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Defying Ocean's End
An Agenda for Action
By Linda K. Glover, Sylvia A. Earle
ISLAND PRESSCopyright © 2004 Island Press
All rights reserved.
Mark Spalding, University of Cambridge
Philip Kramer, The Nature Conservancy
The Caribbean Region, for the purposes of this study, is an area comprising the Caribbean Sea, the Gulf of Mexico and the adjacent areas of the Bahamas and Turks and Caicos. This is a region of considerable geological, biological and political complexity (Figure 1) but also one of internal integrity, a partially closed series of basins having relatively little interplay with adjacent regions. The region is often referred to as the Wider Caribbean, the Intra-Americas Sea or the American Mediterranean. It provides an excellent case study for considering both human impacts in the marine realm and the measures required for their mitigation. Three features underpin this value:
Biological complexity. The region encompasses the heart of Atlantic species biodiversity and is host to considerable numbers of "endemic" species—those found only in a geographically restricted area and nowhere else in the world. This is well documented for shallow coastal communities (Tomlinson 1986, Veron 1995, Spalding, Blasco & Field 1997, Spalding, Ravilious & Green 2001, Spalding et al 2003), and there is now growing evidence for similar patterns in deeper water species (Smith, Carpenter & Waller 2002).
Socio-political complexity. This small region includes 35 countries and territories covering a complete spectrum of political systems and economic regimes, including the world's richest nation alongside some of the world's poorest (Schumacher, Hoagland & Gaines 1996). The history of human activity in the region has had a major role in the current status of the marine environment.
Pressure. Humans have had some influence over parts of the marine environment in the Caribbean for millennia (Jackson et al 2001), but there is no doubt that current pressures are unparalleled (Burke & Maidens 2004). This case study was prepared for the Defying Ocean's End
Conference in Los Cabos, Mexico, in May/June 2003. We commence with an overview of the physical and biological geography of the region, then focus on the range of human stresses affecting the marine environment and finally consider the options for reversing the current trends of rapid environmental decline.
The region under consideration consists of two semi-enclosed basins—the Caribbean Sea and the Gulf of Mexico—as well as the westernmost embayment of the Atlantic Ocean. Bounded to the west and south by Central and South America, the Caribbean is bounded to the north by the Greater Antilles (Cuba to Puerto Rico) and to the east by the great arc of small islands called the Lesser Antilles.
Most of the Caribbean Sea is centered on the Caribbean Plate, a relatively small tectonic plate (section of Earth's crust) that formed between 75 and 90 million years ago. The movements of this plate form subduction zones to the east and west (where one plate slides beneath another) and transform boundaries (where two tectonic plates slide past each other) to the north and south. These tectonic interactions have created the island chains of the Greater and Lesser Antilles as well as the mountainous perimeters across Central America and the southern Caribbean Sea. Most of the remainder of the region lies on the North American Plate (Pindell 1994, Meschede & Frisch 1998).
The Caribbean Sea is itself divided into four smaller basins separated by shallower ridges, while the Gulf of Mexico can be considered a separate fifth deep basin. Deep trenches are found to the north of Puerto Rico and to the south of Cuba and the Cayman Islands. The basins, as well as the deep trenches, are isolated from each other by shallow underwater sills and, although there is some water exchange, each may contain unique and important features (Tomczak & Godfrey 1994, Smith, Carpenter & Waller 2002). With the exception of the Bahamas Banks, the only extensive shallow areas are on the continental shelves, notably around the southern U.S. coast, the northern coast of the Yucatan Peninsula and the Nicaraguan Rise.
Surface water movement is driven by the North Equatorial Current and the North Brazil (or Guiana) Current. These combine to form the Antilles Current, which sweeps up the outer edge of the Lesser Antilles and the eastern side of the Bahamas. There is also considerable flow between the Lesser Antilles islands, and to a lesser degree through the channels of the eastern Greater Antilles, creating a broad westward-flowing Caribbean Current across the Caribbean Sea. This current is weak and there are surface eddies creating a relatively complex pattern of surface flow. Further west, the water flow is concentrated through the Yucatan Channel and becomes much stronger. Once in the Gulf of Mexico, this current flows northwest, then in a broad clockwise circle —the Loop Current—which flows back southward along Florida's west coast, shedding eddies westward into the Gulf before leaving the system east of Florida. The Florida Current, enhanced by flow from the Antilles Current, forms a broad stream of warm water—the Gulf Stream—which flows northeast across the Atlantic (Tomczak & Godfrey 1994, Longhurst 1998).
Rivers have a considerable influence in the southeast Caribbean, where outflows of the Amazon and Orinoco Rivers create slightly lower salinities and more suspended sediments over wide areas (Tomczak & Godfrey 1994).
Deep water flows into the Caribbean Sea through a limited number of deep passes: through the Jungfern Passage east of Puerto Rico into the Venezuela and Colombian Basins, through the Windward Passage east of Cuba into the Cayman and Yucatan Basins and through a number of eastern passages into the Aves and Grenada Basins. The deeper water areas have remarkably consistent temperatures. Little is known about the turnover of deep water in the Caribbean basins, but most estimates suggest periods of hundreds of years (Tomczak & Godfrey 1994, Tyler 2003).
The Caribbean is perhaps best known for its tropical shallow marine ecosystems. Its coral reefs cover about 20,000 square kilometers (Spalding, Ravilious & Green 2001). This is only about 7% of the world's shallow reefs, but they are of enormous biological and human importance. Biologically they are unique, with very little overlap in species with other parts of the world—except for Brazil and to a small degree West Africa, both of which have related assemblages but fewer species (Cortés 2003). The Caribbean is also a center of diversity and endemism for some mangrove and seagrass species, although a higher proportion of these extends into the Eastern Pacific and across to West Africa. The Caribbean has roughly 22,000 square kilometers of mangrove (amended from Spalding, Blasco & Field 1997), which is about 12% of the global total. No accurate estimate of the extent of seagrass area is available, although the Caribbean is likely to be a highly significant region for these habitats (Onuf et al 2003, Creed, Phillips & Van Tussenbroek 2003). Unpublished estimates from work at the United Nations Environment Programme (UNEP) World Conservation Monitoring Centre suggest as much as 33,000 square kilometers of Caribbean seagrass beds (Spalding et al 2003), which would be about 18 to 19% of the global total.
An explanation for this diversity and endemism in the Caribbean is at least partly linked to the geological history (Table 1) of the region. During the Triassic, about 200 million years ago, all of the world's land areas were connected as one supercontinent known as Pangea. Pangea was then divided into northern and southern continents by seafloor spreading, and the growing gap between them was filled by a globe-encircling equatorial sea known as the Tethys (Wikipedia 2004). Without land masses to disrupt biological migration and genetic flow, much of the subtropical and tropical marine regions of the world shared a similar array of animals, referred to as a pan-Tethyan fauna, until the Miocene (Veron 1995, Saenger & Luker 1997). Since that time, the open-sea connection between the Caribbean and the Pacific became increasingly restricted as the Central American land bridge rose above sealevel, which finally separated the two bodies of water about three million years ago (Guzmán 2003). For the corals, and likely for many other species, considerable changes were then caused by the Pliocene/Pleistocene glaciation periods, including the extinction of many marine species. This was followed by significant biological "radiation events" (the rapid evolution of new species to fill new habitats) in isolation from the rest of the world's tropical seas (Rosen 1984, Veron 1995), which led to the present levels of Caribbean coral diversity and endemism.
The boundaries and even the definitions of marine habitats other than coral reefs in the Caribbean remain poor. "Proxy" measures (scientifically assumed from the extension of known data) can be developed from knowledge of physical features such as surface circulation patterns, surface chlorophyll distributions or bathymetry. The location of upwelling areas—where deep, cold, nutrient-rich waters come to the surface—is extremely important for biodiversity and ocean productivity. Some sites are documented, such as the Yucatan Shelf edge (Merino 1997), the eastern coast of Venezuela and parts of the Gulf of Mexico (Longhurst 1998), but it may be possible to improve our understanding of the location, timing and frequency of these and other upwellings by looking at patterns of surface chlorophyll.
Dividing the Caribbean region into zones based on bathymetry (water depth) indicates that about 10% of the seafloor falls within areas less than 30 meters deep—where we find the highest light penetration and primary productivity—the transfer of sunlight into food through photosynthesis. Waters on the continental shelf edges (to 200 meters depth) cover a similar area. Waters between 2,000 and 5,000 meter depths dominate the region (Figure 2). The basins and deep trenches are clearly separated below 4,000 meters depth, however. Existing studies suggest that levels of biodiversity below the continental shelf edge may be very high and that, even in the deep trenches where relatively fewer types of organisms exist, endemism is a key feature, with important examples of species unchanged since ancient times (Zezina 1997). The few studies undertaken in the Puerto Rico Trench suggest that it has lower than expected diversity, however (Vinogradova 1997, Tyler 2003).
Some of the most important features of the deep seafloor include rocky outcrops with no sediment cover, deep shelf bioherms (limestone rock structures derived from the shells of living organism) and hydrothermal vent areas where hot minerals spew up from cracks in the seafloor. There has been no systematic attempt to map these, but it might be possible to produce maps through a combined literature survey and expert interpretation of features from high-resolution seafloor mapping data.
Species diversity patterns
Increasing scientific efforts are rapidly expanding our knowledge of the historical and present-day distribution of species. Shallow-water species have tended to dominate in several global studies, notably coral reef species (Roberts et al 2002), mangroves (Spalding, Blasco & Field 1997) and seagrasses (Spalding et al 2003). Smith, Carpenter and Waller (2002) have developed detailed maps of species ranges across the Caribbean including deeper waters (Figure 3). This work has shown previously unrecorded levels of endemism, with 227 of all fish species studied—fully 23% of them—being restricted to the Caribbean. Maps of these endemic species show new patterns of endemism concentration (Figure 4), especially in the Straits of Florida, the northern coast of South America, the Caribbean coast of Central America and the northern Gulf of Mexico. Location of all of these centers of highest endemism on continental shelf margins suggests that geological history may have a far more important role in both speciation (development of new species) and dispersal events than had previously been assumed (Smith, Carpenter & Waller 2002).
Human History and Environmental Change
Pre-Columbian period (9000 BC to 1492)
The Central and South American coasts have probably been inhabited since about 9000 BC, originally by nomadic hunter-gatherer tribes from North America. Early migration to Cuba apparently occurred about 6000 BC, but many of the other islands were not inhabited until 4000 BC. Many of these early peoples relied heavily on the sea as a major source of protein, but archaeological evidence suggests that extinction rates of terrestrial species within the Caribbean doubled after humans migrated into the area, despite low human populations in many areas (Wilson 1997).
When the first European explorers arrived near the end of the 15th century, two groups of people dominated the Caribbean Islands—the Arawaks in the Bahamas and Greater Antilles and the Caribs in the Lesser Antilles. In addition, small numbers of descendants from an earlier wave of settlement (the Ciboney people) were found in parts of Cuba and Hispaniola. A much broader range of ethnic groups were found living along the continental coasts. Total numbers of people in the region at that time remains unknown, but estimates range from 0.25 to 6 million on the Caribbean Islands and 15 to 30 million in the bordering countries of Central and South America (Denevan 1992).
As agricultural techniques improved, population densities increased, environmental impacts may have grown, and there is evidence of overfishing on several islands of the Lesser Antilles (Wing & Wing 2001). There may also have been some clearing of mangroves for cassava cultivation, and large animals (megafauna) such as manatees, monk seals and turtles may have been particular targets for hunters (Wilson 1997).
Colonial period (1492 to 1850)
Within 20 years of Columbus's arrival, Spain established colonies on most islands of the Greater Antilles. During this time, enslavement, famine and exposure to new diseases decimated the native peoples. Following a series of military conquests between 1520 and 1550, and the discovery of precious mineral resources (notably gold), Spanish attention was transferred to the mainland. The native population there also suffered devastating losses, such that within a century they numbered less than three million.
Other European countries (Britain, France and the Netherlands) began to take an interest in the region, not only seeking to plunder the Spanish treasure fleets but also to establish colonies on the smaller Caribbean Islands throughout the 17th century. Sugar export, first established in Barbados in 1640, quickly transformed many of the island colonies into major national assets. A plantation economy developed which, like the mining economy on the Central and South American mainland, was heavily dependent on imported slave labor. By the early 1800s, there were an estimated 4.65 million slaves in the Caribbean region (Rogozinski 1999).
From an environmental perspective, this was a period of great change. At first, the decimation of the indigenous populations probably led to a reduction in fishing pressure, a hiatus which may have enabled recovery of fish and megafauna to near natural levels in all areas for a period of perhaps 100 years (Jackson 1997). The demise of these indigenous cultures also meant the loss of traditional knowledge, and we do not know whether any of them ever exercised controls to protect marine resources. The growth of European colonies, and particularly the plantation culture, also brought dramatic changes. As soil fertility declined, more slave labor was required to fertilize the cane fields. Vast areas of forest were cut and steeper marginal areas were logged for timber and fuel for the sugar cane boilers. Massive soil erosion resulted along with climate changes on many of the islands, making them more susceptible to drought.
Excerpted from Defying Ocean's End by Linda K. Glover, Sylvia A. Earle. Copyright © 2004 Island Press. Excerpted by permission of ISLAND PRESS.
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