The World of Coral Reefs
Patterns of diversity
Observations of life on coral reefs reveal a number
of striking patterns in the distribution of species.
At the global level, few species are ubiquitous.
Some may be widespread across one or even two ocean
basins, but many others are restricted to certain oceans or
particular seas.
As a larger picture is built up through looking at
many species, certain patterns emerge. Some regions are
highly distinctive with large numbers of endemic species,
found nowhere else. The total diversity of species is also
uneven, with centers of particularly high diversity, and
with clear gradients in diversity mirroring environmental
gradients.
When looking at finer resolutions, new patterns
emerge. Certain species appear to predominate in near
continental reefs, while others are found on oceanic reefs.
Closer still, and the position on the continental shelf, or
that relative to the prevailing wind or currents, appears to
hold sway in determining the species composition. At the
scale of tens or hundreds of meters, patterns of zonation
are observed across individual reefs, with species adapted
to different depths, exposure, water circulation and so on.
Finally, at the scale of individual points or quadrats,
the pattern of which species are found where seems to
disappear in a random noise. Even here, however, the
factors driving the settlement and survival of individuals
may be far from chaotic, but driven by highly complex
interactions, both in the immediate sense and over the life
history of the individual.
Patterns at the global scale
Corals are clearly the most important organisms when
it
comes to understanding the factors that drive the
distribution of coral reefs. The majority of reef-building
corals fall within the group known as Scleractinia. They
have been the subject of continuing studies by biologists
and taxonomists for many years, and a considerable
amount is now known about their distribution and about
the factors which influence it. Some 794 species of
sclcractinian coral are considered to be reef builders, and
Map 1.2 shows a plot of their distribution, highlighting
the patterns of varying diversity. A number of points can
be observed:
Corals, like the reefs they build, are restricted to a
narrow band of low latitudes, with diversity diminishing
fairly rapidly along latitudinal clines.
There are two distinctive regions of coral distribution,
one centered around the Wider Caribbean (the Atlantic),
the other reaching from East Africa and the Red Sea to the
Central Pacific (the Indo-Pacific).
Diversity is far lower in the Atlantic than in the IndoPacific.
Coral diversity is at its highest around insular
Southeast Asia.
Coral diversity and reef development are very restricted
along the western shores of the Americas and West Africa.
Although only relating to corals, these patterns are
reflected in most other groups found in tropical coastal
Map 1.2: Patterns of diversity in reef-building scleractinian corals
^^S:
* ""'^^^^^^^^^^^^^B Number of species
<50
50
100
200
300
400
500
-^-
Patterns of diversity
Observations of life on coral reefs reveal a number
of striking patterns in the distribution of species.
At the global level, few species are ubiquitous.
Some may be widespread across one or even two ocean
basins, but many others are restricted to certain oceans or
particular seas.
As a larger picture is built up through looking at
many species, certain patterns emerge. Some regions are
highly distinctive with large numbers of endemic species,
found nowhere else. The total diversity of species is also
uneven, with centers of particularly high diversity, and
with clear gradients in diversity mirroring environmental
gradients.
When looking at finer resolutions, new patterns
emerge. Certain species appear to predominate in near
continental reefs, while others are found on oceanic reefs.
Closer still, and the position on the continental shelf, or
that relative to the prevailing wind or currents, appears to
hold sway in determining the species composition. At the
scale of tens or hundreds of meters, patterns of zonation
are observed across individual reefs, with species adapted
to different depths, exposure, water circulation and so on.
Finally, at the scale of individual points or quadrats,
the pattern of which species are found where seems to
disappear in a random noise. Even here, however, the
factors driving the settlement and survival of individuals
may be far from chaotic, but driven by highly complex
interactions, both in the immediate sense and over the life
history of the individual.
Patterns at the global scale
Corals are clearly the most important organisms when
it
comes to understanding the factors that drive the
distribution of coral reefs. The majority of reef-building
corals fall within the group known as Scleractinia. They
have been the subject of continuing studies by biologists
and taxonomists for many years, and a considerable
amount is now known about their distribution and about
the factors which influence it. Some 794 species of
sclcractinian coral are considered to be reef builders, and
Map 1.2 shows a plot of their distribution, highlighting
the patterns of varying diversity. A number of points can
be observed:
Corals, like the reefs they build, are restricted to a
narrow band of low latitudes, with diversity diminishing
fairly rapidly along latitudinal clines.
There are two distinctive regions of coral distribution,
one centered around the Wider Caribbean (the Atlantic),
the other reaching from East Africa and the Red Sea to the
Central Pacific (the Indo-Pacific).
Diversity is far lower in the Atlantic than in the IndoPacific.
Coral diversity is at its highest around insular
Southeast Asia.
Coral diversity and reef development are very restricted
along the western shores of the Americas and West Africa.
Although only relating to corals, these patterns are
reflected in most other groups found in tropical coastal
Map 1.2: Patterns of diversity in reef-building scleractinian corals
^^S:
* ""'^^^^^^^^^^^^^B Number of species
<50
50
100
200
300
400
500
-^-