Download Coral reef scenario cards

Food for thought...
Coral reef food web scenarios:
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Coral bleaching.
Healthy coral
With sea surface
temperatures increasing,
corals are increasingly
showing signs of
‘bleaching’ which causes
them to die.
What effect might this have
on the food web?
‘bleached’ coral
Notes for teachers
Coral reefs already exist at the extreme end of their temperature tolerance.
If this continues to occur, corals world-wide will become bleached. This is the process by which coral
reefs loose their mutually beneficial relationship with algae. As such, corals do not receive enough
energy by feeding on plankton at night alone and subsequently die.
Question - What effect might this have on the food web?
Answer - Because corals function as both a habitat and a food source, butterfly fish and parrot fish
would both decline in abundance. Emphasise also that there are many more organisms reliant on coral
reefs, but for this exercise only a small section of the food web was selected.
As a result, there will be less grazing pressure on seaweeds so they will increase in abundance and outcompete any remaining coral for space. They are likely to also out-compete any soft corals as well.
This will lead to an increased abundance of sea urchins and therefore a greater abundance of squid,
which are a predator of sea urchins.
Before sharks can increase though due to increased squid numbers, sea urchins are likely to run out of
seaweed to graze on keeping their numbers low. This is because they multiply in such great abundances
– faster than the seaweed can spread.
All that will remain is a condition known as a ‘sea urchin barren’ state, where just a few sea urchins
survive – often not enough to reproduce successfully. The odd passing squid, sharks because they are
able to forage across ecosystems, and sea cucumbers will remain.
Food for thought...
Coral reef food web scenarios:
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An increase in sewage pollution.
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Coral reefs attract millions of
tourists every year. This means
more hotels are needed to cope
with the increasing number of
visitors.
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What if the money wasn’t there
to set up a proper waste
treatment system and sewage
was discharged straight onto
the reef?
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What effect might this have on
the food web?
Notes for teachers
An increase in the release of sewage into the marine environment tends to lead to a
condition known as eutrophication.
This is where plan life grows rapidly, sometimes in thick dense mats on top of the water. If
sustained, eutrophication can actually lead to oxygen-poor waters.
Question – what effect might this have on the food web?
Answer – Coral reefs are normally surrounded by nutrient depleted waters. This is because
coral reefs are so efficient and cycling them. However, with increased nutrient levels
seaweeds will grow rapidly, out-competing corals for space.
This will reduce coral cover, and the organisms reliant upon it – both as a habitat and as a
source of food.
If sustained, seaweed growth will undergo succession to plant life that is unpalatable to
grazing fish, so their abundance will also decline. In addition, eutrophication eventually
leads to oxygen depleted and bacteria rich waters. All that will be left in the long term is
bacteria!
Food for thought...
Coral reef food web scenarios:
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Overfishing top predators.
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Sharks and other large
predatory fish have been taken
without restraint from coral
reefs in the past.
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If regulations on fishing were
lifted, what effect might this
have on the food web?
Notes for teachers
Overfishing secondary consumers in the food web will leave primary consumers free to
increase in abundance - resources permitting.
Question – what effect might this have on the food web?
Answer – Coral reefs would become over-grazed, leaving coral reefs damaged.
Coral reefs play an important role as storm barriers, as well as a habitat and source of food,
throughout the tropics.
With a weakened reef matrix, storms will tear the reef apart, and with it reduce the
abundance of all the organisms reliant on its presence. Only migratory species that pass
through coral reefs will remain – such as squid and sharks.
Not only does this have ecological impacts, it also has socio-economic impacts as well
because with a natural barrier against storms gone, low-lying islands such as those find in
the Pacific can suffer severe flooding and damage.