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species value populations habitat

It was once customary to consider all undomesticated species of vertebrate animals as wildlife. Birds and mammals still receive the greatest public interest and concern, consistently higher than those expressed for reptiles and amphibians. Most concern over fishes results from interest in sport and commercial value. The tendency in recent years has been to include more life-forms under the category of wildlife. Thus, mollusks, insects, and plants are all now represented on national and international lists of threatened and endangered species.

People find many reasons to value wildlife. Virtually everyone appreciates the aesthetic value of natural beauty or artistic appeal present in animal life. Giant pandas, bald eagles, and infant harp seals are familiar examples of wildlife with outstanding aesthetic value. Wild species offer recreational value, the most common examples of which are sport hunting and bird watching.

Less obvious, perhaps, is ecological value, resulting from the role an individual species plays within an ecosystem. Alligators, for example, create depressions in swamps and marshes. During periods of droughts, these "alligator holes" offer critical refuge to water-dependent life-forms. Educational and scientific values are those that serve in teaching and learning about biology and scientific principles.

Wildlife also has utilitarian value which results from its practical uses. Examples of utilitarian value range from genetic reservoirs for crop and livestock improvement to diverse biomedical and pharmaceutical uses. A related category, commercial value, includes such familiar examples as the sale of furs and hunting leases.

Shifts in human lifestyle have been accompanied by changes in attitudes toward wildlife. Societies of hunter-gatherers depend directly on wild species for food, as many plains Indian tribes did on the bison. But as people shift from hunting and gathering to agriculture, wildlife comes to be viewed as more of a threat because of potential crop or livestock damage. In modern developed nations, people's lives are based less on rural ways of life and more on business and industry in cities. Urbanites rarely if ever feel threatened economically by wild animals. They have the leisure time and mobility to visit wildlife refuges or parks, where they appreciate seeing native wildlife as a unique, aesthetic experience. They also sense that wildlife is in decline and therefore favor greater protection.

The most obvious threat to wildlife is that of direct exploitation, often related to commercial use. Exploitation helped bring about the extinction of the passenger pigeon (Ectopistes migratorius), the great auk, Stellar's sea cow, and the sea mink, as well as the near extinction of the American bison. In the late nineteenth and early twentieth century, state and federal laws were passed to help curb exploitation. These were successful for the most part, and they continue to play a crucial role in wildlife management.

Introductions of exotic species represent another threat to wildlife. Insular or island-dwelling species of wildlife are especially vulnerable to the impacts of exotic plants and animals. Beginning in the seventeenth century, sailors deliberately placed goats and pigs on ocean islands, intending to use their descendants as food on future voyages. As the exotic populations grew, the native vegetation proved unable to cope, creating drastic habitat changes. Other species, such as rats, mice, and cats, jumped ship and devastated island-dwelling birds, which had evolved in the absence of mammalian predators and had few or no defenses.

Pollution is yet another threat to wildlife. Bald eagles, ospreys, peregrine falcons (Falco peregrinus), and brown pelicans (Pelecanus occidentalis) experienced serious and sudden population declines in the 1950s and 60s. Studies showed that these fish eaters were ingesting heavy doses of pesticides, including DDT. The pesticides left the shells of their eggs so thin that they cracked under the weight of incubating parents, and numbers declined due to reproductive failure. Populations of these birds in the United States rebounded after regulatory laws curbed the use of these pesticides. However, thousands of other chemicals still enter the air, water, and soil every year, and the effects of most of them on wildlife are unknown.

By far the most critical threat to wildlife is habitat alteration. Unfortunately, it is also more subtle than direct exploitation, and thus often escapes public attention. As the twenty-first century approaches, human activities are altering some of the most biologically rich habitats in the world on a scale unprecedented in history. Tropical rainforests, for example, originally covered only about 7% of the earth's land surface, yet they are thought to contain half the planet's wild species. Other rich habitats undergoing rapid changes include tropical dry forests and coral reefs. As extensive areas of natural habitat are irrevocably changed, many of the native species that once occurred there will become extinct, even those with no commercial value.

Any species of wild animal has a set of habitat requirements. These begin with food requirements, adequate amounts of available food for each season. Cover requirements are structural components that are used for nesting, roosting, or watching, or that offer protection from severe weather or predators. Water is habitat requirement that affects wildlife directly, by providing drinking water, and indirectly, by influencing local vegetation. The final habitat requirement is space. Biologists can now calculate a minimum area requirement to sustain a particular species of a given size.

Even in the absence of human activities, populations of wild animals change as a result of variations in birth and death rates. When a population is sparse relative to the number that can be supported by local habitat conditions, birth rates tend to be high. In such circumstances, natural mortality, including predation, disease, and starvation, tends to be low. As populations increase, birth rates decline and death rates rise. These trends continue until the population reaches carrying capacity, the number of animals of a particular species that can be sustained within a given area.

Carrying capacity, though, is difficult to define in practice. Variations in winter severity or in summer rainfall can, between years, alter the carrying capacity for a particular area. In addition, carrying capacity changes as forests grows older, grasslands mature, or wetlands fill in through natural siltation. Despite these limitations, the concept of carrying capacity illustrates an important biological principle: living wild animals cannot be stockpiled beyond the practical limits that local habitat conditions can support.

While all populations vary, some undergo extreme fluctuations. When they occur regularly, such fluctuation are called cycles. Cyclical populations fall into two categories, the three- to four-year cycle typical of lemmings and voles, and the eight- to 11-year cycle known in snowshoe hares and lynx of the Western Hemisphere. The mechanisms that keep cycles going are complex and not completely understood, but the existence of cycles is widely accepted. Extreme populations fluctuations that occur at irregular intervals are called population irruptions. Local populations of deer tend to be irruptive, suddenly showing substantial changes at unpredictable intervals.

There are more species of wild plants and animals in tropical rain forests than on arctic tundras. Such patterns illustrate variations in the complexities of life-forms due to climatic conditions. Measurement of biodiversity usually focuses on a particular group of organisms such as trees or birds. The most basic indication of species richness is the total number of species in a certain area or habitat. A measure of species evenness is more valuable because it indicates the relative abundance of each species. As diversity includes richness and evenness, an area with many species uniformly distributed would have a high overall diversity.

Biodiversity is important to wildlife conservation as well as to basic ecology. Comparisons of species diversity patterns indicate the extent to which natural conditions have been affected by human activities. They also help establish priorities for acquiring new protected areas.



Caughley, G. Analysis of Vertebrate Populations. New York: John Wiley & Sons, 1977.

Kellert, S. Trends in Animal Use and Perception in 20th Century America. Washington, DC: U.S. Department of the Interior, Fish and Wildlife Service, 1981.

Matthiessen, P. Wildlife in America. 2nd ed. New York: Viking Books, 1987.

McCullough, D. The George Reserve Deer Herd. Ann Arbor, MI: University of Michigan Press, 1979.

James H. Shaw

Wildlife Trade (Illegal) - The Trade In Wildlife, Monitoring And Regulating The International Trade In Endangered Species [next] [back] Wildlife - Changing Conceptions Of Wildlife In Darwin's Century, Wildlife In The Twentieth Century, Wildlife In The New Millennium

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