 Holdren and Ehrlich (1974) argued that loss of a species or loss of genetic diversity can mean loss of potential uses (medicines, foods etc.), but also referred to the maintenance of the “public service” functions of natural ecosystems. Ehrenfeld (1970) similarly distinguished between the within ecosystem functioning/stability argument and the potential uses or option value argument. Ehrlich and Ehrlich (1981), in their book Extinction discussed the insurance value of the Earth’s “biological diversity” through the analogy of popping rivets off an airplane wing—we strive to keep all the rivets, because we do not know how many could be lost before the wing no longer functions.