In the twentieth century, our capacity to create substitutes grew immensely. Many synthetic products were invented to take the place of natural ones. Declining soil nutrients could be replaced with artificial fertilizer; aluminum could replace copper; plastic could replace just about everything — wood, stone, metal, glass. Nuclear power appeared poised to offer cheap, near-limitless energy supplies in place of fossil fuels extracted from the earth.

These advances gave rise to a way of thinking that we might call “substitution optimism”: the belief that humans can find substitutes for anything that nature does. But substitution optimists tend to neglect two problems. First, the development of substitutes assumes that the price of scarce goods will rise. What about scarce goods that don’t have a price? In particular, what about the services freely provided by nature? The services of atmospheric cycles and pollution-absorbing forests cost nothing — which mean that as they grow scarcer they do not get more expensive, and do not spur the development of technological replacements. Today, those resources — what we might think of as the earth’s reproductive rather than productive functions — are the ones most under threat. Like human reproductive work, they operate in the background of economic production, providing the basic functions necessary for life.

But it’s also an open question as to whether those kinds of resources actually have substitutes. Plastic chairs can substitute for wooden ones, or plastic bags for paper — but can you build a substitute for an entire forest? Can human technologies or human labor substitute for the nonhuman work done by other organisms? Or are there certain kinds of work that only nature can do?