Which is considered the single largest cause of the loss of biodiversity?

The correct answer is A, habitat loss and fragmentation. When forests are cleared, wetlands drained and grasslands ploughed, species lose the space they need, and the fragments that survive are often too small to support large animals or to allow populations to mix and breed.

Option D, over-exploitation, is the second member of the evil quartet and covers overfishing and the hunting of animals for skin, horn or bone. Option C, invasive alien species, covers organisms brought from elsewhere that crowd out native ones, such as water hyacinth in Indian waterbodies. Option B, co-extinction, follows when a species disappears and takes with it those that depended on it, such as a parasite or a pollinator. All four together are called the evil quartet, and examiners ask both for the list and for which member stands first, so fix habitat loss as the leading cause.