IB Biology SLTopic 3 — Adapting to the EnvironmentPaper 1 & 2Core idea~13 min read
Abiotic Factors & Distribution
Species are not scattered at random. Every organism has a set of conditions it can cope with, and where those conditions run out, so does the species. That single idea explains almost every distribution question you will ever be asked.
📚 What you need to know
Abiotic factors are the non-living conditions in a habitat — light, temperature, water, pH, salinity, oxygen, and so on.
A limiting factor is any biotic or abiotic factor that restricts the growth of organisms.
Every species has a range of tolerance: an optimum, plus a band either side it can still survive in.
Outside that range the species is absent. Near the edges it survives but in low numbers.
Species adapted to extreme conditions often have a wider range, or an optimum shifted away from average.
All the abiotic factors act at once, so distribution is decided by the combination, not by one factor alone.
What counts as an abiotic factor
Abiotic simply means non-living. These are the physical and chemical conditions an organism has no choice but to sit in.
Abiotic factor
Why it matters to living things
Light intensity and wavelength
Plants and algae need light for photosynthesis
Temperature
Controls the rate of enzyme-controlled reactions; too hot and enzymes denature
Water availability
Every living organism needs water to survive
Soil or water pH
Different species need different pH; extremes damage proteins
Salinity
Affects water movement into and out of cells by osmosis
Oxygen concentration
Needed by every organism that respires aerobically
Carbon dioxide concentration
The raw material plants use in photosynthesis
Wind speed
Increases water loss from leaves by evaporation
Turbidity (cloudiness of water)
Cloudy water blocks light reaching plants and algae below
Soil composition and mineral content
Supplies the mineral ions plants need to grow
Do not just memorise the list. For each factor, be able to say what process it affects. “Temperature matters” scores nothing; “temperature affects enzyme activity, so it affects the rate of respiration and growth” scores the mark.
Range of tolerance
No species needs one exact set of conditions. There is a level that suits it best — the optimum — and then a band on either side where things are less than ideal but survivable. That whole band is the range of tolerance.
Push the conditions further and you leave the range altogether. At that point the species is simply not found there.
Read this graph carefully in the exam. The x axis is the abiotic factor, not time, and the y axis is population size, not growth rate.
🤔 Why do extreme specialists win in harsh places and lose in nice ones?
A species built for extremes usually has a wide range of tolerance, or an optimum set well away from average. Marram grass, for example, does best at a much lower water availability than most plants. That lets it live on bare sand where almost nothing else can, so it faces very little competition.
Move it to rich, damp soil and it is outcompeted quickly. Species with an average optimum grow faster there and shade it out. Being able to survive anywhere is not the same as being the best anywhere — the trade-off is why extreme habitats have few species and mild ones have many.
Mangroves are the classic tolerance example. They grow best in salinity of roughly 3 to 27 parts per thousand, but they can survive in fresh water and in salt concentrations up to about 75 ppt. That very wide range is exactly why they can hold a coastline that other trees cannot.
Case study 1: marram grass on a sand dune
Marram grass lives on sand dunes, where the abiotic factors are brutal: low water availability, high salinity and low nutrient levels. Marram is a xerophyte — a plant adapted to survive dry conditions.
A xerophyte has two linked problems to solve:
Dry air means a steep concentration gradient of water vapour between the inside of the leaf and the air outside, so water evaporates from the leaf cells and is lost quickly.
The soil holds very little water, so whatever is lost cannot easily be replaced.
Marram’s answer is clever: instead of trying to seal the leaf shut, most of its adaptations trap a pocket of humid air next to the leaf surface. Humid air outside the stomata means a shallower gradient, and a shallower gradient means slower evaporation.
The waxy cuticle works differently from the rest: it is a physical barrier. The other three all work by trapping moisture.
🧩 How to write a marram answer that scores
Name the feature. Rolled leaf, sunken stomata, hairs, thick waxy cuticle.
Say what it does physically. Traps water vapour next to the leaf surface.
Link to the gradient. This raises the humidity outside the stomata, so the concentration gradient is less steep.
Finish with the outcome. Less water is lost by evaporation and diffusion, which matters because soil water cannot easily replace it.
Case study 2: mangrove trees
Mangrove swamps are tropical coastal habitats that are regularly flooded by sea water. The abiotic factors there are high salinity, low fresh water availability and low oxygen availability, because waterlogged mud holds almost no air.
That gives a mangrove three problems at once:
Submerged root cells cannot take in oxygen for aerobic respiration.
Surrounded by salt water, the tree struggles to obtain fresh water.
Sea water has a higher solute concentration than the root cells, so water tends to leave the roots by osmosis.
Both trees also need oxygen. Any root part above the water line can take it in, which is why prop roots and pneumatophores both stick up.
🧠 Telling the two mangroves apart
Red mangroves refuse the salt at the root. Black mangroves banish it through the leaves. Two Rs, two Bs.
Notice that the prop roots do two jobs at once: they hold a heavy tree upright in mud with no firm grip, and their underwater network shelters young fish and other marine animals. Adaptations rarely do only one thing.
Worked examples
WORKED EXAMPLE
Explain how two leaf adaptations of marram grass reduce water loss. [4 marks]
Adaptation 1: rolled leaf
The leaf rolls up so the surface bearing the stomata is enclosed, which traps water vapour inside and shelters it from the wind.
humid air trapped → shallower water vapour gradient → slower evaporationAdaptation 2: sunken stomata
The stomata sit in pits, so water vapour collects in the pit rather than being carried away.
humid pit → shallower gradient → less diffusion out of the stoma2 adaptations, each followed through to the gradient = 4 marksHairs and the thick waxy cuticle would score equally well. Just do not give two that work the same way without saying so.
WORKED EXAMPLE
A plant species is absent from soil with a pH below 4.5, present in small numbers between pH 4.5 and 5.5, and abundant between pH 5.5 and 7.0. State the critical minimum and describe the zone between pH 4.5 and 5.5. [3 marks]
Step 1: find where the species disappears
It is absent below pH 4.5, so the critical minimum is pH 4.5.
Step 2: name the zone
Between pH 4.5 and 5.5 the species survives but in low numbers, so this is a zone of stress.
Step 3: explain what that means
Conditions here are inside the range of tolerance but away from the optimum, so growth and survival are reduced and the population stays small.
Critical minimum pH 4.5; pH 4.5 to 5.5 is a zone of stress
WORKED EXAMPLE
Mangroves survive in salinity from fresh water up to about 75 ppt. Suggest why they dominate coastal mudflats but are rare in inland forests. [3 marks]
Point 1: why they win on the coast
Their range of tolerance for salinity is unusually wide, so they can survive conditions that would kill most trees.
Point 2: the consequence
Very few other species can tolerate that salinity and low oxygen, so there is almost no competition on the mudflats.
Point 3: why they lose inland
Inland soils are within the tolerance range of many other tree species, which grow faster in those conditions and outcompete the mangroves for light and nutrients.
Wide tolerance plus low competition on the coast; outcompeted inland“Suggest” questions on distribution nearly always want tolerance and competition. Give both.
💡 Exam tip
Every adaptation answer needs feature → what it does → why that helps. Two of the three is half marks.
Use the phrase concentration gradient in any water-loss answer. It is almost always on the mark scheme.
Abiotic means non-living. Predators, competitors and disease are biotic, and both types can be limiting factors.
When reading a tolerance graph, check the axes before anything else, then find where the line hits zero.
If a question asks why a species is absent, the answer is usually “conditions are outside its range of tolerance” — then say which factor.
Remember that factors interact. A species may tolerate high salinity only if the temperature is also suitable.
⚠ Common mix-up
Calling predation an abiotic factor. Anything living is biotic.
Saying rolled leaves “stop water escaping”. They slow it by raising humidity, not by sealing the leaf.
Thinking the optimum is the only place a species survives. It survives across the whole range of tolerance, just in smaller numbers near the edges.
Assuming a wide tolerance means the species will be everywhere. Competition keeps it out of mild habitats.
Mixing up the two mangroves. Red blocks salt at the roots; black excretes it from the leaves.
Forgetting that too much of a factor is also fatal. Waterlogging kills roots by starving them of oxygen.
Up next: Species Distribution (Skills) — how to go out and actually measure where a species lives, using transects, quadrats and kite diagrams.
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