Answer: Habitat loss and fragmentation, over-exploitation and alien species invasion (also co-extinction).
- Habitat loss and fragmentation: the most important cause. Clearing of forests for agriculture, industry, roads and housing destroys habitats; for example tropical rain forests once covering 14% of land now cover about 6%. Breaking large habitats into small fragments harms animals needing large territories and migratory species.
- Over-exploitation: excessive hunting, fishing and harvesting due to human greed. Steller’s sea cow and the passenger pigeon became extinct through over-exploitation; many marine fish populations are over-harvested.
- Alien species invasion: introduced species spread rapidly and eliminate native species. Nile perch introduced into Lake Victoria caused extinction of over 200 cichlid fish species; weeds like Parthenium, Lantana and water hyacinth harm native flora; African catfish threatens native catfishes.
Co-extinction (loss of a host causing loss of its dependent parasites or pollinators) and pollution are other causes.
