarseach year.Finally, these biological treasures give us an underwaterworld to study and enjoy. Coral reefs providethese free ecological and economic services whileoccupying less than 0.1% of the world’s ocean area—amounting to an area about half the size of France.They are true wonders of biodiversity!Bad news. More than one-fourth of the world’scoral reefs have been lost to coastal development, pollution,overfishing, warmer ocean temperatures, andother stresses. And these threats are increasing.One problem is coral bleaching (Figure 7-1, right). Itoccurs when a coral becomes stressed and expels mostof its colorful algae, leaving an underlying ghostlywhite skeleton of calcium carbonate. Two causes areincreased water temperature and runoff of silt thatcovers the coral and prevents photosynthesis. Unableto grow or repair themselves, the corals eventually dieunless the stress is removed and algae recolonize them.Aquatic scientists view coral reefs as “aquatic biodiversitysavings banks” and sensitive biological indicatorsof environmental conditions in their aquaticenvironment. The decline and degradation of these colorfuloceanic sentinels should serve as a warning aboutthe health of their habitats and the oceans that provideus with crucial ecological and economic services.Figure 7-1 A healthy coral reef covered by colorful algae (left)and a bleached coral reef that has lost most of its algae (right)because of changes in the environment (such as cloudy water ortoo warm temperatures). With the algae gone, the white limestoneof the coral skeleton becomes visible. If the environmental stressis not removed and no other alga species fill the abandonedniche, the corals die. These diverse and productive ecosystemsare being damaged and destroyed at an alarming rate.