The main reason for the decrease in biodiversity is

Natural habitat destruction is the main reason, and it is the single largest driver of biodiversity loss in the IUCN and Millennium Ecosystem Assessment frameworks. When forests are cleared, wetlands drained or grasslands converted to farms and cities, the species living there lose food, shelter and breeding grounds together. Most extinctions trace back to this.

• Habitat pollution is a real stress, but it is usually a contributing factor and narrower in scope than habitat loss.
• Introduction of exotic species is a serious threat on islands, such as invasive species in the Galapagos or Australia, but it is a regional driver rather than the global main cause.
• Over exploitation (overhunting, overfishing) is significant for whales and some fish stocks, but it ranks below habitat loss in global assessments.

Core concept: the HIPPO framework (Habitat destruction, Invasive species, Pollution, Population growth, Overharvesting), linked to the Convention on Biological Diversity, 1992. Mains angle: forest cover loss, land-use change and the Forest (Conservation) Act, 1980, which was amended in 2023.

Trap: aspirants pick over exploitation because it sounds most human-driven. Exploitation kills individuals, but habitat destruction removes the whole ecosystem they depend on.