In economic terms, resources (or capital) are regarded as actual or potential wealth that can be applied toward the creation of additional wealth. There are three broad types of capital. First, manufactured capital is industrial infrastructure that can be applied to the production of goods and services. Examples include factories, mines, harvesting equipment, buildings, tools and machinery, computers, and information networks. Second is human capital, or the cultural means of production, encompassing a workforce with particular types of knowledge and skills. And third, natural capital refers to quantities of raw, natural resources that can be harvested, processed, used in manufacturing, and otherwise utilized to produce goods and services for an economy.
There are two types of natural resources: non-renewable and renewable. Non-renewable resources are present in a finite quantity on Earth. Therefore, their stock diminishes as they are mined from the environment. Nonrenewable resources can only be used in an non-sustainable manner. The lifetime of a non-renewable resource is determined by the size of its recoverable stocks in the environment, and the rate of mining. However, some nonrenewable resources can be reused and recycled to some degree, which extends the effective lifetime of the resource. Common examples of non-renewable resources include metal ores, coal, and petroleum.
Potentially, renewable resources can be sustained and harvested indefinitely. However, sustainable use requires that the rate of harvesting does not exceed the rate of renewal of the resource. Most renewable resources are biological and include trees, hunted animals such as fish, waterfowl, and deer, and the products of agriculture. Flowing surface water is an example of a non-biological resource that can potentially be sustainably used for irrigation, to generate hydroelectricity, and as a means of transportation.
It is important to recognize that potentially renewable resources can easily be "over-harvested," or exploited at a rate exceeding that of renewal, resulting in degradation of the resource. During over-harvesting, the resource is essentially being "mined"—that is, it is managed in the same way as a non-renewable resource. Regrettably, this is all too often the case, resulting in collapses of stocks of hunted fish, mammals, and birds; deforestation; declines of agricultural soil capability; and diminished river flows due to excessive withdrawals for use by humans.
Another important characteristic of renewable resources is that they can provide meaningful ecological services even when they are in their natural, unharvested state. For example, intact, natural forests provide biological productivity; cycling of nutrients and water; a sink for atmospheric carbon; control of erosion; cleansing of pollutants emitted into the environment by humans; habitat for diverse elements of biodiversity; aesthetics; and other important ecological services. Some of these services are of potential value in providing resources that humans require, an example being the biomass and productivity of trees and hunted animals. However, most of these are not recognized by the conventional marketplace, although they are certainly important to ecological integrity and environmental health.
The undeniable ecological reality is that humans have an absolute dependence on a continuous flow of natural resources to sustain their societies and economies. Over the longer term, this is particularly true of renewable resources because sustainable economies cannot be supported only by non-renewable resources. Therefore, the only way to achieve a condition of sustainable development is to build an economy that is supported by the wise harvesting and management of renewable resources.