Rate of decay calculations

Ecosystems and material cycles · Indicator species and decomposition · note 4 of 4

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Rate of decay calculationsSpec 9.19B

In short

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 %.

You can measure decay by recording the mass of the biological material at intervals. The rate of decay is the change in mass over time.

rate of decay = change in mass ÷ time

percentage loss in mass = (change in mass ÷ original mass) × 100

Rate of decay

A sample of leaves had a mass of 12.0 g. After 6 days the mass was 7.2 g. Calculate the mean rate of decay in g per day and the percentage loss in mass.

Change in mass = 12.0 − 7.2 = 4.8 g
Rate = 4.8 ÷ 6 = 0.8 g per day
Percentage loss = 4.8 ÷ 12.0 × 100 = 40 %

Answer: 0.8 g per day; 40 % loss

Maths skill:

Rate changes as time goes on, so read from a graph by finding the change over a short interval. A steeper gradient means a faster rate.

Quick check

Name two indicator species for clean water.
Show answer
Freshwater shrimps and stonefly nymphs.
What does blackspot fungus on roses indicate?
Show answer
Cleaner air, because it is sensitive to sulfur dioxide.
How does freezing slow decay?
Show answer
Low temperature slows the enzymes of the decomposers.
Give the conditions for fast composting.
Show answer
Warm, moist and well supplied with oxygen.
State the equation for the rate of decay.
Show answer
Change in mass ÷ time.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

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.

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