BIOLOGY · Stage 9 · Chapter 13: Variation and Natural Selection
Biology 13.4 Extinction! — Habitat Loss, Mass Extinctions and Endangered Species
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Lesson summary
13.4 Extinction! — Habitat Loss, Mass Extinctions and Endangered Species — Alhussan International Academy — Biology (Stages 7–9) notes: Define extinction, explain its natural and human causes, and describe how species become endangered.Extinction — a species is extinct when no in…
- Permanence
- Fossils and descendants
- Natural process
- Current rate
- Environmental change
- New species
Section 1 of 5What extinction is
- Extinction
- a species is extinct when no individuals of that species are alive.
- Local extinction involves a population in one place.
- Global extinction means the organism no longer exists anywhere on Earth, like the dinosaurs.
- Permanence
- once a species is globally extinct it has gone forever.
- Fossils and descendants
- some extinct species exist only as fossils, while others have descendants alive today that are better adapted to their environment.
- Natural process
- extinction is part of life on Earth and makes resources available for better adapted organisms.
- Most species that have ever lived have died out.
- Current rate
- it is estimated to be many times higher than the natural background rate, mainly because of human activity.
Section 2 of 5Natural causes of extinction
- Environmental change
- the climate has changed many times, and tropical organisms became extinct in an ice age through lack of food or being too cold.
- New species
- newcomers may be predators, like the brown tree snakes on Guam,
- or better-adapted competitors, like the invasive plants Himalayan balsam and water hyacinth.
- newcomers may be predators, like the brown tree snakes on Guam,
- New diseases
- environmental changes create conditions for new diseases, which sometimes wipe out whole species.
- A deadly new fungal disease threatens amphibian species around the world.
- Catastrophic events
- a massive volcanic eruption or a giant meteorite strike.
- Land use
- draining land for agriculture destroys ecosystems such as mangroves, marshes and bogs.
Section 3 of 5Human causes of extinction today
- Habitat loss
- forests are cleared for farming, towns and roads, wetlands are drained and coral reefs are damaged.
- Rainforest is destroyed at a rate of about 8000 km2 per year.
- Fragmentation isolates populations in small patches, and habitat loss is the biggest single cause.
- Hunting and fishing
- the human population has grown enormously and we have guns.
- Humans hunted mammoths, dodos and whales to extinction and have overfished the seas.
- The dodo of Mauritius had no predators until people arrived in the 17th century and was extinct in less than 100 years.
- Animals are also killed for food, ivory, fur or trade, as the passenger pigeon was and rhinos are today.
- Pollution
- pesticides, oil spills, plastic, factory waste and sewage destroy rivers and ocean environments.
- Sixteen species of freshwater fish became extinct in 2020 alone.
- Climate change
- plants and animals that cannot adapt quickly or lose their natural environment become extinct.
- Human-caused climate change is destroying coral reefs.
- Invasive species and disease
- both can devastate populations.
- Trend
- extinction rates have increased over the last 150 years, beginning in the Industrial Age, and species are dying out faster than ever.
Section 4 of 5Mass extinctions
- Mass extinction = an event in which many species die out on a global scale over a relatively short time.
- Five so far
- each followed huge environmental changes, often a single catastrophic event such as a massive volcanic eruption or a giant meteorite strike.
- The dinosaur extinction
- about 66 million years ago the dinosaurs and about three quarters of species disappeared, probably after a large asteroid strike and intense volcanic activity.
- The largest
- 252 million years ago over 90% of marine species died.
- Recovery
- new species evolved to fill the gaps over millions of years.
- Sixth mass extinction
- the evidence suggests we are living in it, and many extinctions result from human actions.
Section 5 of 5Why small populations are at risk
- Little genetic variation
- a small population is less able to adapt to changes such as a new disease.
- Inbreeding
- breeding between close relatives produces more harmful recessive conditions.
- Chance events
- a storm or fire can wipe out the remaining individuals.
- Vulnerable features
- specialised diets, low reproductive rates and large territories, as in pandas and tigers.
| Term | Meaning |
|---|---|
| Extinct | No living members remain |
| Endangered | At high risk of extinction |
| Vulnerable | Likely to become endangered if threats continue |
- Tracking
- scientists use population counts and the IUCN Red List to track species and set conservation priorities.
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