Economic value and biological conservation

Mark Sagoff writes in Bioscience (The Economic Value of Ecosystem Services):

The concept of economic value presents a dilemma. If conservationists refer to total value, they must concentrate on just those [ecosystem] service providing units that are in jeopardy. It serves no purpose to “valuate” services that are not threatened. If conservationists refer to marginal value, however, they tie themselves to the familiar conceptual framework of market failure, externalities, common pool resources, discounting, transaction costs, and so on. Conservationists then go down a long and weary road, at the end of which they will find mainstream environmental economists waiting for them.