Environment

The Value of Biodiversity

The direct economic value of biodiversity includes resources for our survival

 Many species have direct value as sources of food, medicine, clothing, biomass (for energy and other purposes), and shelter. Most of the world's food crops, for example, are derived from a small number of plants that were originally domesticated from wild plants in tropical and semiarid regions. As a result, many of our most important crops, such as corn, wheat, and rice, contain relatively little genetic variation  whereas their wild relatives have great diversity.
 In the future, genetic variation from wild strains of these species may be needed if we are to improve yields or find a way to breed resistance to new pests. In fact, recent agricultural breeding experiments have illustrated the value of conserving wild relatives of common crops. For example, by breeding commercial varieties of tomato with a small, oddly colored wild tomato species from the mountains of Peru, scientists were able to increase crop yields by 50%, while increasing both nutritional content and color.

 About 70% of the world's population depends directly on wild plants as their source of medicine. In addition, about 40% of the prescription and nonprescription drugs used today have active ingredients extracted from plants or animals. Aspirin, the world's most widely used drug, was first extracted from the leaves of the tropical willow, Salix alba. The rosy periwinkle from Madagascar has yielded potent drugs for combating childhood leukemia, and drugs effective in treating several forms of cancer and other diseases have been produced from the Pacific yew. 

Only recently have biologists perfected the techniques that make possible the transfer of genes from one species to another. We are just beginning to use genes obtained from other species to our advantage . So called "gene prospecting" of the genomes of plants and animals for useful genes has only begun. We have been able to examine only a minute proportion of the world's organisms to see whether any of their genes have useful properties for humans. By conserving biodiversity, we maintain the option of finding useful benefits in the future. Unfortunately, many of the most promising species occur in habitats, such as tropical rain forests, that are being destroyed at an alarming rate.

Indirect economic value is derived from ecosystem services 

Diverse biological communities are of vital importance to healthy ecosystems. They help maintain the chemical quality of natural water, buffer ecosystems against storms and drought, preserve soils and prevent loss of minerals and nutrients, moderate local and regional climate, absorb pollution, and promote the breakdown of organic wastes and the cycling of minerals. By destroying biodiversity, we are creating conditions of instability and lessened productivity and promoting desertification, water logging, mineralization, and many other undesirable out-comes throughout the world. 

The value of intact habitats

 Economists have recently been able to compare the societal value, in monetary terms, of intact habitats compared with the value of destroying those habitats. Surprisingly, in most studies conducted so far, intact ecosystems are more valuable than the products derived by destroying them. In Thailand, as one example, coastal mangrove habitats are commonly cleared so that shrimp farms can be established. Although the shrimp produced are valuable, their value is vastly outweighed by the benefits in timber, charcoal production, offshore fisheries, and storm protection provided by the mangroves. 

Similarly, intact tropical rain forest in Cameroon, West Africa, provides fruit and other forest materials. Clearing the forest for agriculture or palm plantations leads to stream-polluting erosion as well as increased flooding. Combining all the costs and benefits of the three options, maintaining intact forests has the highest economic value. 

Island Biography

One of the most reliable patterns in ecology is the observation that larger islands contain more species than do smaller islands. In 1967, Robert MacArthur of Princeton University and Edward 0. Wilson of Harvard University proposed that this species-area relationship was a result of the effect of geographic area and isolation on the likelihood of species extinction and colonization. 

The equilibrium model proposes that extinction and colonization reach a balance point 

MacArthur and Wilson reasoned that species are constantly being dispersed to islands, so islands have a tendency to accumulate more and more species. At the same time that new species are added, however, other species are lost by extinction. As the number of species on an initially empty island increases, the rate of colonization must decrease as the pool of potential colonizing species not already present on the island becomes depleted. At the same time, the rate of extinction should in-crease the more species on an island, the greater the likelihood that any given species will perish.
 As a result, at some point, the number of extinctions and colonizations should be equal, and the number of species should then remain constant. Every island of a given size, then, has a characteristic equilibrium number of species that tends to persist through time though the species composition will change as some species become extinct and new species colonize. 

MacArthur and Wilson's equilibrium model proposes that island species richness is a dynamic equilibrium between colonization and extinction. Both island size and distance from the mainland would affect colonization and extinction. We would expect smaller islands to have higher rates of extinction because their population sizes would, on average, be smaller. Also, we would expect fewer colonizers to reach islands that lie farther from the mainland. Thus, small islands far from the mainland would have the fewest species; large islands near the mainland would have the most . The predictions of this simple model bear out well in field data. Asian Pacific bird species  exhibit a positive correlation of species richness with island size, but a negative correlation of species richness with distance from the source of colonists.

The equilibrium model is still being tested 

Wilson and Dan Simberloff, then a graduate student, performed , initial studies in the mid1960s on small mangrove islands in the I Florida keys. These islands were censured, cleared of animal life by fumigation, and then allowed to recolonize, with censuses being performed at regular intervals. These and other such field studies have tended to support the equilibrium model. Long-term experimental field studies, however, are suggesting that the situation is more complicated than MacArthur and Wilson envisioned. Their model predicts a high level of species turnover as some species perish and others arrive. But studies of island birds and spiders indicate that very little turnover occurs from year to year. Those species that do come and go, moreover, comprise a subset of species that never attain high populations. A substantial proportion of the species appear to maintain high populations and rarely go extinct. 
These studies have been going on for a relatively short period of time. It is possible that over periods of centuries, the equilibrium model is a good description of what determines island species richness.