3.2.6Unit 3 — Life on Earth
Indicator Species
5 min read 5 parts Lesson 12 of 31
Part 1 of 5What Is an Indicator Species?
Biologists use living organisms as natural monitors of environmental quality.
Definition
- An indicator species is a living thing whose presence, absence or abundance tells us how polluted an environment is.
Sensitive species
- Some organisms cannot survive pollution, so finding them in a habitat suggests that the environment is clean.
Tolerant species
- Other organisms cope well with pollution, so large numbers of them suggest that conditions are poor.
Why they are useful
- Living organisms reflect conditions over weeks or months, unlike a single chemical test that shows only one moment.
Part 2 of 5Water Pollution
Water Pollution
Cause
- Sewage or fertiliser entering a stream makes microorganisms multiply, and their respiration uses up dissolved oxygen in the water.
Oxygen-sensitive animals
- Stonefly larvae and freshwater shrimps need high dissolved oxygen, so they are found only in clean water.
Pollution-tolerant animals
- Bloodworms and mosquito larvae survive in low-oxygen water, so many of them signal serious pollution.
Reading the results
- Stonefly larvae present means unpolluted water, while many bloodworms and no stonefly larvae means heavily polluted water.
| Indicator | Needs much oxygen? | Presence suggests |
|---|---|---|
| Stonefly larva | Yes | Clean water |
| Freshwater shrimp | Yes | Clean water |
| Bloodworm | No | Polluted water |
| Mosquito larva | No | Polluted water |
Part 3 of 5Air Pollution and Lichens
Sensitivity
- Lichens are very sensitive to sulfur dioxide, a gas released when fossil fuels burn in power stations and vehicle engines.
Clean air
- A high abundance of lichens, especially bushy lichens, shows that the air contains very little sulfur dioxide.
Polluted air
- Mainly crustose lichens, with leafy lichens absent, shows that the air is more polluted with sulfur dioxide.
No lichens
- Where almost no lichens grow on trees or walls, the sulfur dioxide level is likely to be high.
Part 4 of 5Using an Indicator Species in a Survey
1
Choose several sites along the stream or at different distances from a road or power station.
2
Sample each site in the same way, for example by kicking the stream bed for a fixed time with a net.
3
Identify the organisms using a key and record how many of each species you find.
4
Compare the presence and abundance of sensitive and tolerant species between sites.
5
Conclude which sites are cleaner or more polluted, and repeat the survey to make the findings reliable.
Part 5 of 5Exam Checklist & Tips
3.2.6 – Key Points for the Exam
- Indicator species A species whose presence or absence shows the level of pollution in an environment.
- Pollution Harmful substances added to the air, water or soil by human activity.
- Dissolved oxygen Oxygen mixed into water, needed by aquatic animals for respiration.
- Lichen An organism sensitive to sulfur dioxide, used to measure air pollution.
- Abundance How many individuals of a species are found in an area.
Common Mistakes
- Saying that polluted water has no oxygen;
- it has reduced oxygen because microorganisms have used it in respiration.
- Writing that pollution kills lichens directly without naming sulfur dioxide as the pollutant they are sensitive to.
- Mixing up the animalsstonefly larvae show clean water, bloodworms show polluted water.
- Judging pollution from one sample only; several samples at each site make the result more reliable.