Natural capital is made up of the stock of resources and living systems from which flows resource harvest and extraction, as well as the essential ecosystem services of a bioregion. Some of the resources that may be included in the natural capital of a region include timber, oil, coal, wildlife, fresh water, fruits, fish and minerals. Living systems in a region may include wetlands, prairies, coral reefs, rainforests, tundra, or the canopy of an old growth forest. Finally, ecosystem services would include the ability of nature to assimilate waste, turn sunlight into edible plants, and create oxygen.

While value has been assigned to the resources of natural capital (for example, board feet of timber and pounds of fish landings), no value has been given in traditional economic systems to the living systems and ecosystem services that are also an integral part of natural capital. Because no value has been placed on these two categories, they have been over-consumed and over-polluted. Some would argue that placing no value on these components has brought us to the verge of disaster.

Determining a value for ecosystem services is an extremely difficult task for several reasons. First, we do not completely understand, nor are necessarily aware of, the multitude of services that nature provides. Second, a service that cannot be replaced by human-made technology, such as the production of oxygen, theoretically has a limitless value. Robert Costanza and his colleagues, however, have tried to estimate a dollar value for 17 ecosystem services. Their calculations estimate that nature provides an average of US$33 trillion per year in services in 1997 dollars. For perspective, the gross national product total in 1997 was around US$18 trillion per year. The work of Costanza et al as well as many other prominent scientists around the world on determining the value of ecosystem services has been compiled in a book edited by Stanford biologist Gretchen Daily titled "Nature’s Services." The main purpose of the text is to underscore societal dependence on the environment and the economic implications of a declining ecosystem.

The decline of wetlands in California and in the Klamath-Siskiyou bioregion is an example of how both a living system and the ecosystem services it provides have been dramatically reduced because they have been undervalued. Wetlands provide valuable services such as groundwater recharge, bird and fish habitat, and reduction of flood damage and erosion. Yet California has lost 95 percent of its historic wetlands. Our ability to "re-create" these services artificially would be prohibitively expensive, if not technically impossible. Recognizing the value of wetlands and the services they provide may help preserve this essential component of our ecosystem.