Endangered Species, Measuring

Endangered Species, Measuring

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The U.S. Endangered Species Act (ESA) is one of the most important and controversial legislative acts in recent years. This law requires the use of specific measures to protect certain species of plants or animals that are listed as threatened species or endangered species . Before a species is listed as threatened or endangered, biologists must determine if a viable population of the plant or animal in question exists in the wild. This usually means determining the number of existing individuals, the sex of each, the number within breeding age, the breeding success rate, mortality rates, birth rates, whether sufficient genetic diversity exists, and many other factors. To answer these questions, the number of plants or animals must be counted.

Study Methods Used to Estimate Population Size

Study methods include observation and photography, live trapping, and transect sampling. All of the methods result in an estimate of the number of individuals in the population. This number is then compared with what is considered a minimum viable population, which is the smallest number of individuals of a species in a particular area that can survive and maintain genetic diversity.

Observation and Photography. These are the simplest study methods. Observation and photography involve going into an area known to contain individuals from the population in question and simply counting how many can be found. This method works well for species that are not very mobile. For example, there is a species of snail that inhabits a small area on one mountain slope. This area is so small that one person can count the number of individuals present. In other cases, the number of individuals is so small that the entire population can be counted. Whooping cranes exist in such small numbers that the entire population is known from yearly censuses made on both the cranes's wintering grounds and their breeding grounds. Whooping cranes can be counted from observation towers, or they can be photographed from low flying airplanes or helicopters.

Observation can involve senses other than vision. Estimates of bird populations are often made by listening to singing males. The songs are distinctive enough that the species can be identified from their song. Knowledge of their breeding success, the number of offspring per pair, and mortality rates can then be used to determine if the population is viable.

Live Trapping. Sometimes population estimates are made by placing traps in the area being studied. The number of animals captured is then related back to the total number of animals in the area. Live trapping generally does not harm the animals, and it has the advantage of allowing the researcher to gather other information about the species, such as age, sex, and health. Individuals can also be marked so that their movements can be followed. Birds trapped in this way are commonly banded. Bird banding has given biologists and wildlife managers extensive information about bird migration patterns.

Transect Sampling. Transect sampling is a standard statistical technique for determining the population in an area. The researcher or surveyor walks along a straight line (called a transect) through the area of interest and counts every individual that can be seen. Alternatively, the researcher may observe other evidence of the presence of an animal (such as droppings). Under ideal conditions, the number of individuals observed within the transect area has the same proportion to the total number of individuals within the total area.

Suppose a surveyor walks across the area A along the transect L. At some time, the surveyor will be at point Z, and may see an individual at X. The width of the strip that can be seen by the surveyor is 2W, and the distance of the animal or plant from the surveyor is r_i. The angle from the transect line to the observed animal or plant is θ_i. The perpendicular distance from the transect line to the observed individual is y_i. Note that y_i = r_i sin θ_i.

The mathematical model for estimating the size of a population from transect data depends on two assumptions: (1) not all individuals will be detected and (2) the probability of detecting an individual decreases as its perpendicular distance from the transect line increases. These two assumptions are generally expressed as a detection function, g (y ), which represents the probability that an individual will be observed at a distance x from the transect line. The function g (y ) decreases as y increases. Generally, estimating detection functions requires calculus. However, the result is an effective width, a, of the transect area that is different from the actual width. Once a is known, the population density is given by where n is the number of individuals observed, and L is the length of the transect. The basic problem in estimating population density is estimating the parameter a. The parameter a can be estimated accurately by choosing an appropriate probability density function . Division by a makes the probability density function equal to one, which is what is expected of probability functions. Candidate choices for f (y ) include Fourier series , exponential power series , and negative exponentials .

The critical assumption permitting estimation from distance data is that all objects located directly on the line (distance = 0) are certain to be detected, so g (0) = 1. If g (0) = 1, then . The equation for estimating population density can now be rewritten in terms of . The estimator function f (0) can usually be determined by trial and error or by one of several different mathematical models available. Wildlife managers commonly use computer programs that have the various mathematical models and estimator functions included.

Determining if a Species Has a Minimum Viable Population

Once the size of the population has been estimated, researchers must then decide if the population is healthy and can survive on its own, or if it is too small to be viable and requires protection. There are various methods of estimating viability of a population.

One method often used by biologists to estimate the viability of a population of vertebrates is the 50/500 rule. The minimum number of individuals in a breeding population required to prevent an unacceptable level of inbreeding is 50. The number of breeding individuals required for the long-term genetic variability necessary for a healthy population is 500. This rule is established by assuming that a mature male and a mature female are randomly drawn and randomly paired. However, in many populations, one male may dominate a large group of females, excluding other males from the genetic pool. In this and similar cases, a larger population is needed for viability.

Since not all individuals in a population are active breeders, the census population must be at least twice as large as the breeding population. Many researchers use a census population of 1,000 to 10,000 individuals as the minimum necessary for long-term genetic viability.

Other Factors Affecting Minimum Viable Population. When rules such as the 50/500 method are not applicable or when greater precision is desired, one of several analytic approaches to calculating the Minumum Viable Population may be used. One analytic factor to be considered is the sex ratio. If the percentage of males is 50 percent, then the sex ratio is not skewed. However, in many populations the fraction of males can be larger or smaller than 50 percent. In these groups, larger census populations are necessary.

Another factor affecting the size of the census population is the average number of offspring. If a certain species typically produces many offspring, the census population can increase quite rapidly. Relative birth and death rates can also be used to predict population increase or decrease.

Simulations. None of the preceding approaches has been completely satisfactory in estimating populations and as a result, simulations have been suggested as an alternative. Simulations are computer programs that attempt to model population dynamics. Some are applicable to many different populations, while others have been developed for specific situations, such as a program to estimate the population of the grizzly bears of Yellowstone National Park.

The Endangered Species Act

The ultimate goal of counting specific species may be to determine if they should be listed as threatened or endangered. The first endangered species legislation was enacted in 1966. This act established a list of animals that were threatened with extinction. However, federal agencies were limited in their ability to protect these species. The most significant part of this first act was the establishment of the National Wildlife Refuge System, which was designed to protect the habitats of endangered species.

In 1969 the U.S. Congress passed the Endangered Species Conservation Act. This act included invertebrates as well as vertebrates. It also restricted interstate commerce in illegally taken animals. In 1973 the Endangered Species Act (ESA) was passed. The ESA has been called the most comprehensive legislation for the preservation of endangered species ever enacted by any nation. Since 1973, the ESA has been reauthorized and amended twice.

Under the ESA, two departments have the sole authority to list species: The National Marine Fisheries Service is authorized to list marine mammals and fish, and the U.S. Fish and Wildlife Service lists all other species. This division of responsibility between the U.S. Department of Commerce and the U.S. Department of the Interior has caused some difficulties in the consistent application of rules and criteria for listing species. The act was amended in 1982 to state that listing should be based solely on biological criteria.

The ESA identifies two different population conditions related to the viability of species. "Threatened" species are those whose populations are still viable, but may be declining or have a limited habitat, and therefore be in danger of becoming threatened. "Endangered" species have populations that are too small to be considered viable and thus are in danger of becoming extinct.

see also Statistical Analysis.

Elliot Richmond

Bibliography

Norton, B. G. Why Preserve Natural Variety? Princeton, NJ: Princeton University Press, 1987.

Robinson, M. H. "Global Change, the Future of Biodiversity, and the Future of Zoos." Biotropica 24 (1992): 345–352.

Rohlf, D. J. The Endangered Species Act: A Guide to Its Protections and Implementation. Stanford, CT: Stanford Environmental Law Society, 1989.

Tarpy, C. "Zoos: Taking Down the Bars." National Geographic 184 (July 1993): 237.

U.S. Office of Technology Assessment. Technologies to Maintain Biological Diversity. U.S. Government Printing Office, Washington, DC, 1987.

Internet Resources

U.S. Fish and Wildlife Service. "Box Score of Endangered Species." Endangered Species Home Page, 2000. <http://endangered.fws.gov/wildlife.html>.