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Habitat destruction

Paper 1Paper 2Paper 3Paper 4

This topic is examined in Paper 1, Paper 2, Paper 3, and Paper 4.

What is Habitat Destruction?
Habitat destruction is the process by which a natural habitat becomes unable to support its native species. It is the primary cause of biodiversity loss worldwide.

Building on the concept of an ecosystem (a community of living organisms interacting with their physical environment), habitat destruction removes the essential components—such as food, shelter, and breeding grounds—that allow these organisms to survive.

Biodiversity

Biodiversity is defined as the number of different species that live in an area.

  • Key point: It is a measure of variety. A rainforest has high biodiversity because it contains many different types of plants, animals, and microorganisms. A monoculture farm (e.g., only corn) has low biodiversity.
Reasons for Habitat Destruction

Humans destroy habitats primarily to meet economic and population needs. The three main drivers are:

  1. Land Use Change (Agriculture and Housing):

    • Housing: Urban expansion converts forests and wetlands into residential areas.
    • Crop Production: Forests are cleared to create farmland for growing crops (e.g., soy, palm oil).
    • Livestock Production: Large areas of land are cleared for grazing cattle or sheep.

  2. Extraction of Natural Resources:

    • Logging: Trees are cut down for timber, paper, and fuel.
    • Mining: Extracting minerals (e.g., coal, gold) destroys the surface habitat entirely.

  3. Pollution:

    • Freshwater Pollution: Industrial waste or agricultural runoff poisons rivers and lakes, making them uninhabitable for aquatic species.
    • Marine Pollution: Oil spills and plastic waste destroy coastal and ocean habitats.
Impact on Food Webs and Chains
Humans can have a negative impact on habitats by altering food webs and food chains.

  • Food Chain: A linear sequence of organisms where nutrients and energy are transferred from one organism to another (e.g., Grass → Rabbit → Fox).
  • Food Web: A complex network of interconnected food chains.

When a habitat is destroyed, key species may be removed. This disrupts the flow of energy. For example, if the prey (rabbit) loses its habitat, the predator (fox) starves. This can cause a cascading effect, leading to the collapse of the entire ecosystem.

Undesirable Effects of Deforestation

Deforestation (the large-scale removal of forests) is a major form of habitat destruction. Its undesirable effects include:

  1. Reducing Biodiversity:

    • Trees provide shelter and food for countless species. When trees are removed, the number of different species in that area decreases drastically.

  2. Extinction:

    • Species that lose their habitat may die out completely if they cannot migrate or adapt. This is permanent loss of genetic diversity.

  3. Loss of Soil (Soil Erosion):

    • Tree roots hold soil in place. Without them, rain washes away the topsoil (erosion). This leads to soil infertility, making it hard for new plants to grow.

  4. Flooding:

    • Trees absorb water through their roots and release it slowly. Without trees, water runs off the surface quickly into rivers, causing flash floods downstream.

  5. Increase of Carbon Dioxide in the Atmosphere:

    • Photosynthesis: Living trees absorb CO_2 from the air to make food. Fewer trees mean less CO_2 is removed.
    • Decomposition/Combustion: When trees are burned or left to rot, the carbon stored in their biomass is released back into the atmosphere as CO_2. This contributes to global warming.
⚠︎ Confusing Causes and Effects
Error: Students often list 'loss of soil' or 'flooding' as a cause of deforestation.

Correction: These are effects (consequences) of deforestation. The causes are human activities like logging, farming, or housing development. Always distinguish between why it happens (cause) and what happens because of it (effect).

Explaining the Carbon Dioxide Link

When to use: When asked to explain how deforestation increases atmospheric CO_2.

Why examiners accept this: Examiners look for two distinct mechanisms: the reduction of removal and the increase of release. Simply saying 'trees absorb CO_2' is incomplete because it ignores the carbon stored in the wood.

Correct phrasing:

  1. Less photosynthesis: Fewer trees mean less carbon dioxide is removed from the atmosphere.
  2. Release of stored carbon: Burning or decomposing felled trees releases the carbon stored in their biomass back into the atmosphere as carbon dioxide.
Past Paper Style Questions
Q:
Describe what is meant by biodiversity. [2]
A:

Answer:

  1. The number of different species (1)
  2. That live in an area / habitat / ecosystem (1)
Q:
State two reasons why deforestation occurs. [2]
A:

Answer:
Any two from:

  • Space for housing / urban development.
  • Space for agriculture / crop production / livestock grazing.
  • Extraction of natural resources (e.g., timber, minerals).
  • Production of timber or paper.
Q:
Explain how deforestation leads to an increase in carbon dioxide in the atmosphere. [3]
A:

Answer:

  1. Fewer trees means less photosynthesis, so less CO_2 is removed from the air (1).
  2. Burning felled trees releases stored carbon as CO_2 (1).
  3. Decomposition of rotting wood also releases CO_2 into the atmosphere (1).
Q:
Describe the undesirable effects of deforestation other than reducing biodiversity. [3]
A:

Answer:
Any three from:

  • Extinction of species.
  • Loss of soil / soil erosion.
  • Flooding (due to lack of water absorption).
  • Disruption of food webs or food chains.
  • Loss of potential medicines or genetic resources.
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