Some people object to the concept of “natural capital” because they say it reduces nature to the status of a commodity to be marketed at its exchange value. This indeed is a danger, well discussed by George Monbiot. Monbiot’s criticism rightly focuses on the monetary pricing of natural capital. But 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.
Big problems certainly arise when we consider natural capital as expressible as a sum of money (financial capital), and then take money in the bank growing at the interest rate as the standard by which to judge whether the value of natural capital is growing fast enough, and then, following the rules of present value maximization, liquidate populations growing slower than the interest rate and replace them with faster growing ones. This is not how the ecosystem works. Money is fungible, natural stocks are not; money has no physical dimension, natural populations do. Exchanges of matter and energy among parts of the ecosystem have an objective ecological basis. They are not governed by prices based on subjective human preferences in the market.
Furthermore, money in the bank is a stock that yields a flow of new money (interest) all by itself without diminishing itself, and without the aid of other flows. Can a herd of cattle yield a flow of additional cattle all by itself, and without diminishing itself? Certainly not. The existing stock of cattle transforms a resource flow of grass and water into new cattle faster than old cattle die. And the grass requires sunlight, soil, air, and more water. Like cattle, capital transforms resource flows into products and wastes, obeying the laws of thermodynamics. Capital is not a magic substance that grows by creating something out of nothing.
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