Figure 5.2.6. Coral species richness as a function of longitude in three reef zones. Leftmost points are for Indonesia, followed to the right by Papua New Guinea, the Solomon Islands, American Samoa, and the Society Islands.

      Source: Redrawn from Karlson et al. (2004).

      Why Is Diversity So High?

      The causes of the peak of marine diversity in the Coral Triangle and the latitudinal and longitudinal diversity gradients have been much debated. There have been many proposals. An early idea was that the center of diversity was a center of species formation (e.g., Briggs 1994). A second view is that more rapid extinction in outlying areas reduces the number of species in those areas. During ice ages, for instance, the Coral Triangle area probably experienced a much smaller drop in sea surface temperature than high latitudes and the eastern Pacific, so more coral species survived in the Coral Triangle than in those other areas. Another proposal is that currents in the tropical Pacific flow westward, carrying new species with them and causing the accumulation of species in the Coral Triangle area (Jokiel and Martinelli 1992). The proponents constructed a model that showed just this effect. Yet another proposal is that many islands close together allow any local populations that might go extinct to be rapidly replenished by larvae from nearby islands. The classic theory of island biogeography predicts higher numbers of species when an island is closer to a source of additional species (MacArthur and Wilson 1967). In areas with few islands, a population could go extinct more often on an isolated island before larvae from distant islands could reach it by chance and replenish the local population. Local extinctions have indeed been documented among the corals on the widely separated reefs of the eastern Pacific (Glynn 1977). A model using different densities of islands but random currents produces higher diversities in areas with more islands (Blanco-Martin 2002).

      Connell (1978) proposed an ‘‘Intermediate Disturbance Hypothesis’’ to account for high diversity in rainforests and coral reefs. Disturbances of intermediate intensity and frequency open spaces where additional species can settle, whereas without disturbance, superior competitors drive out inferior competitors in a biological succession that ends in lowered diversity. If this theory were used to try to explain diversity gradients, it might suggest that the area of highest diversity is where disturbances are of intermediate intensity and frequency. However, sea surface temperatures in Indonesia and New Guinea are disturbed less by cold events than in areas farther from the equator. Similarly, there are no cyclones near the equator in Indonesia (including Papua) and New Guinea (Figure 5.2.7a,b; Fenner and Riolo, under review). On the other hand, the northern Philippines experiences a moderate to high level of disturbance from cyclones, and yet is part of the Coral Triangle. Cyclones are probably one of the most important natural disturbances on coral reefs (Rogers 1993). Thus, the Intermediate Disturbance Hypothesis is unlikely to explain coral diversity gradients.

      In ecosystems with low diversity, each species may be represented by large numbers of individuals. For example, in the tundra of northern Canada and Alaska, there are millions of Snow Geese (Chen hyperboreus) in the summer, and millions of mosquitoes. There is only one large mammal, the Caribou (Rangifer caribou) and it is also present in large numbers. Northern forests are composed of large numbers of individuals of a small number of tree species. By contrast, in the tropics there are large numbers of species, most of which are rare. In a hectare of rainforest in New Guinea, there can be several hundred species of trees, but few individuals of each species. An example from coral reefs is sea slugs (opisthobranchs): in the western Pacific there are many species (over 500), most of which are very rare. In the tropics, functional groups of species, called guilds, usually have many more species than in the temperate or, especially, polar areas. Research on terrestrial systems indicates that the loss of individual species does not have a great impact on a high-diversity ecosystem, because there are many other members of most guilds that can continue to perform that guild’s functions (Grime 1997; Moffat 1996). The loss of a member of a guild in a low-diversity ecosystem may have a much larger impact on the ecosystem, particularly if there is only one member of that guild so that with its loss the guild function is no longer performed. Similarly, Bellwood et al. (2004) have argued that low diversity coral reefs are more vulnerable to the loss of individual species than diverse ecosystems for these reasons. For example, the loss of a single species of sea urchin, Diadema antillarum, in the Caribbean in 1983–1984 (Lessios 1988; Lessios, Robertson, Cubit 1984), led to major phase shifts on some reefs from coral-dominated reefs to algal-dominated hard grounds. The large numbers of a single species in low-diversity ecosystems also makes them more vulnerable to diseases and specialized predators. A dense population of a single species, as was the case with D. antillarum, makes the transmission of disease easier. The die-off of D. antillarum was the largest marine epizootic ever recorded. Similarly, two of the most common coral species in the Caribbean were Acropora palmata and A. cervicornis. Both form large, dense, single-species thickets of genetically identical organisms, or clones. The lack of genetic diversity means that any disease that can kill one individual can kill the whole clone. Both of these species have been decimated in much of the Caribbean by White Band disease (Aronson, Precht, and Macintyre 1998), and were considered for Endangered Species status (Precht, Robbart, and Aronson 2004; Shinn 2004; Wilkinson 2004); they received protected status on 8 June 2006. On diverse coral reefs, most species are rare, so disease transmission is much more difficult, and the likelihood of epizootics is reduced. This probably contributes to the stability of high diversity coral reefs.

      Figure 5.2.7. a. Tracks of tropical cyclone and severe storm tracks for the Indo-Pacific region. All storms that were classified above a tropical depression in strength (wind speeds > 30 mph for two or more 6-hour periods) from 1945 to 2003 are included. b. Severe storm density in the Indo-Pacific region. Storm density was computed for each 50 50 km cell by summing the number of tracks found within 200 km of the cell and dividing by the total area sampled in each cell.

      Source: Data assembled by Unisys Corporation and Joint Typhoon Warning Center (www.npmoc.navy.mil/jtwc.html) and downloaded from the Pacific Disaster Center website (atlas.pdc.org). Maps by F. Riolo.

      Effects of Fishing

      Fishing can remove fish that are important for reef health. The removal of herbivorous fish in Jamaica left it vulnerable, so when a disease killed the last herbivore (sea urchins), the reef was overcome with algae (Hughes et al. 1987). Large fish such as sharks, Humphead Wrasse (Cheilinus undulatus), and Bumphead Parrot-fish (Bulbometopon muricatum) are particularly vulnerable. Bumphead Parrotfish have been extirpated from several places in the Indo-Pacific (Bellwood et al. 2003; Dulvy et al. 2003). Humphead Wrasses are under heavy pressure over a large area due to the live food fish trade. Populations are reduced to low levels in areas with higher fishing pressure (Sadovy et al. 2003). Areas of Fiji where fishing pressure is greatest are also the areas where Crown-of-Thorns

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