Causes of Biodiversity losses: The Evil Quartet

Ecologists commonly group the main human-driven causes of biodiversity loss into four categories, popularly called 'The Evil Quartet'.

Habitat loss and fragmentation is considered the single most important cause of biodiversity destruction. Large natural habitats are shrinking due to direct conversion of land as well as local degradation from pollution, creating a survival crisis for many species; loss of local habitat particularly threatens migratory birds and animals that need large territories to survive. Tropical rain forest cover, for example, has shrunk from about 14% to about 6% of its original extent over the years.

Over-exploitation reflects a key difference between humans and other animals -- the human tendency to accumulate resources far beyond immediate needs. This has driven serious over-harvesting of natural resources, threatening the survival of many organisms, and can be linked, for instance, to today's growing scarcity of fish stocks. The Dodo bird, Steller's sea cow and the passenger pigeon (Fig. 15.3) are classic historical examples of species driven to extinction through over-exploitation.

Alien species invasion occurs when a new species is introduced into an ecosystem, whether accidentally or deliberately, and goes on to harm the species already living there -- sometimes even driving local species to extinction; such a species is then called an invasive species. Familiar Indian examples include carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia). Introducing the predatory Nile perch into Lake Victoria proved catastrophic for around 200 native species of cichlid fish, and in India the introduction of the African catfish Clarias gariepinus for aquaculture has harmed native catfish varieties -- in both cases, one major reason for the damage is the absence of a natural local predator to keep the invader in check. …