Q.Briefly describe the reasons for decline of bio diversity.

Global biodiversity is declining due to a combination of human-driven pressures, chief among them habitat destruction, over-exploitation, invasive species, and the knock-on effect of co-extinctions.

Major causes of biodiversity decline

1. Habitat loss and fragmentation -- considered the single most important cause; large-scale deforestation, wetland drainage, urbanisation, and conversion of natural land to agriculture and infrastructure destroy habitats outright or break them into small, isolated fragments that cannot support viable populations (e.g., loss of tropical rainforests, which cover ~2% of Earth's surface yet harbour more than 50% of its species).
2. Over-exploitation -- unsustainable hunting, fishing, and harvesting of wild species for food, trade, or sport has driven many species to extinction or near-extinction; historic examples include the passenger pigeon and Steller's sea cow, and ongoing threats include overfishing of marine stocks and poaching of large mammals.
3. Alien/invasive species introduction -- species introduced deliberately or accidentally into new regions (e.g., water hyacinth, Nile perch, lantana) often lack natural predators there, and outcompete, prey upon, hybridise with, or transmit disease to native species, sometimes causing local extinctions.
4. Co-extinctions -- when a species becomes extinct, other species intimately associated with it in an obligatory way (a specific host, prey, pollinator, or symbiont) may also become extinct, amplifying biodiversity loss beyond the initially affected species. …