Human influences on the environment questions

Human influences on the environment syllabus

Questions on human influences on the environment can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 4.12 understand the biological consequences of pollution of air by sulfur dioxide and carbon monoxide
• 4.13 understand that water vapour, carbon dioxide, nitrous oxide, methane and CFCs are greenhouse gases
• 4.14 understand how human activities contribute to greenhouse gases
• 4.15 understand how an increase in greenhouse gases results in an enhanced greenhouse effect and that this may lead to global warming and its consequences
• 4.16 understand the biological consequences of pollution of water by sewage
• 4.17 understand the biological consequences of eutrophication caused by leached minerals from fertiliser 4.18B understand the effects of deforestation, including leaching, soil erosion, disturbance of evapotranspiration and the carbon cycle, and the balance of atmospheric gases

How to answer human influences on the environment questions

1. Identify the population, community or environmental factor before interpreting the ecological relationship.
2. Quote values from graphs or tables and distinguish correlation from a supported causal explanation.
3. For sampling methods, describe placement, repeats, measurements and how the result is made representative.
4. Trace energy or matter through each trophic level or cycle without implying that energy is recycled.

Human influences on the environment example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Evaluating evidence for fertiliser pollution

Adapted from November 2020 (R), Paper 1B · Question 10(b)

Question 1

Farmers may add chemical fertilisers to their soil.

These fertilisers may leak into rivers.

A scientist measures the oxygen content of water in two different locations of the same river during the month of April.

In location A he finds that the mean dissolved oxygen was 6 mg per litre and at location B he finds that the mean dissolved oxygen was 3 mg per litre.

He concludes that the use of fertiliser in the field has affected the oxygen content of the river.

Discuss his conclusion.

Final answer

Fertiliser from the field could be washed into the river and cause an algal bloom. When the algae die, bacteria decompose them and respire, using dissolved oxygen. The lower mean oxygen content at location B (3 mg per litre, compared with 6 mg per litre at A), downstream of the field, supports the scientist’s conclusion. However, other factors or fertiliser from other fields could also have caused the decrease, so the results do not prove that this field’s fertiliser was responsible.

Mark scheme points

Any four of the following points are needed for the 4 marks; the alternatives are not all required.

1. M1 Fertiliser leaches or is washed from the field into the river.
2. M2 Fertiliser causes algal or plant growth, an algal bloom, or eutrophication.
3. M3 Dead algae are decomposed or broken down by bacteria or other decomposers.
4. M4 Bacterial respiration reduces the oxygen content of the water.
5. M5 Means are calculated or readings are repeated, supporting reliability or validity.
6. M6 Measurements are taken at the same time of year, in April, supporting a valid comparison.
7. M7 The river flows past the farm between locations A and B.
8. M8 Other factors or fertiliser from other fields could also explain the reduced oxygen.

Explanation

Fertiliser can leach into the river and supply minerals that encourage algae to grow. When the algae die, decomposers break them down. The decomposers respire and use oxygen dissolved in the water, which can lower its oxygen content.

Location B is downstream of the field, and its mean oxygen content is lower than at A, so the results are consistent with this process. But the comparison alone cannot show that the fertiliser from this particular field caused the decrease: other factors or fertiliser sources could also affect the river.

Common mistakes

• Do not say that algae consume the oxygen. Explain that bacteria or other decomposers break down dead algae and use oxygen in respiration.
• Do not claim that the difference proves the field’s fertiliser caused the decrease; acknowledge that other factors or fertiliser sources may be involved.

Example 2: Explaining changes downstream of a sewage outlet

Adapted from January 2018, Paper 2B · Question 3

Question 2

Untreated sewage entering a river affects the numbers of organisms living in the river.

The graph shows the changes in the numbers of fish, algae and bacteria in a river before the sewage outlet and after the sewage outlet.

Algae are protoctists that can carry out photosynthesis.

Explain the changes in the numbers of fish and algae from the point at which sewage enters the river.

Final answer

Fish numbers decrease because bacteria decompose the sewage and respire, reducing the dissolved oxygen available for fish respiration. Algae numbers initially decrease because less light is available for photosynthesis. Farther downstream, algae numbers increase as more light and carbon dioxide are available for photosynthesis, mineral ions support their growth, and fewer fish feed on them.

Mark scheme points

1. M1 The sewage is decomposed or broken down.
2. M2 Bacteria or other microorganisms carry out the decomposition.
3. M3 The dissolved oxygen concentration decreases.
4. M4 Bacterial respiration uses oxygen; less oxygen is then available for fish respiration, so fish numbers decrease.
5. M5 Algae numbers decrease because less light is available for photosynthesis.
6. M6 To explain an increase in algae, give an accepted reason: more light or carbon dioxide for photosynthesis, more mineral ions, or fewer fish feeding on the algae.

Explanation

Bacteria break down the sewage. As they respire, they use oxygen dissolved in the river water. Fish need this oxygen for respiration, so the reduced oxygen supply leads to fewer fish.

Less light means algae photosynthesise less, so their numbers can fall. Further downstream, algae can increase when conditions allow more photosynthesis and when mineral ions are available. Fewer fish also means fewer organisms feeding on the algae.

Common mistakes

• Do not just say that sewage kills fish: explain that bacterial respiration reduces dissolved oxygen, leaving less for fish respiration.
• For algae, link the decrease to less light for photosynthesis; for an increase, give a valid reason such as mineral ions or fewer fish feeding on them.

Practise Human influences on the environment questions

Build a focused practice set from this part of the Edexcel IGCSE Biology syllabus.