5 – Coral Reef Habitats and Assemblages
49
were generated by, or covered by corals. These different
temporal and spatial scales of habitat variability combine
synergistically to provide an enormous range of different types of potential habitat for a huge diversity of
organisms on the GBR.
N CONSEQUENCES OF HABITAT VARIATION
AND HABITAT ASSOCIATION FOR CORAL
REEF ASSEMBLAGES
We defined habitat as the place where an organism lives,
yet discussed habitat patches and zones (e.g. reef slope,
rubble grooves) relatively independently of any particular organism. This is because defining a habitat
based on where an organism lives introduces a curious
circularity in its definition. For example, the damselfish
Pomacentrus coelestis is primarily found living on rubble. This introduces a problem because it implies there
could be ‘empty’ habitats in the environment: if a patch
of rubble doesn’t contain P. coelestis then is the patch
suitable habitat for the species? So the challenge lies in
defining habitats even though they may be unoccupied. Of course in the example above, this is not too
difficult if we view suitable habitat as a probability of
occupation, rather than strict occupancy of any single
particular piece of space. While there is spatial and
temporal variability in habitats, there is also greater
spatial and temporal variability in species abundance.
In general, rubble patches on reefs contain P. coelestis,
and conversely P. coelestis is usually found on rubble
patches. The abundances of course may vary on leeward v. seaward sides of the reef, across the continental
shelf, and latitudinally because of increasingly larger
scales of environmental variation. This introduces
another problem though: if habitat is the place where
an organism generally lives, then we need to identify
the scales at which an organism perceives and responds
to the habitat characteristics, and also identify what the
important characteristics are.
Determining what exactly is important about the
habitat to an organism can be surprisingly difficult for
some species. In an obligate species such as a coral goby
or coral crab, it is easy—a particular species of coral
needs to be present. For species with a wider range of
movement such as wrasses, then simple correlations
between species abundance and different types of coral
cover can be misleading. For example, many fish species
appear to be positively correlated with soft coral cover,
but removal of soft corals at small scales may result in
no change to the fish assemblage. The apparent correlation may simply be due to wrasses preferring habitats
within a diverse range of substratum types, coupled
with soft corals also being positively correlated with a
diverse range of substratum types. Prior knowledge of
the habitat characteristics that are actually important to
organisms is essential for identifying and predicting
responses of organisms to habitat changes.
Identifying coral reef habitats, environments and
cross shelf and latitudinal gradients is important both
for pure ecological studies, and management of the
GBR. Many ecological questions and management
solutions require the habitats of a range of organisms
to be identified or protected, so measuring habitat
diversity is an important task because it is closely
linked to species diversity. This is often known as a
community-level approach, although a community in a
strict sense is a complete suite of interacting species.
On coral reefs ‘resident’ species also interact with nonresident species. For example planktivores eat pelagic
species, and pelagic fishes such as jacks (Carangidae)
may eat resident fishes despite not being considered
‘coral reef’ fishes themselves. Additionally, some fish
species such as some wrasses feed on hard reef and
soft bottom substratum, thus linking the hard and soft
bottom ‘communities’ energetically. In reality, the
community-level approach is really an assemblage-level
approach, in which we consider a co-occurring subset
of the entire range of interacting species, so we will
use the word assemblage in preference to community.
Measuring or managing the habitats of a wide range of
species raises some interesting problems. First, if habitat can only be defined as the place where an organism
lives, do we need to identify the typical range at which
each of the organisms in the community perceive their
habitat? Obviously we could not do this easily with
such a diverse flora and fauna, with each species interacting at different scales with their habitat and each
other. While this may be an important nuisance to
coral reef ecologists, for GBR managers the solution is
rather easier. Simply recognising habitat as a spatial
49
were generated by, or covered by corals. These different
temporal and spatial scales of habitat variability combine
synergistically to provide an enormous range of different types of potential habitat for a huge diversity of
organisms on the GBR.
N CONSEQUENCES OF HABITAT VARIATION
AND HABITAT ASSOCIATION FOR CORAL
REEF ASSEMBLAGES
We defined habitat as the place where an organism lives,
yet discussed habitat patches and zones (e.g. reef slope,
rubble grooves) relatively independently of any particular organism. This is because defining a habitat
based on where an organism lives introduces a curious
circularity in its definition. For example, the damselfish
Pomacentrus coelestis is primarily found living on rubble. This introduces a problem because it implies there
could be ‘empty’ habitats in the environment: if a patch
of rubble doesn’t contain P. coelestis then is the patch
suitable habitat for the species? So the challenge lies in
defining habitats even though they may be unoccupied. Of course in the example above, this is not too
difficult if we view suitable habitat as a probability of
occupation, rather than strict occupancy of any single
particular piece of space. While there is spatial and
temporal variability in habitats, there is also greater
spatial and temporal variability in species abundance.
In general, rubble patches on reefs contain P. coelestis,
and conversely P. coelestis is usually found on rubble
patches. The abundances of course may vary on leeward v. seaward sides of the reef, across the continental
shelf, and latitudinally because of increasingly larger
scales of environmental variation. This introduces
another problem though: if habitat is the place where
an organism generally lives, then we need to identify
the scales at which an organism perceives and responds
to the habitat characteristics, and also identify what the
important characteristics are.
Determining what exactly is important about the
habitat to an organism can be surprisingly difficult for
some species. In an obligate species such as a coral goby
or coral crab, it is easy—a particular species of coral
needs to be present. For species with a wider range of
movement such as wrasses, then simple correlations
between species abundance and different types of coral
cover can be misleading. For example, many fish species
appear to be positively correlated with soft coral cover,
but removal of soft corals at small scales may result in
no change to the fish assemblage. The apparent correlation may simply be due to wrasses preferring habitats
within a diverse range of substratum types, coupled
with soft corals also being positively correlated with a
diverse range of substratum types. Prior knowledge of
the habitat characteristics that are actually important to
organisms is essential for identifying and predicting
responses of organisms to habitat changes.
Identifying coral reef habitats, environments and
cross shelf and latitudinal gradients is important both
for pure ecological studies, and management of the
GBR. Many ecological questions and management
solutions require the habitats of a range of organisms
to be identified or protected, so measuring habitat
diversity is an important task because it is closely
linked to species diversity. This is often known as a
community-level approach, although a community in a
strict sense is a complete suite of interacting species.
On coral reefs ‘resident’ species also interact with nonresident species. For example planktivores eat pelagic
species, and pelagic fishes such as jacks (Carangidae)
may eat resident fishes despite not being considered
‘coral reef’ fishes themselves. Additionally, some fish
species such as some wrasses feed on hard reef and
soft bottom substratum, thus linking the hard and soft
bottom ‘communities’ energetically. In reality, the
community-level approach is really an assemblage-level
approach, in which we consider a co-occurring subset
of the entire range of interacting species, so we will
use the word assemblage in preference to community.
Measuring or managing the habitats of a wide range of
species raises some interesting problems. First, if habitat can only be defined as the place where an organism
lives, do we need to identify the typical range at which
each of the organisms in the community perceive their
habitat? Obviously we could not do this easily with
such a diverse flora and fauna, with each species interacting at different scales with their habitat and each
other. While this may be an important nuisance to
coral reef ecologists, for GBR managers the solution is
rather easier. Simply recognising habitat as a spatial