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Biology 061020.1

Pollution, deforestation and conservation

How human activity damages ecosystems, and what can be done.

Review these first

Learning objectives

What you need to be able to do

Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.

  • 20.1.1Describe the causes and effects of water and air pollution, including eutrophication.
  • 20.1.2Describe the effects of deforestation and the need for conservation.

6 minute read

Pollution, deforestation and conservation

Eutrophication

This sequence is examined often and must be given in order:

  1. Excess fertiliser (nitrate) runs off farmland into a river or lake.
  2. The nutrients cause algae to grow rapidly, forming an algal bloom on the surface.
  3. The bloom blocks light from reaching plants below, so they cannot photosynthesise and they die.
  4. Decomposing bacteria feed on the dead plants and multiply rapidly.
  5. These bacteria respire aerobically, using up the dissolved oxygen in the water.
  6. Fish and other aquatic organisms die from lack of oxygen.

The crucial insight is that the fish are not poisoned by the fertiliser — they suffocate because bacteria consumed the oxygen.

Deforestation

Effects include habitat loss and reduced biodiversity, soil erosion once roots no longer bind the soil, flooding as less water is intercepted, and increased atmospheric carbon dioxide — both because fewer trees photosynthesise and because burning the timber releases it.

Air pollution

  • Carbon dioxide and methane are greenhouse gases, causing enhanced global warming.
  • Sulfur dioxide causes acid rain, damaging trees and acidifying lakes.

Conservation

Reasons to conserve species include maintaining biodiversity, protecting food chains from collapse, preserving a gene pool for future crop breeding and medicines, and the value of ecosystems in themselves. Methods include protected areas and national parks, captive breeding programmes and zoos, seed banks, controlling fishing with quotas and net-size limits, and replanting forests.

Think of it like this

Eutrophication is a party that eats all the food: the algae arrive first and thrive, but the bacteria clearing up afterwards consume all the oxygen — and everyone else in the room suffocates.

Worked examples

Method, step by step

Fertiliser runs off a field into a lake and fish begin to die. Explain the sequence of events.

  1. 1Nitrates in the fertiliser enrich the water, causing algae to grow rapidly and form a bloom.
  2. 2The algal bloom blocks light, so submerged plants cannot photosynthesise and die.
  3. 3Decomposer bacteria feed on the dead plants and multiply rapidly.
  4. 4These bacteria respire aerobically, using up the dissolved oxygen in the water.
  5. 5Fish die because there is too little oxygen left for their respiration.

Nitrates cause an algal bloom, which blocks light and kills plants. Bacteria decomposing them multiply and use up the dissolved oxygen in respiration, so the fish suffocate.

Common misconceptions

  • Saying fertiliser poisons the fish. The fish die of oxygen deprivation caused by decomposing bacteria, not by chemical toxicity.
  • Leaving out the bacteria step in eutrophication, which is the step that actually removes the oxygen.
  • Thinking deforestation only matters for animals. Soil erosion, flooding and CO₂ increase are equally examinable consequences.

In the exam

  • Write eutrophication as a numbered sequence. Mark schemes award each linked step, and jumping from "fertiliser" to "fish die" scores almost nothing.
  • For deforestation, give a range of effect types — biodiversity, soil, water and atmosphere — rather than four versions of "animals lose homes".