Indicator species and decomposition — Edexcel GCSE Biology
Indicator species and pollution, factors affecting the rate of decomposition in food preservation and composting, and rate of decay calculations.
Revision notes
4 short notes, in the order of the specification. Each one in short:
Indicator species and pollution
Higher tierIndicator species are organisms whose presence or absence gives evidence of the level of pollution in an area. Bloodworms and sludgeworms suggest polluted water, while freshwater shrimps and stonefly nymphs suggest clean water. Lichens and blackspot fungus on roses indicate air quality. They are cheap and easy to use, but less accurate than instruments.
The rate of decomposition depends on temperature, water content and oxygen availability, because microorganisms need warmth, water and usually oxygen to grow and respire. Food preservation slows decomposition by changing these conditions: fridges and freezers keep food cold, drying or adding salt or sugar removes water, and vacuum packing or sealed cans keep oxygen out.
Composting uses decomposers to break down plant material and waste into compost, which returns nutrients to the soil. Decomposition is fastest when the heap is warm, because decomposer enzymes work faster up to an optimum; moist, because decomposers need water; and well aerated by turning, so decomposers can respire aerobically.
The rate of decay is the change in mass of biological material divided by the time taken. Percentage loss in mass = (change in mass ÷ original mass) × 100. For leaves going from 12.0 g to 7.2 g in 6 days, the rate is 0.8 g per day and the loss is 40 %.
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Quick check questions
Name two indicator species for clean water.
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Freshwater shrimps and stonefly nymphs.
What does blackspot fungus on roses indicate?
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Cleaner air, because it is sensitive to sulfur dioxide.
How does freezing slow decay?
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Low temperature slows the enzymes of the decomposers.
Give the conditions for fast composting.
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Warm, moist and well supplied with oxygen.
State the equation for the rate of decay.
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Change in mass ÷ time.
Frequently asked questions
What are indicator species?
Higher tier Indicator species are organisms whose presence or absence gives evidence of the level of pollution in an area. In water, bloodworms and sludgeworms suggest pollution, while freshwater shrimps and stonefly nymphs suggest clean water with plenty of oxygen. For air, the species of lichen present and blackspot fungus on roses show the level of sulfur dioxide.
How does food preservation slow decay?
Food preservation slows decay by making conditions poor for microorganisms. Fridges and freezers keep food cold, so enzymes in the microorganisms work more slowly. Drying, or adding salt or sugar, removes water, which microorganisms need to grow. Vacuum packing and sealed cans keep out oxygen, which most decomposers need for aerobic respiration.
What conditions make compost decompose fastest?
Compost decomposes fastest when it is warm, moist and well aerated. Warmth speeds up the decomposers' enzymes up to an optimum, although very high temperatures denature them. Decomposers need water, but waterlogged material has little air. Turning the heap provides oxygen, so the decomposers can respire aerobically and decomposition is faster.
How do you calculate the rate of decay?
The rate of decay is the change in mass divided by the time taken. For example, if leaves fall from 12.0 g to 7.2 g in 6 days, the change in mass is 4.8 g, so the rate is 0.8 g per day. The percentage loss in mass is 4.8 ÷ 12.0 × 100 = 40 %.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026