IB Biology HLB4.1 — Adaptation to EnvironmentPaper 1 & 2Core idea~12 min read
Abiotic Factors: Marine & Terrestrial
Coral reefs cover a tiny fraction of the ocean floor, and they sit in a narrow band around the tropics. Rainforest and desert never swap places. Neither pattern is an accident — both come straight out of abiotic factors and range of tolerance, just applied at the scale of the whole planet.
📘 What you need to know
Coral is a mutualistic relationship between a coral polyp (an animal) and zooxanthellae (algae).
Some polyps secrete calcium carbonate, which builds up over thousands of individuals into a reef.
Reef-building corals are limited to the tropics and subtropics.
They are limited by water depth, pH, salinity, water clarity and temperature, and have a narrow range of tolerance for all of them.
A biome is a large community of plants and animals that has developed as a result of environmental factors.
Biomes are named after their dominant vegetation type and are set mainly by temperature and rainfall.
A climograph plots those two factors and shows which biome will develop.
How a coral reef is built
A coral is not one organism. It is two, living together in a symbiotic relationship where both benefit:
The coral polyp is an animal. It provides shelter and protection for the algae living inside its tissue.
The zooxanthellae are algae. They photosynthesise and produce carbon compounds such as carbohydrates, which the polyp uses.
Some species of polyp also secrete calcium carbonate, which builds up in layers to form a hard outer structure. Over long stretches of time, those deposits from thousands upon thousands of individuals form a coral reef.
The grey cup is the hard calcium carbonate skeleton. A reef is millions of these, laid down layer on layer over centuries.
Hold on to one sentence: the coral depends on the algae for food. Nearly every reef question — depth, clarity, temperature, bleaching — comes back to protecting that photosynthesis.
Why reefs are only found where they are
The distribution of reef-building corals is limited to the tropics and subtropics. Non reef-building species do live in other regions, but they do not build reefs.
The reason is a narrow range of tolerance for several abiotic factors at once. All of them have to be right at the same time, and that combination only happens in a thin band of the world’s oceans.
Abiotic factor
Effect on reef formation
Water depth
Corals grow only in shallow water, where enough light reaches them for the zooxanthellae to photosynthesise.
pH
The hard layer is calcium carbonate, which dissolves if pH falls too low. Corals also need carbonate ions to build it, and at lower pH the extra hydrogen ions combine with carbonate ions to form hydrogencarbonate ions — so fewer carbonate ions are left for reef-building. Carbon dioxide from burning fossil fuels dissolves in the oceans and lowers the pH.
Salinity
Corals are marine animals and need salty water, roughly 32–42 parts per thousand. Freshwater run-off from land dilutes the salt and limits growth.
Water clarity
Light has to penetrate the water. Sediment washed off the land, and water pollution, both cloud the water and limit coral growth.
Temperature
Tolerance is roughly 20–28 °C, with best growth above 23 °C. Short spells of higher temperature can be survived, but sustained warming makes polyps expel their algae — this is coral bleaching.
Bleaching, properly explained. The coral is not painted white by the heat. It expels its zooxanthellae, and it is those algae that give it its colour. Without them the white calcium carbonate skeleton shows through — and the coral has lost its food supply.
Biomes: the same idea on land
Definition
A biome is a large community of plants and animals that has developed as a result of environmental factors.
Biomes cover large geographical areas.
They are usually named after their dominant vegetation type — an area dominated by tropical rainforest is the tropical rainforest biome.
Examples: temperate rainforest, tropical rainforest, temperate deciduous forest, boreal or coniferous forest (taiga), grassland such as temperate grassland or savanna, tundra, and desert (hot or cold).
Temperature and rainfall do most of the work
Average temperature and rainfall patterns are the two big factors that decide which biome develops. For any given combination of those two, a particular biome results.
You can plot this on a graph called a climograph, with mean annual rainfall and mean annual temperature on the axes.
Read it like a map reference: go across to the rainfall, up to the temperature, and see which block you land in.
Three warnings about climographs
The graph shows sharp boundaries, but in reality there is a gradual shift from one biome to the next.
Rainfall and temperature are not the only factors. Soil type and grazing by animals also matter, especially near the boundaries.
Neither rainfall nor temperature is really an independent or dependent variable here, so either can go on either axis — and temperature may run high-to-low or low-to-high. Always read the axes before you answer.
Worked examples
WORKED EXAMPLE
Explain why reef-building corals are found only in shallow water. [3]
Start from the partnership
Corals depend on zooxanthellae living in their tissue for carbon compounds.
What do the algae need?The algae need light to photosynthesise.Now link it to depthLight is absorbed as it passes through water, so at depth there is not enough of it.Only in shallow water is there enough light for the algae to supply the polypthe answer is about the algae, not the polyp — that is the whole point.
WORKED EXAMPLE
Explain how rising atmospheric carbon dioxide reduces the growth of coral reefs. [4]
Step 1: get the CO2 into the seaCarbon dioxide from burning fossil fuels dissolves in the oceans.Step 2: what that does to the waterThis lowers the pH of the sea water.Step 3: the chemistry the examiner wantsThe extra hydrogen ions combine with carbonate ions to form hydrogencarbonate ions.Step 4: the consequence for the coralFewer carbonate ions are available, so less calcium carbonate can be laid downyou can add that existing calcium carbonate also dissolves at low pH.
WORKED EXAMPLE
Use the climograph above to predict the biome at a mean annual rainfall of 250 cm and a mean annual temperature of 25 °C, and at 100 cm and 0 °C. [2]
First point: across to 250 cm, up to 25 °CThat lands in the block for high rainfall and high temperature.Tropical rainforestSecond point: across to 100 cm, up to 0 °CModerate rainfall, cold temperature.Taiga (boreal forest)check the axis direction first — temperature is not always plotted upwards.
💡 Exam tip
Read the axis labels and values carefully before drawing any conclusion from a climograph. This is the examiner’s own warning.
Learn the coral limiting factors as a list of five: depth, pH, salinity, clarity, temperature.
Always route reef answers through the zooxanthellae. Depth and clarity questions are really light questions.
For bleaching, say the polyps expel their algae. “The coral dies of heat” will not score.
Use range of tolerance language when explaining why reefs are restricted to the tropics.
Name biomes by their dominant vegetation, and give the alternative name where there is one (taiga = boreal forest).
⚠ Common mix-up
Calling coral a plant. The polyp is an animal; the algae inside it are not.
Saying bleaching kills the coral instantly. It loses its algae and its food supply; it can recover if conditions improve quickly enough.
Confusing lower pH with the water being warmer. They are two separate threats with two separate mechanisms.
Thinking corals need fresh water. They are marine and need salinity in a fairly narrow band; run-off harms them.
Treating climograph boundaries as real lines on the ground. Biomes blend into one another.
Assuming temperature is always on the y-axis and always increasing upwards. It is not.
Forgetting non reef-building corals exist outside the tropics. Only reef-builders are restricted.
Up next: Biomes — what actually lives in each one, and why deserts on opposite sides of the world end up looking so similar.
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