The loss of biodiversity is the most difficult problem we face.
Loss of species is permanent.
Ingenuity can replace a whale-oil lamp with an electric light bulb, but it cannot replace the whales after we hunt them to extinction. Extinction itself is a natural process. Extinctions are dangerous for humans, but it is not immediately clear just how dangerous. As time goes on, an occasional rivet will pop out. No single rivet is essential for maintaining flight, but eventually if we pop enough rivets, a crash seems certain to occur.