IB Biology SLTopic 3 — Adapting to the EnvironmentPaper 1 & 2Core idea~12 min read
Abiotic Factors: Marine & Terrestrial
Look at a world map of coral reefs and the pattern is almost suspiciously neat: a band around the tropics, hugging the coasts, and nothing anywhere else. That map is a range of tolerance drawn on a globe.
📚 What you need to know
Coral is a symbiotic partnership between an animal (the coral polyp) and an alga (zooxanthellae).
The polyp gives shelter and protection; the algae photosynthesise and supply carbon compounds.
Some polyps secrete calcium carbonate, which builds up over thousands of individuals into a coral reef.
Reef-building corals are limited to the tropics and subtropics. Non reef-building species live elsewhere.
Reef formation is limited by water depth, pH, salinity, water clarity and temperature.
Coral has a narrow range of tolerance for all of these, which is exactly why its distribution is so restricted.
A coral is two organisms, not one
This is the fact that makes the rest of the page make sense. A coral polyp is a small animal, related to sea anemones. Living inside its tissues are enormous numbers of single-celled algae called zooxanthellae.
Each partner gives the other something it cannot get alone. The polyp provides a safe place to live and protection from being eaten. The algae photosynthesise and pass on carbon compounds such as carbohydrates, which supply most of the polyp’s energy.
Layer upon layer of calcium carbonate, laid down by thousands of polyps over thousands of years, is what becomes the hard structure of a reef.
Keep the partnership in your head as you read the rest of this page. Almost every limiting factor traces back to it: the algae need light, the polyp needs carbonate, and both need the right temperature.
Why reefs sit where they do
Coral has a narrow range of tolerance for several factors at once, and the reef can only form where all of them are satisfied. That intersection is a thin band around the tropics.
Water depth
Corals only grow at shallow depths, because light has to reach the zooxanthellae in strong enough amounts for them to photosynthesise. Water absorbs light quickly, so below a certain depth there is not enough energy to keep the partnership going.
Notice the yellow light rays getting shorter as you move into deeper water. That fading is what sets the depth limit.
The other four factors
Factor
Effect on reef formation
pH
The hard skeleton is calcium carbonate, which dissolves if the water is too acidic. Corals also need carbonate ions to build it, and those become scarce at low pH
Salinity
Corals are marine animals and need salty water, roughly 32 to 42 parts per thousand. Fresh water running off the land dilutes the sea and limits growth
Water clarity
Light must be able to pass through. Sediment from land run-off and pollution make the water cloudy and cut light to the algae
Temperature
Corals tolerate roughly 20 to 28 °C and grow best above about 23 °C. Sustained high temperatures make polyps expel their algae, causing bleaching
Water depth
Only shallow water lets enough light through for the zooxanthellae to photosynthesise
Ocean acidification, step by step
This is the chain examiners like, because it links chemistry to a living consequence. Take it one arrow at a time.
Both halves score marks: carbonate becomes less available to build with, and existing calcium carbonate dissolves faster at low pH.
🤔 What actually happens in coral bleaching
Corals can cope with short spells of warmer water. What they cannot cope with is sustained high temperature. When the sea stays too warm, the polyps expel the zooxanthellae living in their tissues.
The algae were providing the colour, so the coral turns white — hence “bleaching”. More importantly, the algae were also providing most of the polyp’s food. A bleached coral is not dead yet, but it is starving. If conditions improve quickly it can take the algae back; if they do not, it dies.
Do not confuse bleaching with death. Bleaching is the loss of the algal partner. Death follows only if the coral is not recolonised in time. Saying “the coral dies immediately” loses the mark.
The same logic on land
Coral reefs are the marine example, but the reasoning is identical for terrestrial habitats. A species is found where its tolerance ranges for every relevant factor overlap, and nowhere else.
On land the leading factors are usually temperature and rainfall, because between them they decide what kind of vegetation can grow. That is the whole basis of the next page.
Worked examples
WORKED EXAMPLE
Explain why reef-building corals are only found in shallow water. [3 marks]
Step 1: name the partner that needs the light
Coral polyps contain zooxanthellae, algae that carry out photosynthesis.
Step 2: what depth does to light
Light is absorbed as it passes through water, so intensity falls as depth increases.
Step 3: the consequence for the coraltoo little light → less photosynthesis → fewer carbon compounds passed to the polyp
Without that supply the polyp cannot grow or secrete calcium carbonate, so no reef forms.
The algae need light, so the coral needs shallow waterThe mark is lost by writing “corals need light”. Corals do not photosynthesise; their algae do.
WORKED EXAMPLE
Sea temperatures at a reef stay 3 °C above normal for several weeks. Explain the likely effect on the coral. [3 marks]
Step 1: relate to the range of tolerance
Corals tolerate roughly 20 to 28 °C. A sustained rise takes conditions above the upper limit of that range.
Step 2: the response
The polyps expel their zooxanthellae, so the coral loses its colour and appears white. This is coral bleaching.
Step 3: why it matters
The algae supplied most of the polyp’s carbon compounds, so without them the coral is starved of energy and will die if the algae are not regained.
Bleaching, then starvation and death if conditions do not improve
WORKED EXAMPLE
A river carrying soil and fertiliser enters the sea beside a reef. Suggest two reasons why coral growth falls near the river mouth. [4 marks]
Reason 1: water clarity
Sediment carried by the river makes the water cloudy, so less light reaches the zooxanthellae and photosynthesis is reduced.
Reason 2: salinity
Fresh water from the river dilutes the sea water, lowering the salinity below the range corals can tolerate.
Reduced clarity cuts light; reduced salinity falls outside the tolerance rangeA third valid answer would be nutrients from fertiliser feeding algal growth that shades the coral.
💡 Exam tip
Always mention zooxanthellae by name when explaining light or depth. It shows you know which partner photosynthesises.
Say symbiotic or mutualistic for the partnership, and say what each partner gains.
Link every limiting factor to a process: photosynthesis, skeleton building or osmotic balance.
For distribution questions, use the phrase narrow range of tolerance and then name the factors.
Quote figures where you have them: about 20 to 28 °C, about 32 to 42 parts per thousand.
Bleaching answers need expel the algae, loses colour and loses its food supply. All three.
⚠ Common mix-up
“Coral is a plant.” The polyp is an animal. Only its algal partner photosynthesises.
“Bleaching means the coral is dead.” It means the algae have gone. Death may follow, but not straight away.
Saying acid dissolves the reef and stopping there. Low pH also removes the carbonate ions needed to build it.
Confusing turbidity with depth. Both cut light, but they are different factors and questions distinguish them.
Assuming all corals build reefs. Non reef-building species live outside the tropics, including in cold water.
Giving only one factor when a question asks why distribution is restricted. It is the combination that limits reefs.
Up next: Biomes — scaling the same idea up to whole continents, and why two unrelated plants on opposite sides of the world end up looking almost identical.
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