Coral reef ecology
Written by Alex Rose
In order to fully appreciate the importance of a balanced ecosystem such as a
coral reef, we must first understand its trophic structure; the organisms that
make up each trophic level, and the functions of each level in the maintenance
of a healthy reef.
Coral reefs, just like any other ecosystem on our planet, rely on a variety of biotic and
abiotic factors to keep them healthy and functional. Without stable temperature, pH,
light/dark cycles, water flow, salinity, and chemical composition of sea water, coral reefs
could not exist, but without a stable trophic cascade, coral reefs could not survive.
Trophic structure in any environment refers to the different levels of the food chain and
illustrates the transfer of energy from one level to the next in the form of a pyramid;
energy is always lost as it travels “up” the food chain.
Figure 1: Coral reef trophic pyramid, showing its trophic levels. On each level, several
important groups of species are shown (Diagram by Alex Rose).
There are three categories of organisms in every ecosystem: producers, consumers, and
decomposers. Producers, often referred to as primary producers, consist of organisms
that are capable of making their own food and are consequently always photosynthetic
(and in some cases chemosynthetic). Consumers are placed higher on the trophic
pyramid than producers, and they can be herbivorous, omnivorous or carnivorous.
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Primary consumers are herbivorous, whereas secondary consumers prey on herbivores
and tertiary consumers eat other carnivores. Decomposers are responsible for breaking
down dead and decaying plant and animal matter into components that are once again
usable for growth by producers. None of these three categories of organisms can exist
without the others in order to complete the cycle of production, consumption, and
decomposition.
"There are three categories of organisms in every ecosystem: producers,
consumers, and decomposers".
This article will concentrate on breaking down the creatures of a typical coral reef
ecosystem into these three groups and considering their importance to the stability and
success of a reef.
Figure 2: The astonishing diversity of phytoplankton
is visible only under a microscope. One trait all
phytoplankton share, however, is chlorophyll—the
green pigment that converts energy from the sun
into
food
(Images
copyright
Smithsonian
Environmental Research Center).
Producers
Primary producers, or autotrophs, make up the base
of all food chains. Autotrophs are capable of
synthesizing complex organic compounds such as
glucose from a combination of simple inorganic molecules and light energy in a process
known as photosynthesis. Other much less common autotrophs (some bacteria) derive
their energy from the oxidation of inorganic compounds such as hydrogen sulfide, ferrous
iron, and ammonium and are referred to as chemoautotrophs. A good example of marine
chemoautotrophs are the bacteria that inhabit deep sea hydrothermal vents; they are the
primary producers in this hostile environment and are able to convert heat, methane,
and sulfur into energy through a process called chemosynthesis and can survive in water
temperatures of 750 °F (400 °C) and a pH of 2.8.
These extremophile bacteria are very important in certain deep sea ecosystems, but are
certainly not the main primary producers in the ocean. Most primary production occurs
within the first 70 meters (230 feet) of water, an area referred to as the euphotic zone.
Primary productivity is measured in grams of carbon produced per square meter of ocean
surface per year (g C/m2/yr) and total ocean productivity is estimated to be between 75
and 150 g C/m2/yr. Some common autotrophs in a coral reef ecosystem are
phytoplankton, coralline algae, filamentous turf algae, the symbiotic zooxanthellae in
corals, and many species of seaweed. Phytoplankton is one of the most important
primary producers in the world and includes a wide variety of organisms such as:
diatoms which are the most productive type of phytoplankton and have tests
(exoskeletons) made of silica, dinoflagellates and silicoflagellates which move by way of
flagella, coccolithophores which have tests made of calcium carbonate, cyanobacteria,
and other extremely small phytoplankton species referred to as nanoplankton (2.0-20
µm) and picoplankton (0.2-2.0 µm).
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Figure 3: The green sea turtle, Chelonia
mydas, is a true herbivore and therefore
a primary consumer (photograph: Hans
Leijnse).
Consumers
- primary consumers
Phytoplankton and other single-celled
primary producers are eaten by primary
consumers. Primary consumers are
herbivorous and they encompass a wide
range of marine creatures including zooplankton, invertebrate larvae, benthic grazers
(e.g. bivalves, gastropods, tunicates, sponges, polychaete and feather duster worms),
some corals, sea urchins, some crabs, green sea turtles, and herbivorous fish.
Zooplankton are undoubtedly the most abundant primary consumers in the water
column; copepods, amphipods, ciliates, and tintinnids are all common types of
zooplankton. Due to the large community of primary consumers on coral reefs,
phytoplankton levels in coral reef waters can be 15-65% lower than in adjacent open
ocean waters. Benthic grazers and some coral species feed by filtering phytoplankton out
of the water while other vertebrate and invertebrate grazers eat algae and seaweed;
many species of parrotfish, surgeonfish and blennies have a diet that consists entirely of
coralline, filamentous, and calciferous algae.
- secondary consumers
The animals in this trophic level feed on primary consumers and are consequently
carnivorous. Secondary consumers in a reef ecosystem can be divided into four main
groups: (1) plankton feeders, (2) corallivores - organisms that feed on coral tissue, (3)
feeders on other benthic invertebrates,
and (4) piscivores - fish eaters.
Plankton feeders can be small sessile
invertebrates like barnacles, corals like
sun
polyps
(Tubastrea
sp.)
and
gorgonians, small damselfish or 15-ton
whale sharks.
Figure 4: A beautiful purple gorgonian,
having polyps which actively feed on
zooplankton. This makes these corals
carnivores, or secondary consumers
(photograph: Hans Leijnse).
Corallivores can be sub-divided into polyp eaters, coral scrapers, mucus feeders, and
coral tissue generalists. Many species of butterfly fish, file fish, and damselfish specialize
in eating coral polyps; their elongated rostrums make it easy for them to pick individual
polyps off a large coral animal. Some common coral scrapers are specific species of
triggerfish, parrotfish, blennies, puffers, and butterfly fish. Some animals that feed on
coral mucus are coral guard crabs and shrimps, and ornate butterfly fish (Chaetodon
ornatissimus). Coral tissue generalists are the most destructive of the corallivores and
include the Crown-of-thorns sea star (Acanthaster planci), parasitic nudibranchs (e.g.
Phistella sp.), parasitic snails (e.g. Epitomium and Drupella sp., the latter often found to
be feeding on Stylophora pistillata in the Red Sea), and acoel polyclad flatworms.
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Benthic invertebrates like mollusks, gastropods, worms, and crustaceans are eaten by
many kinds of fish (e.g. goatfish, wrasses, triggerfish, etc.) and other larger
invertebrates; cone shells specialize on specific invertebrates, Tritons (Charonia tritonis)
eat Crown-of-thorns sea stars, cephalopods primarily eat shelled invertebrates, and
arthropods
(e.g.
lobsters,
mantis
shrimp,.etc.) eat a wide range of benthic
invertebrates.
Piscivores are carnivores that eat fish;
many species of fish are piscivores as
well as some mollusks and arthropods.
Figure 5: This snail actively feeds on
algae, and is a primary consumer.
Species like these are consumed by
secondary consumers such as various
fish species (photograph: Hans Leijnse).
- tertiary (top) consumers
Tertiary consumers are large reef fish at the top of the food chain that eat many smaller
fish. Some examples of top consumers in a coral reef ecosystem are sharks, barracudas,
and moray eels. Marine mammals such as dolphins and seals, and sea birds, if present,
are considered tertiary consumers, too.
Decomposers (and Detrivores)
Decomposers serve an extremely important function in all ecosystems; they break down
dead biological matter and waste products and convert them into usable energy while
returning important materials to the environment. The main decomposers in coral reefs
are bacteria; these bacteria play an integral part in the nitrogen cycle whereby ammonia
(NH4) is converted into nitrite (NO2) by bacteria in the genus Nitrosomonas, after which
nitrite is then converted into nitrate (NO3) by bacteria in the genus Nitrobacter. The
ultimate result is that levels of toxic wastes are kept very low and that waste products
are converted into components that are available to producers in a readily-usable form.
Decomposers are particularly important in coral reef environments considering the heavy
bio-load. Detrivores, or scavengers, play a similar role in recycling dead or waste
material; sea cucumbers and some species of snails, crabs and bristle worms consume
dead organisms and/or decaying plant and animal matter (detritus).
"Decomposers serve an extremely important function in all ecosystems;
they break down dead biological matter and waste products and convert
them into usable energy while returning important materials to the
environment."
Concluding remarks
Coral reefs are complex ecosystems that require a balanced trophic structure to function
properly and efficiently. Imbalances can occur in this intricate trophic cascade from the
top down or the bottom up. For an example of bottom-up effects, nutrient-rich
agricultural run-off can cause a massive increase in primary productivity (e.g. algal
blooms), the effects of which often cannot be buffered by consumers fast enough to
prevent a coral reef ecosystem from collapsing. A good example is the uprise of the
Crown-of-thorns population on the Great Barrier Reef several years ago, which was
Coral reef ecology
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linked to high algal blooms by some researchers. High phytoplankton levels may have
allowed more Crown-of-thorns' larvae to survive, which had devastating effects on some
reefs.
As for a top-down effect, the over fishing of a keystone tertiary consumer like some reef
sharks can allow some fish populations to grow so rapidly that their food sources cannot
be replenished fast enough to support their growing numbers, resulting in an eventual
population collapse. Another example is the overharvesting of another important
predator, the Triton snail, which may have contributed to the Crown-of-thorns uprise.
The magnitude of these potential consequences reinforces the fact that we need to be
more aware of how our actions can affect the environment in both negative and hopefully
positive ways.
Figure 6: The coral reef, a delicate ecosystem in which all of the organisms live together
in harmony. This harmony is severely disrupted when humans come into play;
overharvesting important species from a coral reef can have detrimental effects.
Conversely, stimulating the growth of key predators may lead to the extinction of other
species. Human interference in a functional ecosystem rarely leads to positive outcomes
(photograph: Leo Roest).
This article is part of the science outreach project entitled Coral Science, © 2008-2009
Coral Publications. Visit us at www.coralscience.org.
References
Baum, J.K., R.A. Myers, D.G. Kehler, B. Worm, S.J. Harley, and P.A. Doherty. 2003.
Collapse and conservation of shark populations in the Northwest Atlantic. Science. 299:
389-392.
Dawes Clinton J. 1998. Marine botany, second edition. John Wiley & Sons, Inc., New
York.
Coral reef ecology
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Harris Graham P. 1986. Phytoplankton ecology: structure, function and fluctuation.
Chapman & Hall, London: 1-15.
Sorokin YI. 1995. Ecological Studies: Coral Reef Ecology Vol. 102. Springer-Verlag,
Berlin.
Smithsonian
Environmental
Research
<http://www.serc.si.edu/labs/phytoplankton/guide/index.jsp>
Center
website.
Steeman-Nielsen, E. 1951. Measurement of production of organic matter in the sea by
means of carbon-14. Nature 267: 684–685.
Yahel, G, Post AF, Fabricius K, Marie D, Vaulot D, and A. Genin. 1998. Phytoplankton
distribution and grazing near coral reefs. Limnol Oceanogr 43(4): 551-563.
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