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Adaptations, Interdependence and Competition » Abiotic and Biotic Factors
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.7.1.2, 4.7.1.3
- The seven abiotic (non-living) factors that can affect a community
- The biotic (living) factors that can affect a community
- How to explain what a change in a factor does to a community
- How to read charts, graphs and tables about these factors
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Unlock This CourseTwo kinds of factor
Every community is shaped by its surroundings. A factor is anything that can change how well organisms survive and reproduce. Factors come in two kinds: non-living ones and living ones.
Key terms:
- Abiotic factor: a non-living factor that can affect a community.
- Biotic factor: a living factor that can affect a community.
Abiotic factors
The spec lists seven abiotic factors. You need to know all of them and be able to say how a change in each one affects organisms.
🌡 Temperature
Temperature affects the rate of the reactions in organisms. A very cold winter can kill organisms that cannot cope with it.
💧 Moisture levels
All organisms need water. In a drought, soil dries out, plants wilt and the animals that feed on them have less food.
⚗ Soil pH and mineral content
Some plants grow well only in acidic soil, others only in alkaline soil. Plants also need mineral ions such as nitrates from the soil, so poor soil means poor growth.
🌬 Wind intensity and direction
Strong wind increases water loss from plants and can damage them. Trees on an exposed hill are often smaller and bent in one direction.
🌿 Carbon dioxide levels for plants
Plants need carbon dioxide for photosynthesis. If the level falls, they make less glucose and grow more slowly.
🐟 Oxygen levels for aquatic animals
Fish and other water animals take in oxygen that is dissolved in the water. If the oxygen level drops, for example when a river is polluted, they struggle to respire and may die.
Biotic factors
Biotic factors are the living things that affect a community. The spec lists four.
- Availability of food: if the food supply falls, the animals that eat it have less to eat, so their numbers fall.
- New predators arriving: a new predator eats prey that have had no time to adapt to it, so the prey population can fall quickly.
- New pathogens: a new disease-causing microorganism can spread through a population that has no defence against it. Dutch elm disease, caused by a fungus, killed most of the elm trees in Britain.
- One species outcompeting another: if one species is better at getting a limited resource, the other gets less of it. Its numbers fall until they may be too low for enough individuals to find a mate and breed.
Worked example: grey squirrels and red squirrels
Grey squirrels were introduced to Britain from North America. They compete with red squirrels for food and are better at getting it. Red squirrels get less food, so fewer survive and breed. In many areas the reds became too few to breed and have disappeared, and only grey squirrels are left.
Explaining the effect of a change
An exam question usually gives you a change and asks what happens to a community. Use a chain of three steps.
- Name the factor that changed and say whether it went up or down.
- Say which organism is affected, and how.
- Link on to the next organism that depends on it.
Worked example
A new factory pollutes a river and the oxygen level falls. Fish need oxygen to respire, so fewer fish survive. Herons that feed on the fish then have less food, so their numbers may fall too.
Reading charts, graphs and tables
You may be given data on how a factor affects organisms. Read the title and the headings, look for the pattern, then explain it using a named factor.
Worked example
A table shows the number of mayfly larvae in four streams with different oxygen levels (units of oxygen dissolved in the water).
Stream A: oxygen 9, larvae 48.
Stream B: oxygen 7, larvae 35.
Stream C: oxygen 5, larvae 20.
Stream D: oxygen 3, larvae 6.
As the oxygen level falls, the number of larvae falls. Oxygen is an abiotic factor. Stream D has the lowest oxygen and the fewest larvae, so low oxygen is probably limiting how many can survive there.
Use the numbers in your answer. "The number of larvae fell from 48 to 6 as oxygen fell from 9 to 3" is stronger than "fewer larvae".
Common mistakes
Calling a living factor abiotic, or the other way round. A simple check: if it is alive, it is biotic. Another mistake is writing that a change "affects the community" without saying how. Always name the organism and say whether it goes up or down, and why.
Exam-style question
A scientist counts the daisies growing in four areas of a lawn that get different amounts of water. Area 1 is the wettest and area 4 is the driest.
(a) Area 1 had 40 daisies and area 4 had 12 daisies. Describe the pattern shown. [1 mark]
(b) Moisture level is an abiotic factor. Explain what is meant by an abiotic factor. [1 mark]
(c) A new species of insect that eats daisies arrives in the lawn. Name this type of factor and explain what could happen to the daisies. [2 marks]
Model answer
(a) The drier the area, the fewer daisies there are (or: the number of daisies falls from 40 to 12 as the area gets drier). (1)
(b) A non-living factor that can affect a community. (1)
(c) A biotic factor (1). The insects eat the daisies, so the number of daisies could fall (1).
Exam tip
In part (c), the word "biotic" is not enough on its own. State what the new insect does to the daisies and which way their number goes.