The four causes of loss

Four causes

Why do species disappear? The ecologist Jared Diamond named four main causes. He called them the evil quartet. They are habitat loss and fragmentation, over-exploitation, alien species invasions, and co-extinctions.

1. Habitat loss and fragmentation

This is the most important cause of the loss. When a forest is cut, the animals and plants lose the place that they live in.

• Tropical rain forests once covered more than 14 per cent of the land of the Earth. Now they cover no more than about 6 per cent.
• The Amazon rain forest, which is called the lungs of the planet, is being cleared. Some of the land is used to grow soya beans. Some is turned into grassland for beef cattle.
• Pollution damages the habitat too. This is habitat degradation.

When a large habitat is broken into small pieces, it is fragmented. Mammals and birds that need large territories suffer, and so do the migratory animals. Some animals cannot cross the gap between the pieces.

The edge effect

This part is not in the NEET syllabus, but A-Level and IB explain it.

The edge of a forest piece is different from its middle. It is drier, lighter and windier. The core of the piece keeps the real forest conditions. Suppose the edge is 1 km wide. A forest of 10 km by 10 km has a core of $8 \times 8 = 64$ square kilometres, which is 64 per cent. If it is cut into four equal pieces, each piece is 5 km wide and has a core of $3 \times 3 = 9$ square kilometres. The four cores together are 36 square kilometres, which is 36 per cent. The area of the forest is the same, but the core is much smaller.

2. Over-exploitation

People depend on nature. But when we take too much, a species can go. NCERT gives these cases.

• The Steller's sea cow and the passenger pigeon were hunted until they were gone.
• Many marine fish populations are harvested far beyond what they can replace. Many of them are now declining.

3. Alien species invasions

A species that comes from another place is an alien species. Some are brought by accident, and some on purpose. A few of them become invasive, which means that they spread and harm the native species.

• The Nile perch was introduced into Lake Victoria in East Africa. It was followed by the extinction of more than 200 species of cichlid fish of the lake.
• The weeds carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) damaged our native species of plants and the environment.
• The African catfish, Clarias gariepinus, was brought for aquaculture. It is a threat to the native catfishes of our rivers.

4. Co-extinctions

When a species goes, the species that depend on it may go too. This is co-extinction.

• A host fish species that becomes extinct takes its unique group of parasites with it.
• In a plant and pollinator mutualism, where each has evolved with the other, the extinction of one leads to the extinction of the other.

Try it in Python

The first program works out the core of a forest cut into pieces. Run it to see how fast the core falls.

Practise with new numbers

Common mistakes

• Mixing up an alien species with a native one. Lantana, carrot grass and water hyacinth are aliens that spread in India.
• Saying that habitat loss only means cutting down trees. It also means pollution, and the breaking of a habitat into pieces.
• Putting the Nile perch in India. It was introduced into Lake Victoria in Africa.
• Forgetting that co-extinction needs a close tie. The species that go together depend on each other, like a host and its parasites.

Remember these

• The evil quartet: habitat loss and fragmentation, over-exploitation, alien species invasions, co-extinctions.
• Rain forests: more than 14 per cent of the land once, no more than about 6 per cent now. The Amazon is cleared for soya and cattle.
• Over-exploitation: Steller's sea cow, passenger pigeon, marine fish.
• Alien species: Nile perch (more than 200 cichlid species lost), Parthenium, Lantana, water hyacinth, Clarias gariepinus.
• Co-extinction: a host and its parasites. A plant and its pollinator.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Name the four causes of the loss of biodiversity.

2. Which is the most important cause of the loss, according to NCERT?

3. What happened to the cichlid fishes of Lake Victoria after the introduction of the Nile perch?

4. Give one example of co-extinction.

5. A forest of 10 km by 10 km is cut into 16 pieces. Why does the core fall so much even though the area is the same?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 13, Biodiversity and Conservation, section on the causes of biodiversity losses.
• OpenStax Biology 2e (2018), chapter 47, Conservation Biology and Biodiversity, section on threats to biodiversity.
• Campbell Biology, 12th edition (2021), Pearson, chapter 56, Conservation Biology and Restoration Ecology, section on habitat loss and invasive species.