FEATURE CORAL REEFS AND BIODIVERSITY: A CRITICAL AND THREATENED RELATIONSHIP By J.E. Maragos, M.P. Crosby and J.W. McManus THE IMPORTANCEOF coral reef ecosystems may dramatically increased negative and synergistic im- be seen in their numerous ecological, aesthetic, pacts from poorly managed anthropogenic activi- economic, and cultural functions. Atoll and barrier ties. Globally, scientists are now working together reef islanders recognize that healthy reefs are es- and with other groups to promote assessment, mon- sential for the support, creation, and repair of the itoring, other research, protection, and restoration coral islands on which they live. Coral reefs also of coral reefs. Establishment of coral reef manage- protect coastlines from shoreline erosion and serve ment initiatives at the local community, national, as a living pantry for the subsistence harvest and and regional levels are essential for long-term sus- consumption of many reef organisms. The cycle of tainable use and conservation of these critically im- reef accretion and erosion maintains beaches and portant habitats. The focus of these initiatives provides habitat for seagrasses and mangroves. should be on assisting culturally, economically, and Coral reefs are important recreational resources for politically diverse peoples around the world in their most of the worlds people having the privilege of development of integrated coastal zone manage- The ability of coral living near them. In the modern era, coral reefs ment with emphasis on local community involve- passes and channels provide safe navigation chan- ment and leadership. reef ecosystems to nels for boats, and harbors are often sighted on Introduction exist in balanced har- reefs because they provide natural protection from Most of what we have learned about coral reefs heavy wave action. Coral reefs are fast becoming mony with other natu- has been gathered by scientists during the past 150 the main attraction fbr visitors to many tropical is- years, beginning with Charles Darwin and James rally occurring land and coastal destinations. Coral reefs are also D. Dana in the mid-19th century (Darwin, 1842: the favorite sites of many governments and devel- competing/limiting Dana, 1872). Darwin first postulated that the subsi- opers for construction materials, and reef rock is dence of volcanic islands can result in the evolu- physicochemical and mined in many countries to provide armor stone tion of fringing reefs to barrier reefs and atolls and building materials. Few aspects of these activi- (Fig. 1, a-d). Dana, as geologist aboard the U.S. biological agents has ties, especially modern uses, are beneficial to reefs, Exploring Expedition (1838-1842) which circum- and scientists and other reef users are beginning to been severely chal- navigated the globe, was able to publish the first realize that coral reefs are fragile and are now definitive global distribution of coral reefs. He also lenged in the last threatened in many areas in the world from chronic addressed some of the major factors--the need for anthropogenic reef disturbance. The ability of coral several decades by warm seawater temperatures (generally >21°C) and reef ecosystems to exist in balanced harmony with light--that contribute to vigorous reef growth and the dramatically other naturally occurring competing/limiting described more species of corals than any other physicochemical and biological agents has been se- increased negative scientist beforehand or afterward, Other pioneering verely challenged in the last several decades by the reef scientists through the early twentieth century and synergistic included Stanley Gardiner, Alexander Agassiz, Al- impacts from poorly fred Mayor, Thomas Vaughan, Maurice Yonge, J.E. Maragos, Program on Environment. East West Center, and Cyril Crossland. Important earlier expeditions managed anthro- Honolulu. HI, USA. M.P. Crosby, Ocean and Coastal Re- and laboratories focusing on coral reefs research source Management, National Oceanic and Atmospheric Ad- pogenic activities, ministration, 1305 East West Highway, Rm. 11437, Silver occurred on Bermuda, the Great Barrier Reef, the Spring, MD 20910, USA. J.W. McManus, Coastal & Coral Dry Tortugas, and in Palau. The more recent use of Reef Resource Systems Program, International Center for Liv- drilling equipment, submersibles, scuba equipment, ing Aquatic Resources Management. Manila, Philippines modern laboratory equipment, and other technolog- (ICLARM #1247). All correspondence should be directed to M.P. Crosby. This manuscript reflects the opinions of the au- ical innovations ushered in the era of modern coral thors and is not meant to represent the official policy of any reef research and inquiry, beginning with works of government agency. John Wells, Joshua Tracey, and Harry Ladd, and OCF.XNOOaAP~Y°~OI.9, No. 1o1996 83 co-workers on several atolls in the Marshall Islands causes the islands {or continental coast) to shrink beginning in the 1940s, and with Thomas Goreau, in size, leaving a gap (the lagoon) between the off- in Jamaica, beginning in the late 1950s. Drilling shore reel and island shoreline. If island subsi- expeditions to Funafuti (in Tuvalu), in 1898 and dence continues to be gradual, allowing upward Enewetak (in the Marshalls) in the 1940s and early reel growth, the volcanic island eventually disap- 1950s revealed that present day atolls have sus- pears and is buried under accumulating lagoon tained continuous upward reef growth of tip to sediments, creating an atoll with perilneter coral _>1,000 m over periods of 50 million years or more. reefs surrounding a lagoon. Secondary reefs also Coral reefs are accumulations of the biogenous form in the lagoons of barrier reel's and atolls in- remains of carbonate secreting reel" organisms-- cluding pinnacles, patch reefs, and lagoon fringing Not only is the both plants and animals that formed in warm tropi- reefs around islands. Passes often cut through bar- cal (and in some cases, semitropical) seas. The rier and atoll reefs allowing exchange of lagoon coral reef structure principal reef builders are referred to as "'her- and ocean waters, and natural channels often serve itself composed of matypic" corals. These corals exist in a symbiotic as gaps along fringing reefs where rivers and relationship with microscopic unicellular algae streams discharge sufficiently to discourage reel" and built by a diver- know as -zooxanthellae." The coral-algal symbi- growth. otic association aids the formation of massive reefs Living healthy coral reefs (i.e., Fig. _,'~ a-e) ~,,en- sity of organisms, but because of the unique advantages of the associa- erally grow upward and outward (or seaward) the reef structure tion. The symbiotic algae are protected in the tis- from the subinerged slopes of islands, continents, sues of the coral animals that are themselves pro- or older reefs, and are able to thrive in nutrient- serves as the basis tected by highly specialized stinging cells, the poor transparent oceanic waters. If reef growth for one of the highest nematocysts. These animals can also depend on the reaches sea level, then only outward growth is algae lbr most or all of their food supply, whereas possible because the marine organisms forming diversity ecosystems the algae can depend on the animals lk)r nutrients coral reefs cannot normally live above mean low in the world . and carbon dioxide for photosynthesis. This symbi- tidal levels. Continuous periods of outward reef otic relationship is widely believed to greatly in- growth creates reel" flats. The inner portions of crease calcification and improve energy and nutri- progressively wider reef fiats begin to accumulate ent fluxes between the animals and the plants, sand and other sediments that eventually may be allowing more rapid coral growth and the corre- transported to shore to form the white sand sponding opportunity for more rapid reef growth. beaches that characterize many coral reef coasts. However, the exact nature of this relationship has However, the seaward slope and upper edge of the recently been questioned (Marshall, 1996L reef fiat usually supports the principal populations The coral reef structure itself is a thin venecr of of diverse reef-building organisms--corals, cal- living reef organisms that overlays the remains of careous/encrusting/coralline algae, and sometimes generations of progressively older reef organisms. mollusks. The diversity of reef-building organisms In fact, most of the major coral reefs observed contributing calcium carbonate materials to coral today are living veneers that have covered previ- reefs also includes echinoderms, sand producing ously existing substrates only within the hist green and red algae, and foraminiferans, with 10,000 years. The accumulations are cemented to- worm tubes, barnacles, and other skeletal remains gether to form wave-resistant structures, capable sometimes adding to the bulk of reef structures. of continued growth in the face of heavy and pre- vailing wave action and currents, especially akmg Coral Reef Biodiversity and Importance seaward reef slopes. Not all tropical coasts are Not only is the coral reef structure itself com- able to support coral reefs. Active volcanic islands posed of and built by, a diversity of organisms, but or continental coasts may be too inhospitable or the reef structure serves as the basis for one of the unstable for corals and other reef organisms to set- highest diversity ecosystems in the world (Talbot, tie and grow. If the shorelines of