The Near Future of Coral Reefs
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Environmental Conservation 29 (4): 460–483 © 2002 Foundation for Environmental Conservation DOI:10.1017/S0376892902000334 The near future of coral reefs TIMOTHY R. MC CLANAHAN* The Wildlife Conservation Society, PO Box 99470, Mombasa, Kenya Date submitted: 3 August 2001 Date accepted: 11 June 2002 SUMMARY numerous resources to millions of people. They are a In this paper the current status of coral reefs, predic- unique marine ecosystem in being characterized by a tions concerning the ecological state of coral reefs to geologic component, the deposition of calcium carbonate by the 2025 time horizon and the research needs that can corals, molluscs, foraminfera and algae (Kleypas et al. 2001). help understanding and management activities that These geologic structures leave good fossils that have might alleviate detrimental ecological changes are allowed scientists to track evolutionary change over millions of years (Veron 1995; Wood 1999). Relative to many other evaluated and discussed. The present rate of CO2 emissions will produce an atmospheric concentration ecosystems, evolutionary change on coral reefs is well docu- in 100 years not experienced during the past 20 million mented and ancient reefs have been a focus for numerous years and water temperatures above those of the past studies of past global change (Pandolfi 1999; Budd 2000). interglacial 130 000 years before present. Human influ- Reefs as an ecosystem and geologic structure have been ences on water temperatures, seawater chemistry remarkably persistent over time, but the species compo- (toxic substances, nutrients and aragonite saturation), sition of reefs has changed over time with most of the the spread of diseases, removal of species and food present reef species originating about 1–10 million years web alterations are presently changing reef ecology. A before the present (BP) (Veron 1995; Budd 2000). The significant ecological reorganization is underway and Scleractinia, or modern stony corals, have, however, under- changes include a reduction in calcifying and zooxan- gone a major radiation since the Cretaceous 65 million years thellae-hosting organisms, their obligate symbionts, BP with their species numbers increasing until the present and species at higher trophic levels, with an increase in (Veron 1995). With an estimated 22 ice ages over the past generalist species of low trophic level that are adapted 1.8 million years (Muller & MacDonald 1997), there has to variable environments. Late-successional fleshy been sufficient time for multiple cycles of global climate brown algae of low net productivity or non-commer- change. Past glacial cycles have been driven by changes in cial invertebrates such as sea urchins, starfish and the Earth’s eccentricity and obliquity, or Milankovitch frequencies, with atmospheric CO concentrations lagging coral-eating snails will dominate many reefs. These 2 changes will be associated with a loss of both net around 600 years behind temperature rises (Fischer et al. benthic and fisheries production and inorganic 1999; Zachos et al. 2001). An unprecedented experiment involving raising atmospheric CO independently of the carbonate deposition; this will reduce reef complexity, 2 species richness, reef growth and increase shoreline Earth’s natural orbital cycles is now being undertaken and erosion. To avert these changes management is the initial ecological responses to this change are now being needed at both global and local levels. Both levels need seen. to reduce greenhouse gases and other waste emissions Coral reefs have persisted through changes in water and renew efforts to improve resource management temperature and sea level that have accompanied glacial and including restrictions on the use of resources and glob- other cycles (Pandolfi 1996), but species extinction has alization of resource trade, run-off and waste occurred over these cycles and has been associated with production, and balancing potential reef production synergistic losses in habitat and climate change (Pandolfi and resource consumption. 1999; Budd 2000). The present atmosphere is one of the most CO2-rich in recent geologic history with the current level Keywords: carbon dioxide, coral reefs, fishing, global change, being greater than at any time in the last 420 000 years. The human influences, management, pollution, sea level level projected for the year 2100 is greater than that seen in the last 20 million years (Fischer et al. 1999; Albritton et al. INTRODUCTION 2001; Sandalow & Bowles 2001). Carbon dioxide and other Coral reefs are shallow subtidal ecosystems of the tropical greenhouse gases are reversing (Barnett et al. 2001; Levitus et oceans formed at the edge of the land and sea that provide al. 2001) the slow cooling that has been occurring over the past 50 million years (Lear et al. 2000). The warmest * Correspondence: Dr Timothy R. McClanahan Tel: ϩ254 11 temperatures of the last 150 000 years were only about 1°C 485570 Fax: ϩ254 11 472215 e-mail: [email protected] above today’s and by the year 2100 temperatures are The future of coral reefs 461 Table 1 Environmental variables, the minimum (Min), maximum horizon of the year 2025 (Foundation for Environmental (Max), mean and standard deviation of sites (SD) for 1000 coral Conservation 2001) and conclude by recommending areas for reef sites on ReefBase (taken with permission from Kleypas et al. future research and management. 1999a). PSU ϭ parts per thousand. Variable Min Max Mean SD Temperature (˚C) ENVIRONMENTAL FORCING FACTORS mean 21.0 29.5 27.6 1.1 minimum 16.0 28.2 24.8 1.8 Physicochemical factors maximum 24.7 34.4 30.2 0.6 Physicochemical factors influence coral reefs organisms, and Salinity (PSU) their abundance and distributions are influenced by their minimum 23.3 40 34.3 1.2 tolerance to variation in these factors. A summary of environ- maximum 31.2 41.8 35.3 0.9 mental factors collected from 1000 reefs indicates that there Nutrients (µmol lϪ1) is a significant range for each variable, but there is also rela- NO 0. 3.30 0.25 0.28 tively minor variation around the means (Kleypas et al. 3 1999a; Table 1). For example, temperatures in both space PO4 0. 0.54 0.13 0.08 and time range from 16 to 34°C, but standard deviations are Aragonite saturation (Ω ) arag quite low. This suggests that some extremes are tolerated or mean 3.28 4.06 3.83 0.09 adapted to but the condition is one of minor variations Max depth of light penetration (m) around the means of temperature, salinity, phosphorus and mean Ϫ9. Ϫ81. Ϫ53. 13.5 aragonite saturation relative to temperate, intertidal and minimum Ϫ7. Ϫ72. Ϫ40. 13.5 estuarine ecosystems. With increasing variability in physico- Ϫ Ϫ Ϫ maximum 10. 91. 65. 13.4 chemical factors, such as temperature, coral communities persist but often with reduced species richness (Veron & predicted, by global climate models, to increase by an average Minchin 1992; T.R. McClanahan & J. Maina, unpublished of a 3°C (range 1.4–5.8°C) (Albritton et al. 2001). In data 2002) and changed ecological functions such as calcifica- addition, the rate of environmental change is likely to be tion and reef growth (Cortes 1993). Seawater nitrogen considerably accelerated in the near future. Multiple syner- concentrations and light penetration are more variable and gistic environmental disturbances are likely to interact to high variation in these factors is, therefore, likely to be toler- create stressful conditions unique to the current epoch (for ated more than that of environmental factors with low example, human population growth and resource use, warm variation. Dependent on the factor that is changing, environ- water and the dissolution of CaCO3 in seawater). mental variation will have variable consequences for reef Coral reefs are formed under conditions of warm water ecology. (Ͼ18°C), high light, high aragonite seawater saturation, A few environmental factors vary together, for example stable full salinity and low dissolved seawater nutrients temperature and aragonite are strongly (r2 ϭ 0.76) and (Table 1). Corals and calcifying algae are often found in nitrogen and phosphorus weakly (r2 ϭ 0.30) correlated conditions that differ somewhat from these conditions (Kleypas et al. 1999a). In contrast, temperature, salinity and (Kleypas et al. 1999a; Sheppard 2000a), but these communi- light do not vary together across the measured sites (Table 1) ties do not form massive structures, such as barrier and atoll suggesting tolerance to variation between these environ- reefs, that survive over geologic periods of sea-level change. mental variables. Aragonite becomes increasingly saturated Determining the most important of these environmental with increasing temperature but, importantly, aragonite factors has been difficult because they often vary together, saturation decreases with increasing atmospheric CO2 making their influence on reefs difficult to tease apart concentration (Langdon et al. 2000). Light penetration and (Kleypas et al. 1999a). Understanding the influence of these turbidity may greatly influence the depth to which calcifica- factors is important for their management as the future of tion occurs, but are less likely to influence calcification in coral reef organisms and reef distribution may depend on shallow waters of less than 7 m (Bosscher 1993). how these environmental factors vary over the current age of Consequently, factors that are generally abundant may global climate change. Global climate change, through the become limiting at another depth or place. accumulation of greenhouse gases, is most likely to influence Phosphorus and nitrogen concentrations may be an water temperatures, monsoon and El Niño Southern example where high variation in one factor can have little Oscillation (ENSO) climate systems and the aragonite satu- consequence, whereas small variation in the other can ration concentrations of sea water. These changes, although produce larger changes in reef ecology. The weak relation- currently changing the climate and oceans, are forced by the ship between phosphorus and nitrogen concentrations may more immediate influences on coral reefs, human population be due to the high temporal and spatial variation in nitrogen growth and resource use.