on the Great Barrier Reef in Australia, and nearly all corals were eaten and killed on some reefs in the midsection of the Great Barrier Reef. Outbreaks were reported on many reefs in the Indo-Pacific, with some of the worst around Okinawa, Japan (Moor 1989). Reports of outbreaks are much less frequent today, and reefs around Okinawa are now largely free of outbreaks. Outbreaks continue to occur on the Great Barrier Reef periodically. The first outbreak of COTS reported in Indonesia occurred in 1995, on the reefs of the Seribu Islands south of Papua (Tomascik et al.1997). In the Raja Ampats, only 3 of 45 (6.7%) sites had any COTS at all (McKenna, Boli, and Allen 2002). A few outbreaks of the snail Drupella have been reported in the Indo-Pacific (Moyer, Emerson, and Ross 1982), but they have not done as much damage as Crown-of-Thorns starfish. In the Raja Ampats study, Drupella were only observed at one site.
The Coral Triangle: The Peak of Diversity
The amazing diversity of species is perhaps the most notable aspect of the coral reefs of Papua Province. Papua is situated within the area that has been called the ‘‘Coral Triangle’’ (e.g., Allen 2002a; Wells 2002). This is the area of the highest diversity coral reefs in the world (Figure 5.2.4). It includes the Philippines, central and eastern Indonesia, and Papua New Guinea. There are more coral species in this area than in anywhere else in the world (Hughes, Bellwood, and Connolly 2002; Stehli and Wells 1971; Veron 1995), and the same is true of fish (Allen 2002a; Hughes, Bellwood, and Connolly 2002; Chapter 4.8), mollusks (Gosliner 2002; Wells 2002), and sponges (van Soest 1997). A recent expedition to the Solomon Islands found that the diversity of corals, fishes, and mollusks there is equally high, indicating that it too is part of the Coral Triangle (Green, Veron, and Allen 2004; Wilkinson 2004). Further, the number of mollusks in the Coral Sea area (Solomons to Great Barrier Reef to Vanuatu) is nearly as high as in the Coral Triangle (Wells 2002). In addition, recent work has found that diversity levels on outer barrier reefs of the northern Great Barrier Reef were marginally below that typical in the Coral Triangle (Fenner, unpublished). Coral diversity on mid-shelf reefs is even higher (Done 1982), so it is likely that the northern Great Barrier Reef has coral diversities within the range of diversities of the Coral Triangle.
Figure 5.2.4. Map of coral species diversity. Coral diversity decreases in all directions from the Coral Triangle (indicated in the darkest shade), which contains 581 coral species.
Source: Reproduced from Veron (2000) with kind permission from the author.
DIVERSITY GRADIENTS
The number of species decreases in all directions from the Coral Triangle (Figure 5.2.4). The diversity gradient to the north and south from the Coral Triangle is called the Latitudinal Diversity Gradient. This is perhaps best illustrated by the gradient in species diversity in southern Japan, with 342 species of coral in the small islands just north of Taiwan, decreasing almost in a straight line to zero species around Tokyo (Figure 5.2.5). A similar diversity gradient extends eastward from the Coral Triangle, with the number of species decreasing to the east in the Pacific until in the eastern Pacific there are a total of only about 33 species over a very large area. This gradient is called the Longitudinal Gradient. Both the Latitudinal and Longitudinal gradients have been documented in several groups of organisms. For instance, Indonesia has 90 species of Crinoids (feather stars), and going north, Palau has 30 and Guam has six. Going east, the Marshall Islands have 14 species and Hawai’i has none. Similarly, there are 536 species of sea slugs (opisthobranchs) known from the north coast of Papua New Guinea, 410 in Guam, 244 in Hawai’i, and 183 in Pacific Panama (Gosliner 1992). The diversity gradient for corals and fish shows a less steep gradient going west from the Coral Triangle in the Indian Ocean. Recent work in the Red Sea by Emre Turak, Lyndon Devantier, and J. E. N. Veron (reflected in the maps in Veron 2000) has doubled the number of species known there from 150 to 300, showing that the Indian Ocean Longitudinal Gradient is not as steep as previously thought.
Figure 5.2.5. Coral diversity in Japan decreases with increasing latitude (n 10, R2 0.96).
Source: Redrawn from Veron (1992).
A recent study (Karlson, Cornell, and Hughes 2004) compared coral species richness on a longitudinal gradient, comparing the number of species in Indonesia, Papua New Guinea, the Solomon Islands, American Samoa, and the Society Islands of French Polynesia. There were three sites in each country, with one of the sites in Indonesia being an island site to the west of the Vogelkop Peninsula of Papua. They separated transects on reef slopes, reef crests, and reef flats, and had an equal number of transects at each location and each reef zone. They found that on reef slopes, there was a high diversity from Sulawesi to the Solomon Islands, with no gradient between these sites. American Samoa and the Society Islands had significantly lower diversity on reef slopes. On reef crests, the diversity was highest in Indonesia, decreasing significantly in Papua New Guinea, decreasing further in the Solomon Islands, and lowest in American Samoa and the Society Islands. Reef flat diversity decreased a lesser amount from Indonesia to the Solomon Islands and on to American Samoa and the Society Islands (Figure 5.2.6). Thus, while the reef slope data indicate that the reefs of Papua New Guinea and the Solomon Islands are as diverse as those in Indonesia, the reef flat data and especially the reef crest data indicate that Indonesia has the highest diversity of all these areas, and that the longitudinal diversity gradient begins between Papua and Papua New Guinea. The authors did not separate the data for Papua from the other two Indonesian sites on Sulawesi. Borel Best et al. (1989) proposed that western Indonesia is outside of the Coral Triangle. Allen (2002b) found a lower diversity of fishes at Weh Island off the western end of Sumatra, which is consistent with this proposal. Thus, the best current data indicates that Papua is in the area of highest coral diversity, but diversity begins to decrease to the east of Papua.
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
