In the marginal cost and benefit functions slope in opposite directions on the assumption that the less land in conservation the more valuable conservation becomes per unit while alternative uses become fewer or less valuable so the opportunity costs fall. The result is to argue that monetary valuation will increase conservation in a world where previously there were no benefits taken into account. The under provision of land for conservation will be corrected and an optimal allocation achieved when adding one more unit of land produces less value in species/ecosystem benefits than it would cost in lost alternative development opportunities.