B14.3 More about Extinction

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: environmental change, catastrophic events, mass extinctions and the human impact. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• List the factors that can cause a species to become extinct
• Describe how environmental changes over geological time can cause extinction
• Explain, step by step, how an asteroid impact could have caused the extinction of the dinosaurs
• Describe how human activities are increasing the rate of extinction
• Use data about mass extinctions

Key words

• Geological time: The very long time scale of Earth’s history, measured in millions of years.
• Mass extinction: An event in which a large proportion of the species on Earth die out in a short time (geologically).
• Catastrophic event: A sudden disaster with huge effects, such as a massive volcanic eruption or an asteroid collision.
• Climate change: A long-term change in the average temperature and weather patterns of a region or the whole Earth.
• Habitat destruction: Removing or damaging the place where organisms live, e.g. clearing forest for farmland.

Why species become extinct

Species that are well adapted to their environment survive and reproduce. If the environment changes, or a new organism arrives, a species may no longer be well adapted. If none of its individuals can survive and breed, it becomes extinct.

Environmental change over geological time

Conditions on Earth are always changing, usually slowly. If a species cannot evolve quickly enough, or move somewhere suitable, it may die out.

• Climate change: Earth has had ice ages and warm periods. The end of the last ice age, about 11 000 years ago, saw many large cold-adapted mammals die out, such as woolly rhinos.
• Changes in sea level: rising seas flood coastal habitats; falling seas drain shallow seas.
• Changes in habitat: forests can become grassland or desert, removing food and shelter.

Worked example: a mountain plant in a warming climate

A small flowering plant grows only near mountain tops, where it is cold. As the climate warms, other plants spread uphill and out-compete it.
It cannot move any higher, because it is already at the top.

Answer: its population shrinks and it may become extinct

Single catastrophic events

Some extinctions happen much more quickly, after a single disaster:

• Massive volcanic eruptions release huge amounts of lava, ash and gases such as carbon dioxide and sulfur dioxide, changing the climate and causing acid rain.
• Collisions with asteroids throw dust and debris into the atmosphere and start fires.

When many species die out at about the same time it is called a mass extinction. There have been at least five in Earth’s history.

What destroyed the dinosaurs?

About 66 million years ago, the dinosaurs (apart from the ancestors of birds) and about three-quarters of all species died out. Most scientists think the main cause was a large asteroid that hit what is now Mexico.

• A thin clay layer in rocks of this age all over the world, rich in iridium (a metal rare on Earth but common in asteroids); What it suggests Dust from a huge asteroid spread over the whole planet
• A buried crater about 180 km wide, of the right age, on the coast of Mexico; What it suggests The impact site of a large asteroid
• Soot in the same rock layer; What it suggests Widespread fires
• Huge lava flows in India from around the same time; What it suggests Volcanic eruptions may also have changed the climate

Key fact: Large animals were hit hardest: they need lots of food, breed slowly and cannot shelter underground. Small animals that could burrow and eat seeds, insects or dead material were more likely to survive.

Humans and extinction today

The rate of extinction is increasing as the human population grows. Many scientists think humans are causing a new mass extinction.

• Habitat destruction: clearing rainforest for farming, building towns and roads, draining wetlands.
• Pollution: of air, water and land, including the release of carbon dioxide that is causing climate change.
• Introducing species to new places, where they act as new predators, competitors or pathogens.
• Over-hunting and over-fishing.

Common mistakes

• Saying the asteroid killed every dinosaur directly. Most died later because photosynthesis fell and food chains collapsed.
• Thinking environmental change always causes extinction. A species may survive if it can evolve, move away, or already has suitable variation.
• Forgetting that birds evolved from dinosaurs – not every dinosaur line died out.
• Describing evidence without saying what it suggests. Always link the evidence to the conclusion.

Revision questions

Try these, then check your answers below each one.

Question 1. About how long ago did the dinosaurs (apart from the ancestors of birds) become extinct? (Choose from: 66 million years ago, 6.6 million years ago, 660 million years ago, 66 000 years ago.)

Answer: 66 million years ago

Working: About 66 million years ago, in a mass extinction at the end of the Cretaceous period.

Question 2. The flow chart shows how an asteroid impact could cause extinctions. What should go in the box marked “?” (Choose from: Less photosynthesis, so plants die, More photosynthesis, so plants grow faster, Carnivores eat more herbivores, Temperatures rise, so ice melts.)

Answer: Less photosynthesis, so plants die

Working: Dust and soot block sunlight, so plants cannot photosynthesise enough and die. Herbivores then starve, and carnivores lose their food.

Question 3. True or false? Human activities such as destroying habitats and introducing new species are causing species to become extinct today.

Answer: True

Working: True. Habitat destruction, pollution, hunting and introducing predators, diseases or competitors are increasing the rate of extinction.

Related lessons

• Previous lesson: B14.2 Fossils and Extinction
• Next lesson: B14.4 Antibiotic Resistant Bacteria
• In this chapter: B14.1 Evidence of Evolution
• In this chapter: B14.5 Classification
• In this chapter: B14.6 New Systems of Classification

Continue in the full course

This lesson is section B14.3 (AGCSC1403) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.