Ch 2 · Habitat Destruction · O Level Biology 5090
Topic 2 of 4 in Human Influences on Ecosystems — O Level Biology 5090 — 38 lessons.
Introduction
A habitat is the place where an organism lives, and habitats are disappearing faster than at any time in human history. Deforestation is the example examiners return to most often, because its consequences link so many parts of the syllabus: biodiversity, food webs, soil, water and the carbon cycle. A strong answer traces each consequence back to what a tree actually does.
Key ideas — Why habitats are destroyed
The syllabus names three groups of reasons.
🧑🏫 Teacher's explanation
Imagine a complex, multi-story apartment building where every resident has a specific job—the plumber lives on floor 2, the baker on floor 4, and the electrician on floor 1. If you decide to knock down the middle floors to make a giant open-plan lounge, you haven't just changed the layout; you've destroyed the 'habitat' of the plumber and the baker. Even if the building still stands, the community is gone.
📖 Definitions
🔍 Worked example — Easy
A farmer clears a patch of tropical rainforest to create pasture for cattle. State two ways this action directly contributes to habitat destruction.
🔍 Worked example — Standard
Explain why deforestation leads to an increase in atmospheric carbon dioxide levels.
🔍 Worked example — Exam-style
A marine ecosystem is affected by agricultural run-off containing high levels of nitrates. Explain how this pollution can lead to a loss of biodiversity in the marine habitat.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe two ways deforestation affects the atmosphere. [2] (b) Explain why deforestation can lead to an increase in flooding in downstream areas. [2] [4 marks]
Key ideas — Altering food webs
Habitats can also be damaged without bulldozers. By altering food webs and food chains, humans can have a negative impact on habitats. Removing one species by hunting, overfishing or pesticide use, or introducing a foreign species, changes the numbers of everything linked to it (lesson 24.19.2). Removing top predators lets herbivores multiply and strip the vegetation; killing pollinating insects with insecticide stops plants setting seed; an introduced species with no natural predators outcompetes native ones. In each case the habitat's structure and the species it can support are changed.
🧑🏫 Teacher's explanation
Imagine a complex game of Jenga. Each wooden block is a species in a food web. If you pull out a block from the bottom, the whole tower might wobble or collapse. If you pull out a block from the middle, the blocks above it might fall. In an ecosystem, species are connected through feeding relationships. When humans remove or add a single species, it creates a 'domino effect' that can change the entire landscape.
📖 Definitions
🔍 Worked example — Easy
A farmer uses a broad-spectrum insecticide to kill aphids on his crops. Explain how this might lead to a decrease in the population of local insectivorous birds.
🔍 Worked example — Standard
In a marine ecosystem, humans overfish a specific species of large predatory fish. Predict the effect on the population of the herbivorous fish that this predator used to eat.
🔍 Worked example — Exam-style
A non-native species of snake is introduced to an island. This snake has no natural predators on the island and eats the eggs of native ground-nesting birds. Describe the impact on the island's biodiversity.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State two ways deforestation can lead to a decrease in biodiversity. [2] (b) Explain how deforestation can lead to an increase in the concentration of carbon dioxide in the atmosphere. [2] [4 marks]
Key ideas — The undesirable effects of deforestation
Deforestation is the clearing of forest for timber, fuel, farmland or building. Its five examined effects all follow from what trees do.
🧑🏫 Teacher's explanation
Think of a forest like a massive, complex city. The trees are the skyscrapers, the roots are the foundation/drainage systems, and the animals are the citizens. If you suddenly bulldoze the skyscrapers, the citizens lose their homes (biodiversity loss/extinction). If you remove the drainage systems, the streets flood. If you burn the buildings, the smoke pollutes the air.
📖 Definitions
🔍 Worked example — Easy
A forest area is cleared for cattle ranching. State two ways this directly reduces the biodiversity of the area.
🔍 Worked example — Standard
Explain how deforestation leads to an increase in the concentration of carbon dioxide in the atmosphere.
🔍 Worked example — Exam-style
A farmer notices that after clearing a forest for crops, the soil becomes much less fertile and the nearby river frequently overflows. Explain the link between deforestation and these two specific environmental issues.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State two reasons why humans clear forests. [2] (b) Explain how deforestation can lead to an increase in the concentration of carbon dioxide in the atmosphere. [3] [5 marks]
Worked example
Question. A hillside forest was cleared to grow crops. In the following years the villages in the valley below suffered more frequent floods, and the crop yields on the hillside fell. Explain these observations. [5]
🔍 Worked example — Easy
A farmer converts a woodland into a large wheat field. State two ways this might decrease the biodiversity of the area. [2]
🔍 Worked example — Standard
Explain why deforestation leads to an increase in atmospheric carbon dioxide levels. [2]
🔍 Worked example — Exam-style
A hillside forest was cleared to grow crops. In the following years the villages in the valley below suffered more frequent floods, and the crop yields on the hillside fell. Explain these observations. [5]
⚠️ Common errors
Common mistakes
Practice
Answers