Aesthetic value Species and habitats may be perceived as amenities, to be valued for their beauty and potential for recreation. We can experience joy when we see a tropical landscape or a seal pup, and similarly value Central Park, Yosemite Park or the California Condor as part of our heritage. This amenity argument is not free of limitation: it can be criticized for being rather vague, brazenly anthropocentric, and too inconsistent, since aesthetic appeal is a highly subjective category and can undergo remarkable changes over time, driven by contingent cultural and economic forces. Indeed, amenity is largely associated only with "charismatic megavertebrates," rather than fungi, nematodes, or soil microorganisms. The difficulty in determining aesthetic values is evident in debate over recreational development in potential wilderness areas; but the importance of the concept remains evident.
Direct value of natural resources for humans So far, this has been basically the only way living resources have been valued on the marketplace: biomass is harvested and molded into a product that can be bought or sold, according to market laws and constraints. Resource-based systems (fisheries, forest, and agricultural lands) are basically valued according to this mechanism. In 1988, fisheries provided 100 x 10 kg of food worldwide (FAO 1988), and wild species contributed ~4.5% to the U.S. Gross Domestic Product (Prescott-Allen and Prescott-Allen 1986). The loss of such resources is the most evident and pressing aspect of biodiversity loss. From an investment perspective, biodiversity provides both realized and potential direct services. Many of today's pharmaceuticals have been discovered from the study of natural products. R.S. McCabel, President of the Herb Research Foundation, states that about one in 125 plants that are thoroughly studied yield a major new medicine (personal communication at the Biodiversity Conference, Washington, D.C., 3-4 April 1995). In contrast, only one in >10,000 chemicals synthesized in laboratories turns out to be a drug of potential benefit for humankind (Dobson 1995). Moreover, Miller and Tangley (1991) report that prescription drugs containing active ingredients from angiosperm plants contributed ~ $14 billion peryear to the U.S. economy and $40 billion per year worldwide between 1965 and 1990. F. Grifo, of the American Museum of Natural History, has shown that 118 out of the top 150 prescription drugs in the United States are derived from natural products: 74% are based on plants, 18% on fungi, 5% on bacteria, and 3% on vertebrates (Rosenthal and Grifo 1996). Nine of the 10 top prescription drugs in the United States are based on natural plant products. The World Health Organization estimates that > 80% of the world's human population relies upon traditional plant medicine for primary health care.
Indirect value through maintenance of ecosystem services Biodiversity keeps the planet habitable and its ecosystems functional. The diversity of species and their communities provides essential ecological services of many types (Freedman 1995), including nutrient cycling, biological productivity, trophic function, cleansing of water and air, control of erosion, provision of atmospheric oxygen and removal of carbon dioxide, control of the vast majority of agricultural pests and organisms that can cause disease, pollination of many crops, and "maintenance of nature's vast "genetic library," from which humanity has already drawn the very basis of civilization"(Ehrlich and Ehrlich 1991). Biodiversity is tightly intertwined with the ecosystem's ability to withstand stress and disturbance, such as drought, disease, and global warming.