Q.What could have triggered mass extinctions of species in the past?

Species don't vanish for one reason — the NCERT chapter groups the drivers behind today's accelerated extinctions into what it nicknames the Evil Quartet, and each one has a real, memorable example behind it.

Habitat loss and fragmentation is, in the chapter's own words, the single most important cause. Tropical rainforests once covered more than 14% of Earth's land surface; today they cover no more than 6% — cleared for soya-bean farms and cattle grazing in places like the Amazon. It isn't just that the trees disappear: when a forest is cut into scattered patches, large-territory and migratory animals lose the continuous range they need, even if some green cover technically remains.

Over-exploitation is what happens when a genuine need turns into greed. Two textbook casualties: Steller's sea cow, hunted to extinction within 27 years of its discovery, and the passenger pigeon — once billions strong in North America, gone by the early 1900s from relentless hunting. Today the same pattern plays out in marine fish stocks pushed toward collapse.

Alien species invasion is what happens when a species arrives somewhere it never evolved to be, with no natural checks on it. The chapter's flagship case: the Nile perch, introduced into Lake Victoria, single-handedly drove more than 200 endemic species of cichlid fish to extinction. Closer to home, weeds like Parthenium (carrot grass), Lantana, and the water hyacinth Eichhornia have overrun Indian ecosystems, and the African catfish Clarias gariepinus, introduced for aquaculture, now threatens native Indian catfish. …