Patterns
of Coral Reef Diversity Coral
reefs are areas of immense natural diversity. They accommodate some
of the highest densities of animals on earth and have more species
per unit area than any other marine habitat. Reef organisms are
not distributed evenly throughout the ocean, as anyone who has dived
both Caribbean and Indo-Pacific reefs will testify. The number of
different species differs dramatically between the two regions due
to the many different pressures that have moulded each community
over many millions of years. Today’s Diversity Today’s two main regions of reef diversity centre on the Caribbean and Indo-Pacific. The number of fish and coral species is highest around the many islands of South East Asia, specifically Indonesia, the Philippines and Papua New Guinea. This highly diverse area supports 600 species of coral and 4,000 fish compared to 62 and 1,400 species respectively in the Caribbean. In fact for the majority of reef organisms there are 10-30% as many species in the West Atlantic as Asia and no fish species are shared between the two oceans. Whilst the number of species in the Caribbean appears low they are highly unique compared to those of the Pacific. millions of years ago… Hundreds
of millions of years ago one large land mass named Pangaea consisted
of an amalgamation of today’s continents. During the Cretaceous
period, 144-65 million years ago, North and South America began
to drift away from Europe and Africa thanks to the movement of the
earth’s crust. At this time the Americas were not joined by
the land bridge that today includes Panama, Costa Rica and Mexico.
A huge tropical sea spanned much of the globe’s circumference
from Asia round to Europe with abundant coral reef habitat. As the
land bridge between the two American continents began to form, water
currents were disrupted isolating fish populations in today’s
Atlantic and Pacific Oceans. A land bridge finally joined the landmasses
approximately 3.5 million years ago. Immediately after the formation of the Caribbean Sea the fish species present mirrored those found in the eastern Pacific. Soon after the break up of the water bodies the communities on the Caribbean side began to encounter very different environmental conditions. The area available for coral growth in the Caribbean is relatively small due to the presence of great rivers such as the Mississippi and Amazon that bring large amounts of sediment into the ocean as they enter. This sediment blocks sufficient light from reaching corals and prevents their growth at the northern and southern extremes of the Caribbean. Caribbean reef communities were further moulded by ice ages that reduced the temperature of the water and killed off many of the organisms that require balmy tropical waters. Moulding the Caribbean The
Caribbean’s distinct environmental pressures helped to shape
the organisms into an entirely different community. Natural selection,
or survival of the fittest, weeds out the species and individuals
that are poorly suited to a given environment and helps those with
traits that improve their survival. Over millions of years these
pressures would have gradually created Caribbean reefs that share
no species identical to those in the Pacific, for example the Hamlets
that only occur in the Caribbean. Species that split from a common
ancestor into two or more spe Pacific Diversity Species
diversity is considerably higher within insular South East Asia
than the Caribbean. The species count for fish or corals in one
Indonesian bay can easily exceed the number of species found in
the entire West Atlantic. The region contains an extraordinarily
large area of shallow coral reef habitat, which appears to be imperative
for high species diversity. Historically, the Pacific has been more
climatically stable than the Atlantic and large reef area has provided
many refuges for organisms if conditions became unfavourable. The
broad north-south range of reef habitat also enabled organisms’
ranges to shift as local conditions altered. There is some debate over the explanation for such diversity in the Asian region. One explanation is that this is a hotbed for evolutionary change and species are created here subsequently spreading to other regions of the Pacific. Another possibility is that in this region the ranges of many species overlap from the Indian or Pacific Oceans causing high diversity. The former of these two possibilities appears most likely as genetic analysis of one group, the Parrotfishes, shows Asia to be their original geographical home from where they spread outwards to a global distribution. Endemism As
well as having the highest number of species in the Indo-Pacific,
South East Asia also has the greatest number of endemic species.
These are species with a restricted geographical range often indicating
the species evolved in that location and subsequently isolated there.
Certain areas of the ocean are more prone to high levels of these
restricted range species. The Red Sea has 41 species of endemic
fish such as the common Masked Butterflyfish and Red Sea Anemonefish,
New Caledonia has 43 including the Three Stripe Butterflyfish and
Half Band Angelfish, the Great Barrier Reef has 33 such as the Great
Barrier Reef Anemonefish and Madagascar has 31 including the Reunion
Angelfish: these fish species are found nowhere else. The ‘coral
triangle’ of Indonesia, New Guinea and the Philippines on
the other hand outshines these areas with 90 species of fish that
inhabit only these reefs. Focus: Ghost pipefishes The
six or so species of Ghost pipefishes, that inhabit Indonesia and
surrounding countries, are masters of disguise in their specific
habitats. Each is highly adapted to blend into their immediate environment. A
potential explanation for their splitting is that as they became
more adapted to their particular Human Impact Global patterns of species diversity are naturally in a continuous state of flux as organisms colonise new areas and exploit new opportunities. Man’s technological advances have had unexpected affects on reef organisms as they can now reach areas that once were well beyond their reach. The Suez and Panama canals link seas and their inhabitants in ways that would never happen naturally and could cause dramatic disruptions as neighbours meet for the first time. Black tip reef sharks are for the first time arriving in the Mediterranean from the Red Sea, Pacific nudibranchs are arriving in the ballast water of ships in the Caribbean and Asian Lionfish live in the northern Caribbean thanks to released aquarium subjects. These illegal aliens have the potential to severely disrupt their new home and the diversity that has evolved in isolation. Conclusion With very limited resources available to conservation efforts the identification of regions that contain a high species diversity or many endemic organisms may help pinpoint areas of conservation priority. Whilst scientific data indicates that protection of South East Asian reefs will preserve the most species per unit area there continues to be little conservation impetus in the area. Due to evolutionary processes several areas with higher than average species diversity have arisen that should receive specific conservation attention. Jargon
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cies
are known as sister species. An example of this are the small Pacific
reef Groupers that closely resemble Caribbean Hinds. Both species
are in fact Groupers, although Caribbean species have altered slightly
over time. 
Adapting to these specific habitats appears to have fuelled the
splitting of a single species into several. As populations of ghost
pipefish began to inhabit distinct areas of the reef including algae,
Feather Stars and debris they became increasingly camouflaged through
natural selection. Each species of Ghost pipefish occupies a given
habitat; Ornate is found in Feather Stars and Soft Corals, Robust
lives among seagrass and debris such as leaves and twigs, the Velvet
ghost pipefish mimics a small upright sponge and the Halimeda Ghost
pipefish lives among Halimeda algae.
habitat
on the reef the different populations no longer came into contact,
which prevented them crossbreeding. Another possibility is that
whilst the coexisting populations became more externally adapted
to specific habitats and interbred the offspring would be insufficiently
camouflaged in any habitat and would fall prey to predators and
would not pass on their genes.