How do scientists determine whether a species is endangered or extinct?

A very small mammal or insect endemic to a particular area might be considered endangered while several million are still known to be alive, but a lesser number of another species, spread out over a broad area, would not give rise to concern.

Determining whether or not a species is endangered is a complex task for which no fixed criteria exist to be applied across the board. Just as the species and subspecies of animals and plants are multitudinous, so are the variables that enter into an evaluation of their status.

In the small scale example, the nature of the habitat is significant, for if the specific area were destroyed by pollution, or a natural disaster, all existing members of the species would go with it.

Just such a situation surrounds a particular subspecies of wren endemic to San Clemente Island off the coast of California. Just prior to World War II, farmers and ranchers brought goats and pigs to raise on the small island, which stretches 13 miles and is only 1 to 3 miles wide.

During the war, the Navy used the island as a bombardment range to test artillery, which naturally destroyed some vegetation. In addition, the goats and pigs multiplied so rapidly and devoured so much of the grass and lower vegetation that by 1955 the wrens and several other species of birds were in desperate need of shelter. With the plants that used to provide a cover gone, the wrens became endangered, and finally extinct, for no one has seen one since 1960. Five or six species of plants are also severely threatened.

The fecundity of a species is another factor weighed in determining whether or not it is endangered: how frequently its members reproduce, what percentage of newborn members generally survive, how many offspring are produced at a time, and so on. Average life span and rate of reproduction are especially important considerations when authorities decide how long to wait after an animal’s or plant’s last sighting before calling it extinct.

Although a decision might be arrived at after only a year in the case of, say, fruit flies, a 50 to 60 year wait might be justified in the case of a species of condor with an average life span of 30 years.

In the United States the director of the Fish and Wildlife Service in the Department of the Interior determines which animal species to call endangered, based on the most up to date and thorough scientific and commercial data available. Those data come from specialized biologists, botanists, and naturalists working in the field, who submit their findings to Washington. According to the Endangered Species Act of 1973, a species can be listed if it is threatened by any of the following:

1. The present or threatened destruction, modification, or curtailment of its habitat or range;

2. Utilization for commercial, sporting, scientific, or educational purposes at levels that detrimentally affect it;

3. Disease or predation;

4. Absence of regulatory mechanisms adequate to prevent the decline of a species or degradation of its habitat; and

5. Other natural or man made factors affecting its continued existence.

If the species does appear to be threatened, the director immediately determines the “critical habitat”, the areas inhabited by the species that contain physical or biological features essential to the conservation of the species and that may require special management programs. “Critical habitat” may also refer to areas outside those occupied by the species which the director sees as necessary for protection of the species.

He focuses on preserving key elements such as feeding sites, nesting grounds, water supplies, necessary vegetation, and soil types, attempting to nourish the species to the point where it can be removed from the endangered list.