Abiotic factors at Higher Level: The same six factors as SL, with different cases: an acid river in Spain, the dark rainforest floor, trout in warming rivers.
Practise this as you read
- Pair the abiotic factor with the biotic process it limits.
- Plan how to measure a factor: instrument, sites, repeats.
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Six factors to know: Many abiotic factors affect where species live. The guide names six: learn what each is and how it affects living things.
The points to remember
- Temperature: speeds up or slows down growth; too hot or too cold kills.
- Sunlight: plants need light for photosynthesis.
- pH: how acidic or alkaline water or soil is; few species live at very low or high pH.
- Salinity: how salty water or soil is; sea species and freshwater species need different levels.
- Dissolved oxygen: the oxygen in water; fish and insect larvae need it to breathe.
- Soil texture: how sandy, silty or clayey a soil is; it decides how much water and air the soil holds.
Remember it as: Temperature, Sunlight, pH, Salt, Oxygen, Soil: T-S-P-S-O-S.
Real example: the Rio Tinto in southern Spain has water with a pH of about 2, as acidic as lemon juice, from old mines and natural rocks. Almost no fish or plants live in it; only a few specialist microbes do.
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Papers often ask which abiotic factor limits plant growth or productivity in a place.
Limiting factors
- A limiting factor is the one in shortest supply: it holds back growth, whatever the others are.
- Give its direction: 'low rainfall', 'low winter temperatures', not just 'rainfall' or 'temperature'.
- Link it to plants: 'so less photosynthesis' or 'so plants grow more slowly'.
- In a desert, low rainfall limits most. In cold places, low temperature and a short growing season.
- Under a closed canopy, low light limits the biotic process of photosynthesis.
- Fire and grazing animals are not abiotic factors: do not give them.
Remember it as: Name it, give its direction, link it to photosynthesis.
Reading it: at the forest floor only about 2% of sunlight arrives, so low light limits photosynthesis and few plant species can grow there. Light is the abiotic factor; photosynthesis is the biotic process.
Link it, and give the direction: 'Temperature' alone, or 'it lowers productivity' alone, is not enough. Write 'low temperatures slow photosynthesis'.
A tolerance curve shows why a species lives in some places and not others.
Reading a tolerance curve
- Each species survives only within a range of each abiotic factor: its range of tolerance.
- It does best at the optimum, the peak of the curve.
- Near the edges it is in stress: it survives but grows and breeds less.
- Beyond the limits it cannot survive, so it is absent from those places.
- So abiotic factors, measured, help explain where a species is found.
Remember it as: Peak = optimum. Edges = stress. Outside = absent.
Real example: in hot, dry summers, parts of rivers in England and Scotland warm above 20 °C; brown trout move to cooler, shaded stretches or die, because warm water also holds less dissolved oxygen.
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Measuring abiotic factors shows how conditions change across an area, and so why species are found where they are.
Measuring abiotic factors
- Use the right instrument for the factor (see the table).
- A data logger with a probe records readings automatically, at set times, for hours or days.
- Along a transect, take readings at regular intervals, such as every 10 m.
- Take readings at several sites or regular times to see any change.
- Repeat each reading and use the mean, so the results are reliable.
- Measure all sites in the same conditions, e.g. the same day and time.
| Abiotic factor | How to measure it |
|---|---|
| Temperature | Thermometer or temperature probe |
| Sunlight (light intensity) | Light meter or light sensor |
| pH | pH probe or meter (or indicator paper) |
| Salinity | Salinity or conductivity meter |
| Dissolved oxygen | Oxygen probe or meter |
| Soil texture | Sieves of different mesh sizes |
| Soil moisture | Soil moisture probe, or weigh soil before and after drying |
| Turbidity (cloudiness of water) | Secchi disc |
Reading it: where the farm drain enters, dissolved oxygen falls and mayfly larvae almost vanish; both recover downstream. A data logger left at Site 2 would show whether the oxygen drops at night too.
How this comes up: Paper 1: suggest how to measure an abiotic factor along a transect [3], or describe how to monitor one [2]. Also: identify the factor that limits growth, from a table [1].
A student is studying why plants change from the bottom of a valley to the top of a hill.
Suggest how the student might measure the changes in one abiotic factor along a transect up the hill.
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