Indicator species and pollutionSpec 9.16B
Some species are sensitive to pollution and some tolerate it. The presence or absence of indicator species gives evidence of the level of pollution in an area.
| Pollution | Indicator species | What it shows |
|---|---|---|
| Polluted water | Bloodworms, sludgeworms | These can live in water with very little oxygen, so many of them suggests the water is polluted (for example with sewage) |
| Clean water | Freshwater shrimps, stonefly nymphs | These need water with plenty of dissolved oxygen, so their presence suggests clean water |
| Air quality | Different species of lichen | Lichens differ in how sensitive they are to sulfur dioxide, so the species present indicate the level of air pollution. Few or no lichens suggest poor air quality. |
| Air quality | Blackspot fungus on roses | The fungus is sensitive to sulfur dioxide, so it is more common where the air is cleaner |
Evaluating the use of indicator species
- Advantages: cheap and easy, needs little equipment, and shows the effects of pollution over a period of time, not just at one moment.
- Limitations: gives only a rough idea of pollution level, because other factors such as temperature or habitat may also affect the species present; it does not identify which pollutant is present; and it is less accurate than measuring with instruments such as an oxygen meter or sulfur dioxide meter.
Decomposition and food preservationSpec 9.17B
Food goes bad because microorganisms (bacteria and fungi) feed on it, grow and respire. The rate of decomposition depends on temperature, water content and oxygen availability. Food preservation slows decomposition by changing these conditions.
| Factor | Effect on decomposition | Preservation method |
|---|---|---|
| Temperature | Warm conditions speed up enzyme reactions in microorganisms, so decomposition is faster. Cold slows these reactions. | Fridges and freezers keep food cold. Heating (for example in canning) kills microorganisms. |
| Water content | Microorganisms need water to grow and reproduce. Less water means slower decomposition. | Drying, or adding salt or sugar, which draws water out of microorganisms by osmosis |
| Oxygen | Most decomposers need oxygen for aerobic respiration. Less oxygen means slower decomposition. | Vacuum packing removes oxygen, and sealed cans keep oxygen out |
Decomposition and compostingSpec 9.18B
Composting uses decomposers to break down plant material and waste into compost, which returns nutrients to the soil. Here the aim is to make decomposition fast, so the conditions are the opposite of food preservation.
- Temperature: warm, because the enzymes of decomposers work faster as temperature rises, up to an optimum. Very high temperatures denature enzymes.
- Water content: moist, because decomposers need water. Too dry slows decomposition, and waterlogged material has little air.
- Oxygen availability: well aerated, for example by turning the heap, so that decomposers can respire aerobically. With little oxygen decomposition is much slower.
Many gardeners turn the compost heap regularly and keep it moist but not soaked.
Rate of decay calculationsSpec 9.19B
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
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
Quick check
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.