Reef Ecology

Isolation and diversity  

Hawaii lies 2,367km north of the equator and 4,025km southwest of the nearest continental land mass, North America.  The Hawaiian islands are the most isolated archipelago in the world, lying far to the north and east of the vast majority of Pacific islands.        

Their geographic isolation has been emphasised by the dominant westerly flowing North Pacific Equatorial Current which has reduced the likelihood of pelagic larvae being carried to the islands.  This considerable isolation, on the northern edge of the tropics, has meant that Hawaiian reefs have not become very diverse.  For example, in Hawaii there are said to be 52 species* of reef building corals; whereas in the western Pacific island of Palua there are over 300.  However, the lack of competition has meant that the fewer species present have been able to spread more widely throughout Hawaii's reefs than would otherwise have been the case.

This isolation has also created opportunities for the development of new species.  Besides the stony corals, there are 500 species of near-shore fish, thousands of marine molluscs and 450 marine algae have been recorded, of which typically 25% are endemic (i.e. unique to the Hawaiian islands and found naturally nowhere else).  This is the highest level of endemism found anywhere in the world.  A list of some of the near-shore fish species found around Hawaii can be found here.

One particular feature of the coral community is the relative paucity of corals from the Acropora genus - major reef builders and the most common and diverse group of stony corals in the rest of the Indo-Pacific region.  Unusually Acropora are very rare in the main Hawaiian islands, and while a few Acropora species have reached as far as the main island of Kauai the epicentre of the Hawaiian population appears to be the French Frigate Shoals.  In these north-western Hawaiian Islands Acropora species are reported to be comparatively abundant.  Their presence is thought to be due to larval settlement drifting in from Johnston Atoll, some 650 km to the south-west, as Hawaiian waters are believed to be too cool for them to reproduce locally. 

Acropora are therefore rarely dominant, the major reef-building species for most of the Hawaiian islands being massive encrusting colonies of Porites and also Pocillopora, with Montipora species also having a significant presence.  Some lists of endemic and indigenous stony coral species, that I have collated from Veron, or been produced by Hawaii's Comprehensive Wildlife Conservation Strategy, can be found here.

Soft corals seem to have an even more restricted presence and variety than stony corals.  Because of their younger age and the general absence of barrier reefs in the windward islands, Hawaiian reefs are generally less productive than other reefs.  With smaller areas of reef and the absence of lagoons that trap and retain both coastal and terrestrial nutrient run-off, Hawaiian reefs tend to be more nutrient poor and therefore lack an abundance of filter feeding animals such as sponges, tunicates, bivalves, and soft corals.  As a result, Hawaiian reefs are more clearly dominated by its restricted range of stony corals.   The few soft coral species that I have found references to are listed here.  

Reef formation and development  

The reefs of the main Hawaiian islands are geologically young and therefore not as well developed as most other reefs in the Indo-Pacific.  As explained in the Geography and Geology section, the age of the Hawaiian islands increases as you move along the archipelago from Hawaii's 'Big Island' towards the north-west.  Coinciding with the increasing geological ages of the islands is the degree of reef development, with a general trend towards increasing reef development as you travel westward.    

However, as the movement of the Pacific plate pushes these islands north-westwards into increasingly cooler waters the islands also begin to slowly sink back towards the ocean floor.  It  was Charles Darwin who explained how these geological events interacted with reef development to create atolls. 

Corals first take a hold on the island shoreline and grow and develop into fringing reefs projecting out to sea.  As explained in the Geography and Geology section, the islands are gradually sinking but the reef is still able to grow and maintain its position.  So as the island sinks a gap opens up between the island and the still growing reef, which now forms a barrier around the island.  Eventually, the island submerges completely and all that is left are these barrier reefs which form an atoll.     

You see this pattern of reef development in the Hawaiian Island chain.  Most reefs in the windward islands of Hawaii, Maui, Oahu and Kauai are small and occur relatively close to shore.  As these reefs grow and develop they become fringing reefs, which eventually develop into barrier reefs, which are found on Oahu and on the south shore of Molokai.  As you travel further westward along the chain the barrier reefs give way to the atolls of the French Frigate Shoals, Pearl, Midway and Kure.

Kure is the last of the atolls in the Hawaiian island chain.  Beyond this point is the submerged Emperor Seamount Chain (see the graphic in Geography and Geology) which were once reef-capped volcanic islands.  No atolls are left behind by these submerged islands because of another consequence of the north-westerly drift of the Hawaiian island chain.  Stony coral growth is dependent on nutrients gathered either from the water, or from the zooxanthellate stored within them.  The surrounding waters are renowned for their low nutrient content and productivity, and runoff from the islands bringing nutrients is therefore a significant factor for reef development.  But as the land mass subsides the amount of runoff reduces.  Also the further North the islands drift the cooler the waters become.  These two factors combine to create conditions where the reefs find it increasingly hard to grow and keep up with the rate of crustal subsidence.  Eventually they will pass the 'Darwin point' - where reef growth will be insufficient to keep up with the sinking of the island beneath it.  The atoll will then slowly begin to be pulled deeper and deeper, eventually passing below the regions where zooxanthellate coral can survive.  Kure Atoll will be next to pass the Darwin point.

Overlaid on top of this westward pattern of aging, increasing reef development and then eventual decline, are the effects of wave exposure.  The Zonation page provides more details on the effect of the physical environment on the distribution of corals and other organisms in the Hawaiian islands.

* This figure of 52 stony corals is frequently quoted in the literature but Veron's Corals of the World suggests the presence of more than 70