Indicator species, decay and preserving food

Some organisms tell us how polluted a place is. Decomposers break things down, which we speed up for compost and slow down to keep food fresh.

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Indicator species

An indicator species shows how polluted an environment is by whether it is present or absent.
Air: many kinds of lichen only grow where the air is clean, with little sulfur dioxide. Lots of lichen means clean air.
Water: stonefly larvae and mayfly nymphs need clean, oxygen-rich water. Bloodworms and sludgeworms can survive in polluted water with little oxygen.

Decomposition and food preservation

Decomposers (bacteria and fungi) need warmth, water and usually oxygen. Food is kept fresh by removing one of these:
Refrigeration slows microbial growth; freezing stops it.
Drying removes water.
Salting or adding sugar draws water out of microbes by osmosis.
Vacuum packing removes oxygen.
Canning heats food to kill microbes, then seals it. Pickling in vinegar makes it too acidic for most microbes.

Compost and rates of decay

Gardeners want fast decay to make compost: keep the heap warm and moist, turn it to add oxygen, and use small pieces (bigger surface area).
rate of decay = change in mass ÷ time
Example: grass cuttings lose 0.75 kg in 15 days. Rate = 0.75 ÷ 15 = 0.05 kg per day.
Worked example

Compost goes from 1.20 kg to 0.45 kg in 15 days. Calculate the rate of decay and the percentage of mass lost.

  1. Change in mass = 1.20 − 0.45 = 0.75 kg
  2. Rate = 0.75 ÷ 15 = 0.05 kg per day
  3. % lost = 0.75 ÷ 1.20 × 100 = 62.5%

Answer: 0.05 kg per day; 62.5% of the mass was lost.

Key idea

Lichens show clean air; stonefly larvae show clean water, while bloodworms and sludgeworms tolerate pollution. Food is preserved by cooling, drying, salting, vacuum packing, canning or pickling. Compost decays fastest when warm, moist and aerated. Rate = change in mass ÷ time.

Key words
Indicator species
A species whose presence or absence shows the level of pollution, such as lichens for air quality.
Osmosis
The net movement of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane.

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2 more questions

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  1. Leaves lose 450 g of mass in 25 days. Calculate the rate of decay in g per day.

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    Answer: 18 g/day

    450 ÷ 25 = 18 g/day

  2. Few types of lichen grow on trees in a city centre. What does this suggest?

    1. The air is very clean
    2. The soil is fertile
    3. The air has a high level of sulfur dioxide
    4. The water is clean
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    Answer: The air has a high level of sulfur dioxide

    Lichens are indicators of air quality.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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