5 – Coral Reef Habitats and Assemblages
47
survival of pathogens, which may affect habitat determiners such as Diadema urchins. The interaction between
physical and biological processes is important because
of the different scales of each; physical processes, which
usually occur at a large scale, may effectively force biological processes to occur at a similar scale. This spatial
autocorrelation (i.e. the abundance or ecological interactions of organisms are more similar at locations closer
to each other) has been termed the Moran Effect. The
consequence of this is that minor biological effects that
would appear only to exert local effects can, if generated by physical forcing, occur over a much larger spatial extent than would be predicted.
Small scale disturbances may also have important
and immediate consequences for habitat structure.
Storm damage can range from localised damage to
individual coral heads, up to complete coral removal
across 100 m swaths within windward sides of reefs,
and across several reefs (Fig. 5.4E, F). These types of
disturbances are not necessarily detrimental to a reef’s
overall health. Removal of a coral colony by storm
damage frees up space for recolonisation by other species. It also generates heterogeneity in the substratum,
so habitat responders that prefer bare rock or rubble
can occupy the space that is cleared by the disturbance.
In this way, local disturbances within a coral reef can
actually increase species diversity. In low disturbance
regimes competitively dominant species, which may
be fast growing in disturbed habitats, large, and can
overgrow other species, will form large monospecific
stands. The diversity of corals in these stands, and habitat responders will be low. In contrast, highly disturbed
areas will have a low coral diversity, and the substratum will consist primarily of encrusting algal and coral
forms. These organisms provide little structural heterogeneity, and consequently species diversity in highly
disturbed habitats will be low. However, at intermediate levels of disturbance, the abundance and cover of
competitively dominant species is reduced due to their
higher susceptibility to disturbance, which in turn frees
up space for other coral species. At intermediate disturbance levels, coral diversity increases. The relationship
between coral diversity and disturbance is reflected in
the diversity of many habitat responders (Fig. 5.5D, E).
Fish diversity, for example, tracks coral diversity with the
highest species richness found at intermediate coral densities that in turn are associated with higher levels of
habitat heterogeneity. In contrast with the competitivedisturbance mechanism generating coral diversity, the
tracking of coral diversity by habitat responders is usually
not due to competition, but rather the provision of a range
of habitat types, which enables a range of species with
different habitat preferences to occupy the same area.
Although natural disturbance is a normal part of
coral reef ecology, and an important process that generates habitat variability, anthropogenic disturbances
also exert effects at a range of spatial and temporal
scales. Boat anchors may generate local coral damage.
Artisanal fishing practices such as netting, trampling,
and rock-throwing (although not a major influence in
the GBR) may generate local coral damage in reef systems. Commercial fish extraction practices such as cyanide and explosives, again not common on the GBR but
widespread in other parts of the world, may generate
more widespread physical damage. Extraction of certain fish species may itself have biologically mediated
effects. For example extraction of herbivorous fishes
such as parrotfishes (Scaridae) may lead to increased
growth of macroalgae, which may in turn lead to lower
coral cover and, importantly, reduce the ability of corals to recruit onto bare space. At larger scales, increased
sedimentation and eutrophication may result from
land-based human practices such as deforestation,
agriculture, and sewage outfalls. The resulting changes
in water quality are usually long-lived, and may alter
the trophic regime, light levels, and increase stress to
corals via sedimentation. These stresses may in turn
lead to increased susceptibility of corals to environmental fluctuations such as temperature changes (see
Chapter 9).
Natural and anthropogenic temporal changes generate habitat heterogeneity by generating space that is
devoid of corals. The form of disturbance is important
in determining what the reef mosaic will look like. Moderate disturbances, which fragment large coral colonies
will reduce the amount of physical structure but as
many corals can grow from fragments, this may not
result in much loss of coral cover. Increased levels of
physical disturbance may remove and kill entire colonies, and thus provide bare space. This space will
47
survival of pathogens, which may affect habitat determiners such as Diadema urchins. The interaction between
physical and biological processes is important because
of the different scales of each; physical processes, which
usually occur at a large scale, may effectively force biological processes to occur at a similar scale. This spatial
autocorrelation (i.e. the abundance or ecological interactions of organisms are more similar at locations closer
to each other) has been termed the Moran Effect. The
consequence of this is that minor biological effects that
would appear only to exert local effects can, if generated by physical forcing, occur over a much larger spatial extent than would be predicted.
Small scale disturbances may also have important
and immediate consequences for habitat structure.
Storm damage can range from localised damage to
individual coral heads, up to complete coral removal
across 100 m swaths within windward sides of reefs,
and across several reefs (Fig. 5.4E, F). These types of
disturbances are not necessarily detrimental to a reef’s
overall health. Removal of a coral colony by storm
damage frees up space for recolonisation by other species. It also generates heterogeneity in the substratum,
so habitat responders that prefer bare rock or rubble
can occupy the space that is cleared by the disturbance.
In this way, local disturbances within a coral reef can
actually increase species diversity. In low disturbance
regimes competitively dominant species, which may
be fast growing in disturbed habitats, large, and can
overgrow other species, will form large monospecific
stands. The diversity of corals in these stands, and habitat responders will be low. In contrast, highly disturbed
areas will have a low coral diversity, and the substratum will consist primarily of encrusting algal and coral
forms. These organisms provide little structural heterogeneity, and consequently species diversity in highly
disturbed habitats will be low. However, at intermediate levels of disturbance, the abundance and cover of
competitively dominant species is reduced due to their
higher susceptibility to disturbance, which in turn frees
up space for other coral species. At intermediate disturbance levels, coral diversity increases. The relationship
between coral diversity and disturbance is reflected in
the diversity of many habitat responders (Fig. 5.5D, E).
Fish diversity, for example, tracks coral diversity with the
highest species richness found at intermediate coral densities that in turn are associated with higher levels of
habitat heterogeneity. In contrast with the competitivedisturbance mechanism generating coral diversity, the
tracking of coral diversity by habitat responders is usually
not due to competition, but rather the provision of a range
of habitat types, which enables a range of species with
different habitat preferences to occupy the same area.
Although natural disturbance is a normal part of
coral reef ecology, and an important process that generates habitat variability, anthropogenic disturbances
also exert effects at a range of spatial and temporal
scales. Boat anchors may generate local coral damage.
Artisanal fishing practices such as netting, trampling,
and rock-throwing (although not a major influence in
the GBR) may generate local coral damage in reef systems. Commercial fish extraction practices such as cyanide and explosives, again not common on the GBR but
widespread in other parts of the world, may generate
more widespread physical damage. Extraction of certain fish species may itself have biologically mediated
effects. For example extraction of herbivorous fishes
such as parrotfishes (Scaridae) may lead to increased
growth of macroalgae, which may in turn lead to lower
coral cover and, importantly, reduce the ability of corals to recruit onto bare space. At larger scales, increased
sedimentation and eutrophication may result from
land-based human practices such as deforestation,
agriculture, and sewage outfalls. The resulting changes
in water quality are usually long-lived, and may alter
the trophic regime, light levels, and increase stress to
corals via sedimentation. These stresses may in turn
lead to increased susceptibility of corals to environmental fluctuations such as temperature changes (see
Chapter 9).
Natural and anthropogenic temporal changes generate habitat heterogeneity by generating space that is
devoid of corals. The form of disturbance is important
in determining what the reef mosaic will look like. Moderate disturbances, which fragment large coral colonies
will reduce the amount of physical structure but as
many corals can grow from fragments, this may not
result in much loss of coral cover. Increased levels of
physical disturbance may remove and kill entire colonies, and thus provide bare space. This space will