The Great Barrier Reef
104
pace with the rate of climate change, then ultimately the
current frequency and intensity of coral bleaching and
mortality events would remain constant.
The evidence to support these hypotheses is extremely weak (see Additional reading). Most evidence
points to the evolutionary capacity of corals falling behind current and future rates of climate change. The fact
that the rates of change over the past 100 years are at
least 100 fold higher than the rapid shifts seen when the
earth moved from ice ages to the warm interglacial periods is sobering in terms of any discussion about adaptation. Terrestrial populations are changing rapidly in
response to climate change, with birds and butterfly
populations experiencing extinction at lower latitudes,
and establishment at higher latitudes where habitat is
available. Similar trends, though less well documented,
are occurring in the ocean. Fish normally found at lower,
more tropical latitudes are starting to appear on reefs
at higher latitudes. Responses by marine populations
depend very much on the organisms involved. Reefbuilding by corals is limited by other factors in addition
to temperature (e.g. light, concentration of carbonate
ions; see Chapter 8) and hence increases in sea temperatures due to the global warming will not result in the
appearance of carbonate coral reefs at higher latitudes.
A central requirement for any adaptive response to
have a major impact is the stabilisation of the concentrations of greenhouse gases such as carbon dioxide
and hence, of global temperature. Continuation of midto high-range emissions will perturb the climate for
hundreds if not thousands of years, with major implications for how populations of corals and other coral
reef organisms do or do not respond. For example, if
the world’s international agreements were to stabilise
global temperatures at 2°C above present day conditions (i.e. low emission scenarios), coral populations
would initially decrease as unfit genotypes disappeared
from particular regions, and then would increase as
fitter genotypes proliferated to establish populations
under the new stable global temperature. Thermally
tolerant equatorial genotypes may migrate to higher
latitudes over time (probably over decades), as they
track their preferred warm temperature regime. However, if greenhouse gases like CO 2 do not stabilise, there
is likely be a regime of heatwaves at high latitudes too
severe for even the most heat tolerant genotypes to establish. Constantly changing conditions of such a nonstabilised climate would drive corals (and the reef
ecosystems) into low population densities with the
prospect of massive extinction rates for corals and the
thousands of coral framework- dependent species.
N RAMIFICATIONS FOR CORAL REEF
ECOSYSTEMS AND PEOPLE IN A RAPIDLY
CHANGING CLIMATE
The key question for coral reef ecologists is how changes
in the abundance of reef-building corals will affect those
thousands of other species that are totally dependent on
the coral framework for food, shelter and reproduction
(Fig. 10.5). While defining the full set of relationships
between corals and other reef organisms goes beyond
the space allowed here, it is important to outline some
of the ways that the highly interconnected coral reef
ecosystem are likely to be affected by climate change.
Several studies have now demonstrated that impacts
of coral bleaching and mortality on reef fish populations
include local extinctions, reduced taxonomic distinctiveness and species richness, and a loss of species
within key functional groups. Several studies reveal
that fish diversity is directly affected by the loss of corals. Using data from Pacific and Indian Ocean studies, it
is clear that fish populations appear highly sensitive to
changes in coral cover, with 62% of fish species declining in abundance within three years of disturbances that
resulted in greater than a 10% decrease in coral cover.
Particular species appear to be more sensitive than others, with coralivorous (coral eating) species being the
most sensitive. The response of other less coral dependent reef fish is not clear. Several studies have shown that
the number of herbivorous fish may increase after the
coral mortalities associated with mass bleaching events,
primarily due to the increase in algal turfs, which are
the preferred food of these fish species.
Our current understanding of how coral reef organisms other than fish are influenced by the loss of corals
from reefs is limited. It is clear that other organisms are
equally susceptible to the projected changes in coral cover.
Obligate crab fauna that live in corals (such as Pocillopora
spp., for example) disappear from corals that have
104
pace with the rate of climate change, then ultimately the
current frequency and intensity of coral bleaching and
mortality events would remain constant.
The evidence to support these hypotheses is extremely weak (see Additional reading). Most evidence
points to the evolutionary capacity of corals falling behind current and future rates of climate change. The fact
that the rates of change over the past 100 years are at
least 100 fold higher than the rapid shifts seen when the
earth moved from ice ages to the warm interglacial periods is sobering in terms of any discussion about adaptation. Terrestrial populations are changing rapidly in
response to climate change, with birds and butterfly
populations experiencing extinction at lower latitudes,
and establishment at higher latitudes where habitat is
available. Similar trends, though less well documented,
are occurring in the ocean. Fish normally found at lower,
more tropical latitudes are starting to appear on reefs
at higher latitudes. Responses by marine populations
depend very much on the organisms involved. Reefbuilding by corals is limited by other factors in addition
to temperature (e.g. light, concentration of carbonate
ions; see Chapter 8) and hence increases in sea temperatures due to the global warming will not result in the
appearance of carbonate coral reefs at higher latitudes.
A central requirement for any adaptive response to
have a major impact is the stabilisation of the concentrations of greenhouse gases such as carbon dioxide
and hence, of global temperature. Continuation of midto high-range emissions will perturb the climate for
hundreds if not thousands of years, with major implications for how populations of corals and other coral
reef organisms do or do not respond. For example, if
the world’s international agreements were to stabilise
global temperatures at 2°C above present day conditions (i.e. low emission scenarios), coral populations
would initially decrease as unfit genotypes disappeared
from particular regions, and then would increase as
fitter genotypes proliferated to establish populations
under the new stable global temperature. Thermally
tolerant equatorial genotypes may migrate to higher
latitudes over time (probably over decades), as they
track their preferred warm temperature regime. However, if greenhouse gases like CO 2 do not stabilise, there
is likely be a regime of heatwaves at high latitudes too
severe for even the most heat tolerant genotypes to establish. Constantly changing conditions of such a nonstabilised climate would drive corals (and the reef
ecosystems) into low population densities with the
prospect of massive extinction rates for corals and the
thousands of coral framework- dependent species.
N RAMIFICATIONS FOR CORAL REEF
ECOSYSTEMS AND PEOPLE IN A RAPIDLY
CHANGING CLIMATE
The key question for coral reef ecologists is how changes
in the abundance of reef-building corals will affect those
thousands of other species that are totally dependent on
the coral framework for food, shelter and reproduction
(Fig. 10.5). While defining the full set of relationships
between corals and other reef organisms goes beyond
the space allowed here, it is important to outline some
of the ways that the highly interconnected coral reef
ecosystem are likely to be affected by climate change.
Several studies have now demonstrated that impacts
of coral bleaching and mortality on reef fish populations
include local extinctions, reduced taxonomic distinctiveness and species richness, and a loss of species
within key functional groups. Several studies reveal
that fish diversity is directly affected by the loss of corals. Using data from Pacific and Indian Ocean studies, it
is clear that fish populations appear highly sensitive to
changes in coral cover, with 62% of fish species declining in abundance within three years of disturbances that
resulted in greater than a 10% decrease in coral cover.
Particular species appear to be more sensitive than others, with coralivorous (coral eating) species being the
most sensitive. The response of other less coral dependent reef fish is not clear. Several studies have shown that
the number of herbivorous fish may increase after the
coral mortalities associated with mass bleaching events,
primarily due to the increase in algal turfs, which are
the preferred food of these fish species.
Our current understanding of how coral reef organisms other than fish are influenced by the loss of corals
from reefs is limited. It is clear that other organisms are
equally susceptible to the projected changes in coral cover.
Obligate crab fauna that live in corals (such as Pocillopora
spp., for example) disappear from corals that have