The greatest threat to species diversity is the degradation and loss of critical habitats, often caused by declines in
the abundance of key habitat-forming species. The effects of habitat degradation are particularly evident in coral
reefs, where environmentally sensitive reef-building corals provide complex living habitat that is essential for the
persistence of many species of fishes and motile invertebrates.
The effects of disturbance on coral reefs are not usually homogenous, with some corals suffering greater impacts
than others, even within the same reef area. This patchiness in coral degradation and mortality provides
opportunities for coral-associated species to move and recolonize remaining healthy habitat patches. The coral reef
system is studied by ecologists who are more interested in the interaction of organisms and the interaction of them
with the environment.
CORAL REEFS ARE ECOSYSTEMS
We study coral reefs in order to gauge the ocean's health and the status of other life forms. The health of the coral
reefs and all of its life forms are barometers for ecologists concerned with the future of the oceans.
Coral reefs are ecosystems. An ecosystem is made up of a community of living plants, animals and microbes along
with the essential nonliving parts in the environment and these all interact together as one system.
In ocean ecosystems, complex and dependent relationships exist between many species of plants and animals. A
coral reef is an example of an ecosystem. The structure of the reef is actually made up of calcium carbonate,
which comes from living stony corals (scleractinian) and their structures left behind when they die.
Making up the ecosystem are other vertebrates and invertebrates, as well as plants, which live within the
scleractinian corals. Together they are often referred to as the “Tropical Rainforests of the Oceans”.
For instance, the survival of clown fish depends on the health of the coral reefs and anemones they live within for
protection, reproduction and the availability of the fish they feed on.
Coral reefs are living creatures that can die. In tropical shallow waters, hard corals increase the structures and
shape of the ocean bottom. They provide a habitat for a variety of species and contribute to the survival of the
ocean ecosystem. Loss of hard corals on reefs has a severe effect on biodiversity, ecosystem structure, function
Coral reefs also provide coastal protection for fishing, shipping and other activities conducted along the coast.
Wave energy is significantly dissipated by the friction exerted by the roughness that coral reefs provide. It softens
wave activity along the coastal areas.
However, widespread coral death has occurred following mass bleaching events on some Indo-Pacific reefs
which actually has reduced the reefs and the number of fish species dependent on a coral habitat for survival.
WHY ARE CORAL REEFS DYING?
To survive, corals have a specific needs, including a specific water temperature, nutrients, water salinity, UV
radiation and opacity. Stony corals capture tiny plankton using their nematocyst covered tentacles. During the
daytime corals receive nutrients from single cell algae (zooxanthellae) living symbiotically in the tissues of coral
polyps and through photosynthesis producing fixed carbon products for energy. At night, corals feed on fish
Amazingly, coral are not responsible for the beautiful colors we see in the coral reef systems. They are actually
clear. It is these zooxanthellae algae living in the coral tissue that provide the color.
The stony coral needs calcium carbonate which it gets from the surrounding water in order to build its skeleton.
The skeleton left behind when the stony coral dies is what makes up the reef framework.
Overfishing by man and the pollution we pour into the ocean has had the largest effects on coral reef survival.
Pollution overloads the ocean waters with nutrients such as nitrogen reducing coral tolerance to other harmful
changes. Ozone depletion has caused UV radiation increases from the sun, which is out of the range tolerated by
the living corals.
El Nino Southern Oscillation (ENSO) and other temperature extremes had decreased coral survivability and created
sorporadic disease infestations to occur within the reef ecosystem. Poachers who collect and sell living corals to
the hobbyist has also had increasing effects on the size of reefs.
Some scientists believe that by far the most dominant impact on coral reef destruction is the growing human
population and its direct and indirect effects.
The greatest global-scale threats currently faced by coral reefs appear to be all linked to man-made or man-
mediated changes of climate:
WHAT IS CORAL BLEACHING?
Living corals respond to the stressors mentioned above by bleaching resulting in large scale death of corals.
Bleaching occurs because of a reduction in the numbers of algae (zooxanthella) living within them and this reduces
the nutrients available to corals during the daytime hours. The whitish appearance of the coral occurs because the
cnidarian skeleton is showing through the clear tissues of the corals. Bleaching corals have lost up to 90% of their
DYING CORAL REEFS
Coral reefs in the Caribbean have been sufficiently damaged by bleaching causing an alarming decline in its
structural complexity and at the same time, a drastic reduction in the fish living within the reef structure.
In the Caribbean, the largest changes in coral cover occurred as a result of the disease-induced die-off of
acroporids in the late 1970s and early 1980s.
Before this the erect branching structures of Acropora corals contributed disproportionately to reef complexity.
Although the robust skeletons of A. palmata may have persisted longer than the fragile framework of A.
cervicornis in some locations, the regional trends of declining coral cover and architectural complexity suggest that
most dead Acropora were relatively rapidly broken down and eroded following mortality.
The reduction in other reef organisms, such as sponges and soft corals, also contribute to the reef habit.
Much of coral reef conservation at present focuses on ecological management and control of the cover of coral
However, restoring coral cover on reefs may not necessarily provide the complex structure that makes up the
important coral reef ecosystem. The fish and plants that inhabited the reef also need to be replenished.
However, there could be consequences for social organization and demographic rates if decolonization is
associated with increased density within habitat patches. Organisms that have a strong association with live coral
habitat experience the greatest impact to coral loss and degradation. While there is substantial evidence of a direct
effect on the coral-associated fishes brought on by the loss of live coral habitat, it has only been speculated about
whether these fishes are declining in abundance in these impacted habitats, or if they are moving to alternative
Habitat formed by branching scleractinian corals is an essential resource for many small-bodied reef fishes,
providing refuge spaces among the complex matrix of branches). However, this branching morphology is highly
sensitive to disturbances, such as thermally-induced coral bleaching. While mass bleaching events can cause
considerable coral mortality, bleaching is typically patchy; even on a small scale some colonies will show signs of
bleaching while neighboring corals will appear healthy.
Physical disturbances (e.g., tropical storms) break down the complex reef framework, but bleached and recently
dead corals retain their physical structure. Although it appears that the physical structure of these corals is
important for their survival, many fishes will only associate with live corals and are not found associated with dead
coral skeletons. This suggests that live coral habitat is more important for these fishes than just the physical
structure it provides.
Complex branching corals can support large social groups of coral-dwelling fishes and many fish take refuge
between coral branches at night, or during the day when threatened by predators. Some coral-associated fishes
exhibit such fidelity to their host coral that they rarely venture more than a few meters from the coral and remain
within the same social group for the majority of their life. However, by living in a group these fish experience a
trade-off between; i) reduced predation risk through increased vigilance and dilution of predation risk and ii)
increased intra-group competition for food, shelter and reproduction.
Within social groups there can be strong size-based social hierarchies that mediate competition for essential
resources, such as food and breeding partners. Here, larger individuals (relative to other group members) are
generally higher in the ranking than smaller individuals because of their dominance and superior competitiveness.
Stability of social groups and persistence of individuals within groups can depend on the maintenance of these size-
based hierarchies. However, habitat degradation may cause social groups to break down, potentially leading to
changes in social hierarchies and increases in competitive interactions.
Historical data shows that coral cover is declining in many geographical regions (e.g., Caribbean, Great Barrier
Reef) due to a range of disturbances, including mass coral bleaching.
While bleached and recently dead corals should still offer suitable physical habitat, there is often a rapid and
dramatic decline in the local abundance of coral-dwelling fishes following coral bleaching.
Following a severe bleaching event, coral colonies will eventually die and quickly become colonized by turf algae.
It appears that once dead, even though host corals retain the same physical structure, but they no longer provide
these fish with a suitable habitat.
However, less aggressive fish remain within their degraded coral habitats if they are unsuccessful at joining the
resident community on the neighboring healthy colony. Less dominant fish species from dying coral colonies will
try to approach the neighboring healthy coral colony and attempt to enter the colony, but would be chased back to
the degraded habitat.
Research studies have shown that coral-dwelling fishes decline in abundance following bleaching and loss of live
coral cover Fish associated with host coral colonies that suffer mortality will readily vacate and relocate to
alternative healthy colonies where they use the presence of conspecifics as a cue for suitable habitat. However,
there must exist another coral colony to relocate to. Vanishing coral colonies will mean certain extinction of the
fish who depend upon corals for protection and sustenance.
Allen, G.R. Damselfishes of the world. 1991.
Aronson, R.B. White-band disease and the changing face of Caribbean coral reefs, Hydrobiologia, pp. 25–38.
Carpenter, K.E. 2008. One-third of Reef-building Corals face elevated extinction risk from climate change and local
impacts. Science. pp. 560–563.
Coker, D. 2009. Coral bleaching and habitat degradation increase susceptibility to predation for coral-dwelling
fishes. Behavioral Ecology, pp. 1204–1210.
Cole, A. J. 2009. Effects of coral bleaching on the feeding response of two species of coral-feeding fish. Journal
of Experiemntal Bology and Ecology, pp. 11–15.
Ellison, A.M. 2005. Loss of foundation species: consequences for the structure and dynamics of forested
ecosystems. Frontiers in Ecology and the Environment. pp. 479–486.
Forrester, G.E. 1991. Social rank, individual size and group composition as determinants of food consumption by
humbug damselfish, Dascyllus aruanus. Animal Behavior, pp. 701–711.
Gardner, T.A. 2003. Long-term region-wide declines in Caribbean corals. Science, pp. 958–960.
Kerswell, A. P. 2003. Effects of hypo-osmosis on the coral Stylophora pistillata: nature and cause of low-salinity
bleaching, Marine Ecology, pp. 145–154.
Paddack, M.J. 2000. Recent region-wide declines in Caribbean Reef fish abundance. Current Biology, pp. 590–
595van Oppen, M.J.H. 2009. Coral bleaching, pp. 139–158.
Thomas, C.D. 2004. Extinction risk from climate change. Nature, pp. 145–146.