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.
Excellent must-read article by Herman Daly, the father of ecological economics, on the monetary valuation of ‘natural capital’