Coral reefs are places of immense natural diversity. They accommodate some of the highest densities of animals on earth and have more species than any other marine habitat. However, the species inhabiting coral reefs are not distributed evenly through the oceans.
Richness differs dramatically between regions due to the many different pressures that have molded each community over millions of years. The world’s highest marine biodiversity is found in an area that has become known by conservationists as the ‘Coral Triangle’.
This region of mega-diversity is a roughly triangular area extending from central Indonesia to Papua New Guinea and the Solomon Islands, and northwards to the Philippines. This relatively small area, on a global scale, boasts the world’s richest marine biodiversity. As you travel in any direction from there the number of species decreases.
The richest reefs
I was lucky enough to conduct much of my PhD research, on the biology and conservation of pygmy seahorses, at Wakatobi Dive Resort located at the heart of the Coral Triangle. For more than six months, over a three-year period, I dived the reefs and really got chance to appreciate the amazingly high biodiversity of this region.
Due to the nature of my work, I spent hundreds of hours in one specific location on the reef and as a result got to know the area like the back of my hand, and the local residents like family.
Over the years I had the opportunity to appreciate the change, or lack thereof, on the reef. Astoundingly over the three years, there was almost no perceptible change in the size of sponges, whips and soft corals in my small overhang on the Wakatobi House Reef. I was constantly amazed that, even after six months, I would still encounter something new almost daily.
One day I found a tiny beige frogfish half the size of my little finger nail, the next a nudibranch I had never seen before crawling across my path and the following evening a bright orange shrimp crept from beneath a sponge. Such is the Coral Triangle, the world’s most biodiverse marine habitat.
Today, the two main areas of reef diversity centre on the Caribbean and Indo-West Pacific. The biodiversity in these two areas, however, is quite different: the species richness in the Indo-West Pacific vastly surpasses that of the Caribbean.
The Indo-West Pacific supports at least 600 species of coral and 4,000 fish, compared to 62 and 1,400 species respectively in the Caribbean. The species count for either fish or corals in one Indonesian bay can exceed the number of species found in the entire West Atlantic.
In fact, for the majority of reef organisms there are 10-30% as many species in the West Atlantic as Asia, and no reef-associated fish species are shared between the two bodies of water. Whilst the number of species in the Caribbean appears low, they are in fact entirely different to those of the Pacific due to millions of years of separation.
There is some debate over the explanation for such high Asiatic diversity. One theory is that the area is a hotbed for evolutionary change and species are created here, with some eventually spreading to other regions of the Pacific.
Another possibility is that the ranges of many species from the Indian and Pacific Oceans overlap in the Asian archipelago, causing higher diversity where they coexist. In addition, the geology of the region has origins in many areas, each with their own fauna. The Australian and continental Asian land fragments each contribute their own unique assemblage of organisms.
Whilst the true explanation is probably a combination of these factors, the most simple and palatable reason for high diversity is the huge diversity of habitat types in the Coral Triangle. The profusion of different habitats equals a correspondingly high diversity of organisms to inhabit them: sheltered inner shore habitats have their own set of species, whilst exposed atolls have another.
Beneficially, the Coral Triangle has also avoided the mass extinctions that have blighted other areas over the millennia. Instead, the Coral Triangle has been blessed with long periods of warm, stable conditions fostering the persistence of species.
Fish doctors without borders
Last year I had the opportunity to join Wakatobi’s liveaboard, Pelagian on an itinerary that ventured beyond the resort’s reach and explored other islands in the chain. Whilst the resort has a profusion of different habitats including steep walls, pinnacles, ridges and bays, I had often heard about the muck dives found on Buton island, close to the Sulawesi mainland.
Muck dives are interesting as they host a diverse set of species you are unlikely to see in other habitats, due to their distinct set of environmental conditions. Ghost pipefish, seahorses, frogfish and countless other oddities make this their home. It is a perfect example of the influence habitat diversity can have on overall species richness.
The mangrove forests, shallow seagrass beds and even the open ocean are just a few of the other habitats that contribute their own collection of unique species to the Coral Triangle.
During our tour of the Tukangbesi archipelago, in which Wakatobi Resort lies, I also came across, for only the second time, a pair of Denise’s pygmy seahorses living on a whip coral colony. Unusual associations and biological quirks such as this seem quite commonplace in the Coral Triangle. With the Coral Triangle as a production line for new species, it might be that these pygmy seahorses, or their offspring, are better adapted to life on a whip coral. In many generations time it might be that these pygmies split off to become a new, distinct species.
Endemism: riding the wave
As well as accommodating the highest number of species in the Indo-Pacific, South East Asia also has the greatest number of endemic or indigenous species (species that occur nowhere else). Certain areas of the ocean are more prone to high levels of these restricted range species: Hawaii has 86 species of endemic reef fish, the Red Sea 41, New Caledonia 43 and the Great Barrier Reef 33. The Coral Triangle vastly outshines all these areas, with over ten percent of its almost two and a half thousand species found only there.
In 2007, when I was first at Wakatobi conducting my pygmy seahorse observations, one of the guides found a tiny pipefish-like fish. It was distinct in several ways: it’s miniature stature of less than 3cm in length, strange swimming method resembling a sea dragon and red wisp-like filament on the head all indicated to me this was something new. Later that year the pygmy pipehorse, Kyonemichyths rumengani, was scientifically described. Known initially only from Sulawesi in Indonesia, this tiny fish is now also recorded from Halmahera and Raja Ampat and seemingly making it another Coral Triangle endemic.
Currents play a major role in the movement of organisms around the ocean, and reef communities can become isolated from others depending on local current systems. The East Australian Current, for example, flows from tropical equatorial waters towards the much cooler waters of southern Australia and Tasmania. This effectively backs the reef organisms up against uninhabitable cold waters, isolating the organisms from other populations and fuelling their evolution into distinct species.
Certain species are especially susceptible to such conditions and form a higher than average proportion of the endemics. Those species whose juvenile forms spend long periods drifting in the ocean as miniscule larvae tend not to become isolated as they are able to reach distant reefs during this period, increasing their range. Other species, such as anemonefish and dottybacks, have well-developed young that settle very quickly on the local reef. The young, therefore, do not get chance to move far from their place of birth before settling, and thus have a propensity towards endemism.
Patterns of marine biodiversity around the globe are historically quite stable and species evolve to fill a specific role within their own community. Man’s technological advances have had unexpected affects on marine organisms, as they can now reach areas that were once physically well beyond their reach. The Suez and Panama canals link bodies of water, and their inhabitants, in ways that would never meet naturally. For example, blacktip reef sharks have been found for the first time in the Mediterranean having originated in the Red Sea. Pacific nudibranchs are also arriving in the ballast water of ships in the Caribbean and Indo-Pacific Lionfish have invaded the Caribbean thanks to released aquarium subjects. These illegal aliens have the potential to severely disrupt their new home and the diversity that has evolved without them.
I was very pleased to see the energy that Wakatobi put into protecting their reefs. Great effort is made to educate and work with local communities to prevent destructive practices such as dynamic or cyanide fishing on local reefs. In fact, much of the revenue from guests visiting the resort goes directly into local villages in payment for a strict no-fishing policy on fifteen miles of reef surrounding the resort.
The extremely remote location, extraordinarily high biodiversity and conservation efforts in the area make this a safe haven for many species that are suffering throughout the Coral Triangle.
With very limited resources available to conservation efforts, the identification of regions that contain high species diversity or many endemic organisms may help pinpoint areas of conservation priority. Scientific data indicates that the protection of South East Asian reefs will preserve the most species; however, there are also many other areas deserving of a conservation focus.
Richard Smith is a British marine biologist and photojournalist. As well as writing for many publications internationally, he leads marine life expeditions where the aim is for participants to get more from their diving and photography by learning about the marine environment: www.OceanRealmImages.com