How do human activities harm the environment, and what can be done about it?
Describe the effects of pollution and deforestation and ways to conserve the environment
A focused answer to the O-Level Biology outcome on human impact. The effects of water and air pollution, the enhanced greenhouse effect, the consequences of deforestation, and ways to conserve the environment.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
SEAB wants you to describe how human activities harm the environment through pollution (of water and air) and deforestation, to explain processes such as eutrophication and the enhanced greenhouse effect, and to suggest ways of conserving the environment. You should be able to explain a chain of cause and effect, such as how fertiliser kills fish.
The answer
Water pollution and eutrophication
When fertilisers (containing nitrates) or untreated sewage wash into rivers and lakes, they add extra nutrients to the water. This causes eutrophication, a harmful sequence:
- The extra nutrients make algae and water plants grow rapidly (an algal bloom), often covering the surface.
- The thick algae block out light, so plants below cannot photosynthesise and die.
- Decomposers (bacteria) break down the dead plants and algae and multiply, respiring and using up the dissolved oxygen in the water.
- With little oxygen left, fish and other animals suffocate and die.
Air pollution
Burning fossil fuels releases pollutants:
- Carbon dioxide adds to the greenhouse effect (see below).
- Sulfur dioxide and nitrogen oxides dissolve in rain to form acid rain, which damages trees, lakes and buildings.
- Carbon monoxide is a poisonous gas that reduces the oxygen the blood can carry.
The enhanced greenhouse effect
Greenhouse gases such as carbon dioxide and methane trap heat in the atmosphere. This keeps the Earth warm enough for life. But burning fossil fuels and deforestation add extra carbon dioxide, trapping more heat and causing global warming (a rise in average temperatures), which can lead to melting ice, rising sea levels and changing climates.
Deforestation
Cutting down large areas of forest harms the environment by:
- Increasing carbon dioxide in the air (fewer trees to remove it by photosynthesis, and burning trees releases more), adding to the greenhouse effect.
- Soil erosion: without tree roots to hold the soil, rain washes it away, which can cause flooding and the loss of fertile land.
- Destroying habitats, reducing biodiversity (the variety of living things).
Conserving the environment
Ways to reduce human impact and conserve the environment include:
- Reducing pollution: treating sewage, using cleaner fuels, and controlling fertiliser use.
- Conserving forests: replanting trees (reforestation) and managing forests sustainably.
- Recycling materials and reducing waste, and protecting habitats in nature reserves to maintain biodiversity.
Examples in context
Example 1. A dead lake. A lake near intensive farmland may turn green with algae, then lose its fish entirely. Testing shows very low dissolved oxygen, the result of eutrophication driven by fertiliser run-off, a clear case of human impact.
Example 2. Replanting mangroves. Coastal areas stripped of mangrove forest suffer erosion and flooding. Replanting the mangroves stabilises the soil, protects the coast, and restores habitats for fish and birds, an example of conservation reversing damage.
Try this
Q1. Name the process by which fertiliser run-off leads to fish deaths. [1 mark]
- Cue. Eutrophication.
Q2. State two harmful effects of deforestation. [2 marks]
- Cue. Any two of: more carbon dioxide in the air (less photosynthesis), soil erosion, flooding, loss of habitats and biodiversity.
Q3. Explain how extra carbon dioxide contributes to global warming. [2 marks]
- Cue. Carbon dioxide is a greenhouse gas that traps heat in the atmosphere; extra carbon dioxide traps more heat, raising average temperatures.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksFertiliser washed from farmland into a river can kill the fish. (a) Name this type of water pollution. (b) Describe the sequence of events that leads to the death of the fish.Show worked answer →
(a) This is eutrophication.
(b) The fertiliser (containing nitrates) washes into the river and provides extra nutrients. This causes algae and water plants to grow rapidly (an algal bloom) over the surface. The thick algae block out light, so plants below cannot photosynthesise and die. Decomposers (bacteria) break down the dead plants and algae and multiply, respiring and using up the oxygen dissolved in the water. With little oxygen left, the fish (and other animals) cannot respire and die.
Markers reward eutrophication, the extra nutrients causing rapid algal growth, the blocking of light killing plants, and decomposers using up the oxygen so fish suffocate.
Original4 marksDescribe two harmful effects of cutting down large areas of forest (deforestation) on the environment.Show worked answer →
Two harmful effects: deforestation increases the carbon dioxide in the air, because fewer trees remain to remove it by photosynthesis (and burning the trees releases more), adding to the greenhouse effect. It also leads to soil erosion, because tree roots no longer hold the soil in place, so rain washes the soil away; this can cause flooding and loss of fertile land. It also destroys habitats, reducing biodiversity (the variety of living things). Any two are accepted.
Markers reward two correct effects, such as more carbon dioxide (less photosynthesis), soil erosion, flooding, or loss of habitats and biodiversity.
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