15.7 More About Extinction

A free GCSE Biology lesson for AQA (8461) from OBA School: extinction caused by environmental change and catastrophic events, and the end of the dinosaurs. 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

• Describe how environmental changes over geological time can cause extinction
• Describe how single catastrophic events can cause mass extinctions
• Explain, step by step, how an asteroid impact could lead to the extinction of the dinosaurs
• Use evidence and 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 that has a huge effect, 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 of the whole Earth.
• Asteroid: A large rocky object orbiting the Sun. A large one hitting Earth can cause global effects.

Environmental change over geological time

Conditions on Earth are always changing, usually very slowly. If a species cannot adapt (evolve) quickly enough, or move somewhere suitable, it may die out. Changes include:

• Climate change: the Earth has had ice ages and very warm periods. Species adapted to one climate may not survive another.
• Changes in sea level: rising seas flood coastal habitats; falling seas drain shallow seas where many species lived.
• Changes in habitat: forests can become grassland or desert, removing food and shelter.
• Moving continents: over millions of years the continents move, changing climates and ocean currents.

Worked example: the woolly mammoth

Woolly mammoths were adapted to cold grasslands in the last ice age. As the climate warmed, their grassland habitat shrank and was replaced by forest and wetland.
Mammoth populations became small and isolated. Hunting by humans probably added to the pressure.

Answer: most mammoths died out about 10 000 years ago

Single catastrophic events

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

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

There have been at least five mass extinctions in Earth’s history.

What destroyed the dinosaurs?

About 66 million years ago, the dinosaurs (apart from the ancestors of birds) and around three-quarters of all species died out. Most scientists think the main cause was a large asteroid, about 10 km across, which hit what is now Mexico.

• A thin layer of clay in rocks of this age around the world is rich in iridium, a metal that is 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, and glassy beads formed from melted rock; What it suggests Widespread fires and a violent impact
• 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, eat seeds or scavenge were more likely to survive.

Common mistakes

• Saying the asteroid killed every dinosaur directly. Most died later because food chains collapsed when photosynthesis fell.
• Thinking environmental change always causes extinction. Species may survive if they can evolve, move, or already have suitable variation.
• Forgetting that birds evolved from dinosaurs – not every dinosaur line died out.
• Describing the evidence without saying what it suggests. Always link 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 birds) die out? (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, at the end of the Cretaceous.

Question 2. True or false? Single catastrophic events such as asteroid impacts cause extinction slowly, over millions of years.

Answer: False

Working: False. Catastrophic events act quickly; slow environmental changes act over geological time.

Question 3. Why were large animals most likely to die out after the asteroid impact? (Choose from: They needed lots of food and bred slowly, They could all burrow underground, They were not affected by cold, They only ate seeds.)

Answer: They needed lots of food and bred slowly

Working: Large animals need more food, breed slowly and cannot shelter, so they could not survive when food chains collapsed.

Related lessons

• Previous lesson: 15.6 Fossils and Extinction
• Next lesson: 15.8 Antibiotic Resistant Bacteria
• In this chapter: 15.1 The History of Genetics
• In this chapter: 15.2 Theories of Evolution
• In this chapter: 15.3 Accepting Darwin's Ideas

Continue in the full course

This lesson is section 15.7 (AGCBH1507) of our free GCSE Biology 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 Biology course.