Rivet popper hypothesis was proposed by Stanford ecologist Paul Ehlrich. He considered aeroplane as an ecosystem and all the rivets used to join different parts as a species. If every passanger travelling in plane takes a rivet home causing extinction of a species than it will affect functioning of flight, but removal of the rivets from wings, which drives the plane will affect the functioning directly. Such species which directly affects the functioning of ecosystem are called as keystone species.