humans, ranging from the material needs of tropical populations who exploit their carbohydrate, protein and limestone resources, to recreation for millions of tourists, and a contribution to biogeochemical cycling. Although they transform C02 to CaC03, the process actually contributes to atmospheric C02 (Smith and Buddcmcier 1992). However, the relatively small global area of reefs makes their contribution to the global carbon cyclc small compared with that of other sources and sinks (Smith 1978).
C'oral reefs have more species and co-evolved relationships per unit area than any other marine ecosystem but, with the exception of a few groups such as fishes and corals, most taxa are poorly known (Bohkle and Chaplin 1968; Springer 1982; Achituv and Dubinsky 1990; Butman and Carlton 1993). Reefs have fewer species than tropical forests, with which they are often compared, but a much higher phyletic diversity (Ray and Grassle 1991; Briggs, 1994). In both systems, disturbance plays a major role in the maintenance of diversity (Connell 1978) and rare species, which have either very clumped and localized or overdispersed distributions, are collectively important (Grassle 1973; Connell 1978). Rare species are also, by virtue of small population sizes, the most vulnerable to local extinction.
High species and genetic diversity arc defining characteristics of coral reefs, but there are huge differences in species composition and diversity among reefs on biogeographic scales (Potts and Garthwaite 1991; Pandolfi 1992; Jablonski 1993; Knowlton and Jackson 1994, Vernon 1995). Collectively, coral reefs are rich in phyla and diversity within phyla, and uniform taxono-mically down to the level of Order, individual reefs vary greatly in their composition and diversity at the levels of Family, Genus and Species. Distinctively different coral reef biota occupy the western Atlantic and the Indo-Pacific Oceans, and within each province, there is a center of high biological diversity: the Caribbean Sea and the Western Pacific Arc, respectively (Figure 15.1). Even the most depauperate coral reef is likely to be more diverse and structurally complex than any adjoining benthic community. However, coral-reef scientists have not often addressed whether reefs having naturally low biodiversity differ in ecosystem function from more diverse reefs, or from those which have had their biodiversity lowered by humans.
In recent years, molecular studies have demonstrated a strong genetic basis for much of the morphological and distributional variation seen within coral reef species (Knowlton et ai 1992; Miller 1994). For purposes of this synopsis, however, the numbers of species refer to the conventional taxonomy, while recognizing that the formal species delineations are currently subject to debate Veron (1995).
Different coral reef locations (regions, positions on continental shelves, positions on reefs) are characterized by differences in water quality (Birkeland 1987, 1988) and in the frequency and intensity of natural stressors and disturbances. Examples are exposure to hurricane waves (Scoffin 1993), flood plumes and lethal temperature excursions associated with El Niño Southern Oscillation (ENSO) events (Glynn 1990) or other regional or global climatic fluctuations. Whole human generations can pass without any Papuan or Maldivian reef being disturbed by hurricane-generated waves, or any Red Sea or Western Australian reef coming under the influence of flood waters. By contrast, it is to be expected that sometime before one's children finish their schooling, their favorite snorkel site in Puerto Rico or the central Great Barrier Reef will be damaged by hurricane waves, fresh water or both.
15.1.1 Use, abuse and management of coral reefs
Coral reefs provide essential services to humans (UNEP/IUCN 1988). Large human populations live on islands built solely by coral reefs (e.g. atoll nations of the Indian and Pacific oceans) or by coral reefs in conjunction with other marine sediments (e.g. the Florida Keys). To many coastal and island communities, particularly in the developing countries of central America, the Caribbean, Africa and Asia, coral reef biota are important sources of food and of reef limestone, sands, rubble and blocks for use as building materials. The physical barriers provided by coral reefs protect coasts from erosion by storm waves. Tourism associated with coral reefs provides many countries with significant foreign exchange earnings. For example, in Queensland, Australia, tourism associated with the Great Barrier Reef is the State's second largest industry sector and valued at around $1.5 billion per annum. Beyond these perhaps obvious benefits, coral reef plants, animals and microbes are rich in unusual organic compounds, including antitumor compounds whose potential is just now beginning to be defined (Guan et al. 1993)
However, coral reefs in many parts of the world are degraded or at risk through over-exploitation and abuse (Brown 1987; Salvat ¡987; D'Elia et ai. 1991; Wilkinson 1993). Active management of the use of coral reefs (Kench-ington and Agardy 1989; Kelleher 1994) and research in support of issues defined by users and managers (Crossland 1994) are now well established in various parts of the world.
Our intent is to provide an overview of the influences of biological diversity on ecosystem function and to suggest research that will contribute to our long-term understanding of biodiversity and the management of reefs for sustainable use. We define "biodiversity" as the diversity of genotypes, species, communities, habitats, whole reefs and regions. In all cases, to adequately link biodiversity with ecosystem function, the term "biodiversity" must embrace both the elements of richness and evenness (e.g. as in the species diversity concept), and some notion of abundance per unit area. For example, two reef zones may have identical species richness and relative abundances, but the one with the greater total biomass will contribute more to limestone and/or protein accumulation. Below, we show that the links between "biodiversity" thus defined and scaled, and the function or dysfunction of coral reefs, are complex and poorly understood.
At all scales, biodiversity and ecosystem function are emergent properties of population and community dynamics of plants and animals: temporal fluctuations in the abundance of populations will be reflected in aspects of ecosystem function. Although traditionally characterized as "a well-ordered, climax system" in which predator population explosions do not occur (Odum 1971), coral reefs are extremely dynamic at the level of populations and communities. Current coral-reef paradigms, particularly since the seminal work of Connell (1978), tend to give a greater emphasis to chance, disturbance and eyelidty than those of earlier decades. Coral-reef environments are not always benign, and population explosions and crashes, notably involving reef-building corals, echinoderms and algae, occur commonly on contemporary coral reefs (see references below).
How much of this temporal variability is "natural" and how much a symptom of human influences is the central focus of much current research. Strong arguments have been made (e.g. Endean and Cameron 1990a,b; Done 1992a, and see below) that reductions in coral-reef biodiversity caused by human activities have amplified, and that such activities have possibly even been the primary cause of these fluctuations. Therefore our chapter also considers the system properties of "resistance" ("the opposition offered") and the "resilience" ("ability to return to original form") of coral reefs subject to natural and anthropogenic stress and disturbance.
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