Explain why species become endangered or extinct
Climate change, habitat destruction, hunting, overharvesting, pollution and introduced species can reduce populations and threaten survival.
Section 20.4 · Conservation
Core and Extended; Biology 0610, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked
Before you begin
- Know that organisms interact with other organisms and their environment.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which statement distinguishes extinction from endangerment?
What you will learn
- Explain how six syllabus-listed pressures can endanger organisms or cause extinction
- Distinguish reduced population size from extinction
Trace a pressure to a smaller population
Climate change can change conditions faster than a population can tolerate; habitat destruction removes living space and resources; hunting and overharvesting remove organisms faster than populations can replace them; and pollution can harm organisms or their habitat.
Introduced species may compete with native organisms, eat them, or introduce disease. These pressures can reduce a population, and continued decline may make a species endangered or lead to extinction. A species is extinct when no individuals remain, not simply when it becomes rare.
Explain the route for the named pressure
For each example, state the pressure, how it affects survival or reproduction, and how that can reduce numbers. Do not treat every introduced species as harmful automatically: the effect depends on its interaction with the ecosystem.
Several pressures can act together. If the question gives evidence for only one, explain that factor without claiming it is the sole cause of decline.
Pause and explain
How can overharvesting endanger a species?
Worked example
An introduced predator feeds on an island bird that nests on the ground. Explain how the bird could become endangered.
Show the worked solution
- Identify the new predation pressure.
- Connect increased deaths with a declining population.
Answer The introduced predator may increase deaths of ground-nesting birds. If deaths exceed replacement by reproduction, the bird population can decline and become endangered.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain how habitat destruction can lead to extinction.
Give me a hint
Link loss of habitat with survival and reproduction.
Compare my reasoning
- Organisms lose some living space and resources.
- If the remaining habitat cannot support the population, numbers can continue to decline.
Habitat destruction reduces living space and resources. If the population cannot survive or reproduce sufficiently in what remains, its numbers may fall until no individuals remain.
Look for these in your work
- Habitat loss linked
- Extinction pathway explained
List six factors named in the syllabus that can make organisms endangered or extinct.
Give me a hint
Recall climate, habitat, removal, pollution and introduced organisms.
Compare my reasoning
- Climate change, habitat destruction and hunting.
- Overharvesting, pollution and introduced species.
The six factors are climate change, habitat destruction, hunting, overharvesting, pollution and introduced species.
Look for these in your work
- All six named
- No factor omitted
A student says every introduced species makes native species extinct. Improve the statement.
Give me a hint
Use a conditional claim based on the interaction.
Compare my reasoning
- An introduced organism may compete, prey or spread disease.
- Its effect depends on the ecosystem and the native species.
An introduced species can threaten native species if it competes with them, preys on them or spreads disease, but the outcome depends on its interactions and the ecosystem.
Look for these in your work
- Mechanism named
- Universal claim qualified
Common mistakes
- Using endangered and extinct as synonyms
- Assuming every introduced species has the same effect
- Naming a threat without explaining its effect on survival or reproduction
What can you explain now?
Which statement distinguishes extinction from endangerment?
Compare with the explanation
Extinction means no individuals of the species remain
Endangered species still exist but face a high risk of disappearing.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How do the six listed pressures threaten population size, and how does extinction differ from endangerment?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.