Hotspots of biodiversity and endemism
‘Hotspot’ is a term frequently used by conservation biologists to denote a relatively restricted geographic area containing exceptionally high levels of biodiversity and/or endemism, a concept that has been effective in prioritising conservation activities where resources are limited. With 83% of all the species of the Indo-Pacific the so-called ‘Coral Triangle’ (see below) is clearly the centre of Indo-Pacific coral diversity, with minor centres in Madagascar and the Red Sea / Arabia.
Diversity may be the result of a high level of endemism or the overlap in the ranges of species with wide ranges. The former category contributes only 2.5% and 7.4% of the diversity of the Coral Triangle and the Red / Arabian Sea region respectively, and is a smaller component of all other diverse ecoregions.
Patterns of endemism are created by isolation; either geographic distance (locations remote from centres of diversity), or geographic enclosure. It is a complex issue to address in corals and other taxa that may be widely dispersed because it requires knowledge of where a species does not occur as well as were it does.
Coral TriangleThe Coral Triangle is an area of 5.5x106 km2 of ocean territory of Indonesia, the Philippines, Malaysia (Sabah), East Timor, Papua New Guinea and the Solomon Islands. The highest species richness resides in the Bird’s Head Peninsula of Indonesian Papua, which hosts 574 species, with individual reefs having up to 280 species ha-1, over four times the total zooxanthellate scleractinian species richness of the entire Atlantic Ocean. Within the Bird’s Head, The Raja Ampat Islands has the highest diversity of 553 species.
Importantly, the boundary of the Birds Head diversity centre is not strongly delineated, nor is this region markedly distinct from neighbouring ecoregions to the south and east. Ninety-five per cent of Coral Triangle species are found in one or more adjacent ecoregions (notably other parts of SE Asia (including Malaysia, Thailand and Vietnam) Micronesia, the northern Great Barrier Reef, Vanuatu, New Caledonia and Fiji) although all exhibit marked declines in species richness and ubiquitousness.
Habitat diversity is clearly the key to the reason why the Coral Triangle is so diverse; however there are other semi-independent explanations. The fossil record suggests that the corals of the Coral Triangle are the world’s youngest – less than half the mean age of their Caribbean counterparts. These relatively young genera either evolved in the region of the Coral Triangle or have survived there since going extinct elsewhere. The region deep water in close proximity to reef areas providing minimal dislocation during times of rapid sea-level change.
Distribution maps allow measures of regional similarity to be calculated which take in to account not only the number of species, but also the identity of those species. The result is intuitive: the broad pattern of similarity among regions primarily reflects their degree of geographic proximity. Atlantic corals are very distinct from Indo-Pacific corals. The Caribbean/Gulf of Mexico is a distinctive region, with the corals of Bermuda and the east American coast most closely allied to it. In the Indo-Pacific, the far eastern fauna is also very distinctive. To a lesser extent so also are the corals of the Red Sea. The least distinctive are the regions of the central equatorial Indo-Pacific.
Tropical species are not replaced with subtropical species as latitude increases, but rather there is a gradual attenuation (thinning out) of tropical species. This is correlated with decreasing temperature, but it is also correlated with the poleward direction of the main surface currents. What the currents do is perpetually move planulae away from the equatorial centre of diversity to higher latitudes. This creates a ratchet effect: the corals can go in one direction only as there is no return. Subtropical locations are thus genetically connected to tropical locations, but not the reverse. This makes it possible to ‘lock up’ corals in high-latitude regions and this is the probable reason why there are a large number of geographic subspecies found at high latitudes.
These patterns also show that there is usually a substantial difference in species diversity between reef and non-reef communities. This is not directly related to temperature. The majority of corals can tolerate water temperatures as low as 14°C for protracted periods of time, but corals can only build large consolidated reefs if the temperature normally stays above 18°C. Rather, it is related to the capacity of corals to out-compete macro-algae. In effect, coral species act cooperatively to create habitats which will support a diverse coral community (see Environment).
Explanations of distribution patterns
Corals, more than any other group of marine invertebrates, have attracted the attention of marine biogeographers. This is partly due to the interest created by Darwin’s book about coral reefs and subsequent debates about some of the issues it raised, but also because of the obvious concentration of both corals and reefs in tropical locations. The result has been a progression of publications which contain contour maps giving various revisions of generic-level maps. A wealth of hypotheses has been put forward to give evolutionary explanations for these maps.
The explanation of distributions is best understood as a progression of layers of detail. Family-level distribution provides the ancient basic template. This pattern is the present-day remnant of major global geological and climatological changes, especially continental movements and major extinction events. Super-imposed on this is the generic-level template. This ‘inherits’ the family-level background but is largely the outcome of more recent geological events, especially the obliteration of the Tethys Sea and the closure of the Central American Seaway. Super-imposed on this again is the species-level pattern. This ‘inherits’ the generic pattern but is largely the outcome of Pliocene to modern climates and their effect on ocean currents.
Control of diversity and dispersion
Outlying areas of the Pacific have, to a large extent, subsets of the species of the centre of diversity: most peripheral patterns are therefore the result of outward dispersion from that centre, they are not the result of evolution by regional isolation.
There are essentially two reasons why there is an Indo-Pacific centre of diversity. The first is due to currents. The main currents of the equatorial Pacific flow in a westward direction to the centre, which therefore acts as a catch-all for planula larvae. The second is due to habitat diversity. The centre of diversity is the world’s largest tropical archipelago of islands, Philippines and Indonesia, contains 37% of the world’s coral reefs. These islands have convoluted coastlines offering a wide range of habitats, all in close proximity. These habitats are closely linked by variable currents. Species diversity is maintained by habitat diversity and habitat accessibility: again, this diversity is not an outcome of regional evolution.
There is a balance between species diversity and dispersal capability, a balance maintained or changed by ocean circulation patterns. If currents weaken, diversity will appear to increase because genetic isolation increases, creating pockets of semi-isolated taxa which are indistinct from their relatives. If currents strengthen, diversity will appear to decrease because genetic isolation decreases, causing taxa to become widespread through genetic intermixing. These taxa will be relatively well defined.
In effect, changing currents cause genetic diversity to be endlessly ‘repackaged’ from large numbers of relatively isolated taxa into smaller numbers of relatively widespread taxa, then back again. This occurs in cycles that have variable impacts and frequencies. Substantial biogeographic change may occur at intervals of thousands of years or less and these may be associated with sufficient repackaging to form geographic races or varieties. Major cycles, or the additive effects of many cycles, may create species-level distinctions.
Dispersal of most terrestrial animals is an active undertaking. It requires a capacity for mobility, it allows choice of direction, and there is a high probability of surviving the journey. Dispersal of most marine animals is passive: it is controlled by ocean currents. It requires little effort, there is no choice of direction, and there is a low probability of survival. For these reasons, most dispersal on land is undertaken by adult animals, whilst in the ocean dispersal is usually undertaken by larvae. Reproduction and dispersal, for most marine animals, are therefore closely linked subjects. Only some larger vertebrates – oceanic fish, reptiles and mammals – are able to defy the currents and undertake active dispersion. For the rest of marine life, the pathways of dispersion cannot be controlled, or (except for timing) even selected for. This has enormous significance for all aspects of coral biology, including taxonomy and evolution.
Principal global circulation patterns today. The main currents illustrated are the highways of dispersion, but there are many other pathways created by regional detail and seasonal, annual and long-term variations. All but the greatest of these currents are subject to changes associated with climate, tectonic alteration of coastlines, and fluctuations in sea level
Because most marine organisms (including corals) are dispersed by larvae, the paths of the currents are the paths of gene flow – the paths of genetic connections. Currents are largely responsible for creating, and breaking, the distribution ranges of species. For most species, these ranges are very large, so large that the genetic composition of a species in one part of the range may be very different from that in another. For corals, as with many other major groups of plants and animals, this variation in genetic composition reaches a point where variation within species merges with variation between species. When this happens, geographic patterns are created where the morphological boundaries of species (the limits to what a taxonomist might call a species) become arbitrary.
Importantly, distribution patterns are the outcomes of dispersal by currents: not just the currents of today, but all the currents that existed during the evolutionary history of the species.