Loss of biodiversity and the evil quartet
Human activity is driving extinctions far faster than natural rates. The four main causes, the evil quartet, are habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinctions.
Extinctions today
The world's biodiversity is shrinking, mainly because of human activities. The IUCN Red List (2004) records the extinction of 784 species in the last 500 years: 338 vertebrates, 359 invertebrates and 87 plants. Recent extinctions include the dodo (Mauritius), quagga (Africa), thylacine (Australia), Steller's sea cow (Russia) and three subspecies of tiger (Bali, Javan and Caspian). Amphibians seem especially vulnerable. Earth has had five mass extinctions before. The present sixth extinction is different: species are disappearing 100 to 1,000 times faster than in pre-human times, and humans are the cause. Ecologists warn that if this continues, nearly half of all species may be lost within the next 100 years.
Effects of biodiversity loss in a region
- A decline in plant production.
- Lowered resistance to environmental disturbances such as drought.
- Increased variability in ecosystem processes such as plant productivity, water use, and pest and disease cycles.
The evil quartet: causes of biodiversity loss
| Cause | What happens | Example |
|---|---|---|
| Habitat loss and fragmentation (the most important cause) | Habitats are destroyed, polluted or broken into small pieces. Animals that need large territories and migratory animals suffer most. | Tropical rain forests once covered more than 14% of Earth's land; now no more than 6%. The Amazon, called the 'lungs of the planet', is cleared for soya beans and for cattle ranching. |
| Over-exploitation | 'Need' turns into 'greed', and a species is harvested faster than it can recover. | Steller's sea cow and the passenger pigeon were hunted to extinction; many marine fish are over-harvested. |
| Alien species invasions | An introduced species turns invasive and pushes out native species. | The Nile perch in Lake Victoria wiped out more than 200 species of cichlid fish. In India: Parthenium (carrot grass), Lantana and water hyacinth (Eichhornia); the illegally introduced African catfish (Clarias gariepinus) threatens native catfishes. |
| Co-extinctions | When a species dies out, species tied to it in an obligatory way die out too. | The unique parasites of a host fish die with it; in a co-evolved plant-pollinator mutualism, losing one dooms the other. |
Habitat loss vs habitat fragmentation
Both come from human activity, and together they are the single most important cause of extinction.
Water hyacinth chokes many Indian lakes and backwaters, and Lantana has spread through forests from the Western Ghats to the Himalayan foothills. Highways cutting through forests fragment the ranges of elephants and tigers.
2 marks: “Name the four major causes of biodiversity loss, called the evil quartet.” Habitat loss and fragmentation; over-exploitation; alien species invasions; co-extinctions.
3 marks: “Explain alien species invasion and co-extinction with one example each.” Alien invasion: an introduced species turns invasive and eliminates natives, e.g. the Nile perch wiped out over 200 cichlids in Lake Victoria (or Parthenium, Lantana, Eichhornia in India). Co-extinction: obligate partners die together, e.g. a host fish and its unique parasites, or a plant and its co-evolved pollinator.
5 marks: “Describe the evil quartet with examples. Which is the most important cause?” Explain each cause with an example. Habitat loss and fragmentation is the most important: rain forests fell from over 14% to no more than 6% of land; the Amazon is cleared for soya beans and cattle.
The evil quartet is HOAC: Habitat loss, Over-exploitation, Alien invaders, Co-extinctions.