Definition
Abiotic factor
A non-living condition in an ecosystem, such as light intensity, temperature, moisture, soil pH or oxygen level, that affects the organisms living there.
- Abiotic factors are the non-living, physical and chemical conditions in an ecosystem.
- A change in an abiotic factor can change the size and distribution of the populations in a community.
- If the conditions no longer suit a species, its numbers fall; if they suit it better, its numbers rise.
- Abiotic factors are different from biotic (living) factors, which are covered in the next section.
Key Idea
When an abiotic factor changes, organisms that are well suited to the new conditions do better, while those that are poorly suited decline.
The seven abiotic factors
- Light intensity sets the rate of photosynthesis, so in dim light fewer plants grow and there is less food and shelter for animals.
- Temperature controls the rate of growth and enzyme-controlled reactions, so a colder climate slows plant growth and limits the animals that can survive.
- Moisture level matters because too little water kills many plants, while waterlogged soil drowns their roots.
- Soil pH and mineral content decide which plants can grow, because a shortage of mineral ions such as nitrates, or a very acidic soil, supports fewer species.
- Wind intensity and direction affect plants because strong wind speeds up water loss by transpiration and can stop them growing on exposed ground.
- Carbon dioxide level is important for plants, because more carbon dioxide raises the rate of photosynthesis and so of growth.
- Oxygen level matters for aquatic animals, because the amount of dissolved oxygen in the water decides which species can live there.
Note
For a plant the key gas is carbon dioxide, but for an animal living in water the key gas is dissolved oxygen.
Explaining a change from data
- Exam questions usually give you data or a graph and ask you to explain how a change in an abiotic factor affects a community.
- State which factor has changed and read from the data whether it has risen or fallen.
- Then link that change to the organisms, using their survival, growth or competition to explain why a population rises or falls.
- For example, if light intensity falls, plants photosynthesise and grow more slowly, so the animals that feed on them have less food and their numbers drop.
Common Mistake
- Do not just name the factor that changed.
- Marks come from linking the change to what happens to the organisms.
Distribution across a habitat
- Abiotic factors also decide the distribution of organisms, meaning where in a habitat they are found.
- Daisies grow on an open, sunlit part of a field but not under a shady tree, because the light intensity is different.
- This is why the same factor can give a large population in one spot and almost none a few metres away.
Self review
- Name five abiotic factors that can affect a community.
- Why is dissolved oxygen an important abiotic factor for fish?
- Explain how a fall in light intensity could reduce a population of caterpillars.
- Give one abiotic factor that affects where in a field daisies grow.
