Biology · Biodiversity and Conservation · NEET
The four causes, in NCERT order, are: (i) Habitat loss and fragmentation, (ii) Over-exploitation, (iii) Alien species invasions, and (iv) Co-extinctions. These are the four major human-driven reasons that drive species to extinction. NEET repeatedly asks you to identify all four and to reject wrong add-ons like pollution, mutation, or migration.
'Quartet' means a group of four, and 'Evil' points to the harmful effect on nature. NCERT says the sobriquet (nickname) 'The Evil Quartet' is simply used to describe the four major causes of biodiversity loss. It is only a memory label, not a scientist's name. NCERT does not credit any single person for coining it, so do not attach a name to it in exams.
Habitat loss and fragmentation is the single most important cause driving animals and plants to extinction. This exact line is a favourite NEET question. The most dramatic example is the destruction of tropical rain forests, like the Amazon (called the 'lungs of the planet') being cleared for soya beans and cattle. If a question asks for THE most important cause, always pick habitat loss.
No. Pollution, climate change, mutation, drought, floods, and migration are NOT part of the Evil Quartet. NEET sets traps by inserting 'air pollution' or 'climate change' into the list. NCERT does mention that habitat degradation by pollution also threatens species, but pollution itself is not one of the four named quartet causes. Only the four: habitat loss, over-exploitation, alien species, co-extinctions.
Over-exploitation means humans directly harvest a species too much ('need' turns to 'greed') — examples are Steller's sea cow and passenger pigeon going extinct, and over-fishing of marine fish. Alien species invasion means a non-native species is introduced (on purpose or by accident) and then harms native species — examples are Nile perch in Lake Victoria (killed 200+ cichlid fish), Parthenium, Lantana, water hyacinth (Eichhornia), and African catfish Clarias gariepinus in the Yamuna.
Co-extinction means when one species becomes extinct, another species that is tied to it in an obligatory (compulsory) way also becomes extinct. Example: a host fish and its unique parasites, or a plant and its specialised pollinator/animal partner. It is different because no direct human harvesting or introduction is involved — it is a chain reaction (cascade) triggered by another extinction.
Among 'The Evil Quartet' which one is considered the most important cause driving extinction of species?
Habitat loss and fragmentation, over exploitation, alien species invasion and co-extinction are causes for:
These are regarded as major causes of biodiversity loss: A. Over exploitation, B. Co-extinction, C. Mutation, D. Habitat loss and fragmentation, E. Migration. Choose the correct option:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT uses the Nile perch introduced into Lake Victoria in East Africa as the classic case of alien species invasion. It led to the extinction of more than 200 species of unique cichlid fish. NEET 2026 set a trap saying the Nile perch 'helped population growth of cichlids' — that is false; it caused their extinction.
Habitat loss: Amazon rain forest cleared for soya beans/cattle. Over-exploitation: Steller's sea cow and passenger pigeon extinct; marine fish over-harvested. Alien species: Nile perch (Lake Victoria), Parthenium (carrot grass), Lantana, water hyacinth (Eichhornia), African catfish Clarias gariepinus (river Yamuna). Co-extinction: host fish and its obligate parasites; plant and its specialised pollinator.
Fragmentation is when a large continuous habitat is broken into small isolated fragments by human activity. It badly affects mammals and birds needing large territories, and animals with migratory habits, causing population declines. It is grouped with habitat loss as the single most important cause.
No. Mass extinction is the event/pattern (five past mass extinctions plus the ongoing sixth). The Evil Quartet is the set of four causes that drive the current, human-caused (sixth) extinction. In NEET, keep 'causes' (Evil Quartet) separate from 'events' (mass extinctions).
