Population Change & Extinction
Section: Biology | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is Environmental Change?
- An environmental change is any change in the conditions of a habitat or ecosystem that affects the organisms living there
- Can be natural:
- drought, flood, volcanic eruption, wildfire
- disease outbreak
- natural long-term climate shifts
- Can be caused by human activity:
- habitat destruction, e.g. deforestation or urban development
- pollution of air, water or soil
- introducing a new species to an area
- overhunting or overfishing
- climate change caused by greenhouse gas emissions
- Environmental change can be sudden, like a wildfire, or gradual, like rising sea temperature over decades
How a Population Can Respond
- When the environment changes, a population's size and characteristics can respond in different ways, depending on whether the change is favourable or unfavourable, and how quickly it happens
| Response | What Happens | Example |
|---|---|---|
| Increase | A favourable change lets more individuals survive and reproduce | A milder winter lets more insects survive to breed in spring |
| Decrease | An unfavourable change reduces survival or reproduction | A drought reduces the food supply available to grazing animals |
| Migrate | Individuals move to an area with more suitable conditions | A herd moves to a new grazing ground when a water source dries up |
| Adapt | Over many generations, natural selection favours individuals with characteristics suited to the new conditions | A population gradually shifts towards individuals with fur colours that camouflage better in a changed habitat |
| Become extinct | If the population cannot survive, migrate or adapt quickly enough, every individual dies and the species disappears | A species confined to a single small island cannot escape a habitat destroyed by a volcanic eruption |
The possible outcomes for a population when its environment changes
Extinction
- A species is extinct when there are no living individuals of that species left anywhere on Earth
- Local extinction: a species disappears from one area, but survives elsewhere
- global extinction: a species disappears everywhere — this is permanent
- Extinction risk increases when:
- environmental change happens faster than a population can survive, migrate or adapt
- the population is already small
- the species has a narrow habitat range, or a highly specialised diet
Exam Tip
- If asked to explain why a species became extinct, link your answer to the specific environmental change described in the question, and explain why that species in particular could not survive, migrate, or adapt quickly enough — a generic answer without this link will not score full marks
Human Activities That Drive Environmental Change
- Habitat destruction: deforestation and urban development remove the space and resources a population depends on
- Introducing non-native species: new predators or competitors can out-compete or prey on native species that have no evolved defence against them
- Pollution: chemicals, plastics and oil spills can poison organisms or destroy habitats
- Overhunting and overfishing: removing individuals faster than the population can reproduce reduces population size
- Climate change, caused by greenhouse gas emissions, is one major driver of environmental change
- see the Climate Change topic for how it specifically affects species and ecosystems
Population Change & Extinction - Summary
- Environmental change can be natural or caused by human activity, and can happen suddenly or gradually
- A population can respond by increasing, decreasing, migrating, adapting over generations, or becoming extinct
- A species is extinct once no living individuals remain anywhere on Earth
- Extinction risk is highest when change happens too fast, or when a population is already small or highly specialised
Interactive revision notes, videos and practice questions load below.