Herman Daly reminds us that the concept of natural capital does not necessarily have to be measured in money:

it is worth clarifying that the word “capital” in its original non-monetary sense means “a stock or fund that yields a flow of useful goods or services into the future.” The word “capital” derives from “capita” meaning “heads,” referring to heads of cattle in a herd. The herd is the capital stock; the sustainable annual increase in the herd is the flow of useful goods or “income” yielded by the capital stock–all in physical, not monetary, terms. The same physical definition of natural capital applies to a forest that gives a sustainable yield of cut timber, or a fish population that yields a sustainable catch. This use of the term “natural capital” is based on the relations of physical stocks and flows, and is independent of prices and monetary valuation. Its main use has been to call attention to and oppose the unsustainable drawdown of natural capital that is falsely counted as income.

This is a long article that covers a pretty wide range of loosely related topics. But Herman Daly is always worth a read.