Causes of Biodiversity Loss: Habitat Loss, Overexploitation

Biodiversity is declining faster than the natural rate because of human activities. Four major causes often called The Evil Quartet are habitat loss and fragmentation, overexploitation, alien species invasion, and co-extinction.

These causes work together. Clearing a forest not only removes homes but also opens the way for invasive species and overhunting, leading to chain extinctions. Tackling them is urgent for ecosystem services and human survival.

Key Concepts

Habitat loss is destruction of natural home, and fragmentation breaks it into small isolated patches. Overexploitation is harvesting species faster than they reproduce, like overfishing. Alien or invasive species are introduced organisms that outcompete natives, for example water hyacinth in lakes. Co-extinction is loss of one species causing loss of dependent species, like a pollinator and its plant. Extinction rate today is estimated by biologists to be much higher than background, but exact values depend on group studied.

How It Works

1. Forests are cleared for farms and cities, animals lose food and shelter, populations shrink.
2. Small fragmented populations inbreed and cannot find mates or migrate.
3. Hunting, logging and fishing remove adults quickly, leaving too few to breed.
4. Introduced species have no local predators, spread fast, eat natives or bring diseases.
5. When a host or mutual partner disappears, dependent species also die out.

Worked Example

A forest of 1,000 hectares is cut into 10 fragments of 100 hectares each.

Step 1: Edge effect increases. If edge zone is 10 metres deep, a square 100 hectare fragment (1,000 m x 1,000 m) has interior of 980 m x 980 m = 96.04 hectares.

Step 2: Edge area per fragment = 100 - 96.04 = 3.96 hectares.

Step 3: For 10 fragments edge = 39.6 hectares, while one intact 1,000 hectare block (about 3,162 m side) has interior about 984 hectares and edge about 16 hectares.

Fragmentation more than doubles edge exposure, increasing heat, wind and invasion risk.

Common Misconceptions

• Habitat loss only affects large animals. Insects, fungi and soil microbes lose homes too and they support the whole food web.
• Invasive species are always harmful abroad. Many introduced species stay harmless; only some become invasive, but those cause severe damage.
• Overexploitation is only hunting. It includes overfishing, overlogging and excessive collection of medicinal plants.

Real-World Applications

Protected corridors reconnect fragments so tigers and elephants can migrate. Fishing quotas and seasons prevent overexploitation. Quarantine checks at ports reduce alien species entry. In India, controlling lantana and water hyacinth restores native grasslands and wetlands. Captive breeding helps species hit by co-extinction risk.

Overall, habitat loss fragments homes, overexploitation removes species too fast, invaders outcompete natives, and co-extinction causes linked losses, making combined action necessary.