The Great Barrier Reef
44
N SPATIAL VARIABILITY IN CORAL
REEF HABITATS
It is important to realise that coral reefs are not simply
homogenous areas of corals on which organisms live.
Spatial variability occurs at a range of scales (Fig. 5.3).
At the finest scale, relatively homogenous areas such as
single coral heads, rubble patches, or algal covered rock,
form habitat patches at scales from centimetres to metres
(Box 5.1). These habitat patches may indeed form the
entire habitat of extreme specialists. Coral gobies, for
example, are restricted to a limited species range of live
coral heads and they will not move from a single coral
head. For many organisms though, the habitat patch is
not necessarily the smallest habitat unit. In a patch of
staghorn coral, for example, some species may live at
the base of the coral whereas others may live near the
top. In this way, habitat patches can also be subdivided
in a range of microhabitats. The microhabitat need not be
completely contained within a single habitat patch.
Some species use microhabitats at the transition of different patches. Physical, temporal, and biological variability generates heterogeneity in the environment at
larger scales of tens to hundreds of metres. On coral
reefs this is often termed a habitat zone, that is, a collection or mosaic of different habitat patches, and is usually
characterised by a combination of biotic, physical, and
physiographic factors. On the GBR the seaward side of
reefs (facing the continental shelf break) typically have a
shallow reef crest, which may also be present or absent
on the leeward side of reefs. This is a shallow zone in
which coral growth rates are high, but the corals are also
subjected to wave action and currents (Fig. 5.3). The reef
crest will consist of stands of different coral species,
interspersed with rock covered with encrusting algae.
The reef crest sharply breaks to a reef slope, which
is generally steeper, with a stronger depth gradient on
seaward sides of reefs that are exposed to the prevailing winds, and hence have much more coral growth
with well developed reef crests and slopes (but also
subject to more wave disturbance). This reef slope is
not just a continuous band of corals, however. On
exposed reefs in particular, the reef slope is punctuated
by grooves in which coral rubble accumulates as a
result of wave disturbance and rips. On the leeward
side of the reef crest, a shallow reef-flat zone often containing rubble, sand, and fleshy macroalgae such as
Padina or Sargassum species might occur. This zone is in
very shallow water, exposed to wave action, with little
vertical physical structure and heterogeneity. On reefs
with lagoons, the reef-flat zone will give way to a
lagoonal habitat zone, relatively sheltered from wave
disturbance. The lagoon may often be very coral-rich in
clear lagoons, but possibly devoid of corals in turbid
lagoons. In the centre of the lagoon fine sediments
occur, providing yet another set of microhabitats. Large
Porites colonies (metres wide and high) are rare on the
exposed sides of reefs, but are often common on the
leeward sides of reefs and in lagoons. On the leeward
side of a typical reef, the lagoon will give way to a backreef habitat zone. The back-reef is often rich in corals
because visibility is good; not as good as the exposed
side, but wave disturbance is much lower. This collection of habitat zones forms a landscape or seascape within
which biodiversity exists on a single coral reef and
while there is considerable small scale spatial variability in habitats, at the spatial scale of 100’s of metres to
kilometres the physical structure of the reef is relatively
unchanging and predictable at ecological time scales
and the spatial distribution of habitat responders across
a reef landscape becomes very predictable.
Clearly the integration of habitat patches into zones
in the reef landscape generates an enormous range of
different types of places for different types of organisms to live, even taxonomically related species. And
while these organisms can be characterised as ‘coral
reef’ species, they may not actually be associated with
corals themselves. However, the reefs themselves form
part of a larger seascape of multiple reefs that constitute the GBR (Fig. 5.3).
At this scale, geological, oceanographic, geographic,
environmental, and biogeographic patterns exert additional effects on the individual coral reefs that make up
the GBR (see Chapter 2). Cross shelf variability at scales
of tens of kilometres are associated with large differences in turbidity, wave exposure, and nutrient load.
Physiographic, oceanographic, and current regimes
will be different at different latitudes at scales of hundreds of kilometres due to a combination of continent
44
N SPATIAL VARIABILITY IN CORAL
REEF HABITATS
It is important to realise that coral reefs are not simply
homogenous areas of corals on which organisms live.
Spatial variability occurs at a range of scales (Fig. 5.3).
At the finest scale, relatively homogenous areas such as
single coral heads, rubble patches, or algal covered rock,
form habitat patches at scales from centimetres to metres
(Box 5.1). These habitat patches may indeed form the
entire habitat of extreme specialists. Coral gobies, for
example, are restricted to a limited species range of live
coral heads and they will not move from a single coral
head. For many organisms though, the habitat patch is
not necessarily the smallest habitat unit. In a patch of
staghorn coral, for example, some species may live at
the base of the coral whereas others may live near the
top. In this way, habitat patches can also be subdivided
in a range of microhabitats. The microhabitat need not be
completely contained within a single habitat patch.
Some species use microhabitats at the transition of different patches. Physical, temporal, and biological variability generates heterogeneity in the environment at
larger scales of tens to hundreds of metres. On coral
reefs this is often termed a habitat zone, that is, a collection or mosaic of different habitat patches, and is usually
characterised by a combination of biotic, physical, and
physiographic factors. On the GBR the seaward side of
reefs (facing the continental shelf break) typically have a
shallow reef crest, which may also be present or absent
on the leeward side of reefs. This is a shallow zone in
which coral growth rates are high, but the corals are also
subjected to wave action and currents (Fig. 5.3). The reef
crest will consist of stands of different coral species,
interspersed with rock covered with encrusting algae.
The reef crest sharply breaks to a reef slope, which
is generally steeper, with a stronger depth gradient on
seaward sides of reefs that are exposed to the prevailing winds, and hence have much more coral growth
with well developed reef crests and slopes (but also
subject to more wave disturbance). This reef slope is
not just a continuous band of corals, however. On
exposed reefs in particular, the reef slope is punctuated
by grooves in which coral rubble accumulates as a
result of wave disturbance and rips. On the leeward
side of the reef crest, a shallow reef-flat zone often containing rubble, sand, and fleshy macroalgae such as
Padina or Sargassum species might occur. This zone is in
very shallow water, exposed to wave action, with little
vertical physical structure and heterogeneity. On reefs
with lagoons, the reef-flat zone will give way to a
lagoonal habitat zone, relatively sheltered from wave
disturbance. The lagoon may often be very coral-rich in
clear lagoons, but possibly devoid of corals in turbid
lagoons. In the centre of the lagoon fine sediments
occur, providing yet another set of microhabitats. Large
Porites colonies (metres wide and high) are rare on the
exposed sides of reefs, but are often common on the
leeward sides of reefs and in lagoons. On the leeward
side of a typical reef, the lagoon will give way to a backreef habitat zone. The back-reef is often rich in corals
because visibility is good; not as good as the exposed
side, but wave disturbance is much lower. This collection of habitat zones forms a landscape or seascape within
which biodiversity exists on a single coral reef and
while there is considerable small scale spatial variability in habitats, at the spatial scale of 100’s of metres to
kilometres the physical structure of the reef is relatively
unchanging and predictable at ecological time scales
and the spatial distribution of habitat responders across
a reef landscape becomes very predictable.
Clearly the integration of habitat patches into zones
in the reef landscape generates an enormous range of
different types of places for different types of organisms to live, even taxonomically related species. And
while these organisms can be characterised as ‘coral
reef’ species, they may not actually be associated with
corals themselves. However, the reefs themselves form
part of a larger seascape of multiple reefs that constitute the GBR (Fig. 5.3).
At this scale, geological, oceanographic, geographic,
environmental, and biogeographic patterns exert additional effects on the individual coral reefs that make up
the GBR (see Chapter 2). Cross shelf variability at scales
of tens of kilometres are associated with large differences in turbidity, wave exposure, and nutrient load.
Physiographic, oceanographic, and current regimes
will be different at different latitudes at scales of hundreds of kilometres due to a combination of continent