Extinction

Extinction

Extinction is the irreversible disappearance of all signs of life pertaining to a particular group of organisms. This group can be any of the accepted categories of taxonomic nomenclature, including kingdom, phylum, class, order, family, genus, species, and even subspecies. Most often, extinction is used in reference to the species level organism. The organism may have gone extinct at any time within the past 600 million years of the fossil record or may be in the process of going extinct today. Furthermore, this phenomenon does not pertain only to animals but is also applicable to plants, fungi, protists, bacteria, and archaebacteria.

Many situations can theoretically lead to the extinction of an organism. Determining the cause of extinction is very difficult, especially when information from the fossil record must be used to reconstruct a sequence of events. In many cases, a direct explanation is not forthcoming, and many hypotheses must be examined. Unfortunately, present-day cases of extinction are becoming increasingly common, leading to the need to predict and counteract the disappearance of currently living organisms. Extinction is a natural occurrence well documented in prehuman fossil deposits, and there is evidence that it may allow opportunistic evolution of species that had previously been overshadowed. Human domination of the biosphere, however, is accelerating the extinction rate. For many reasons, human lives are negatively affected by mass extinctions, and humans are struggling to oppose further loss.

Causes of Extinction

Many mechanisms account for extinction. Although organisms depend on other species in their ecosystem for food and population control, this delicate balance may be upset through the pressure of interspecies interactions. If an organism serves as the food source for another, it may be overpredated or overharvested, leading to the death of all members of the taxonomic group. This can occur if the predator/herbivore evolves keener sensory or cognitive capabilities, making it a more efficient consumer. It can also be the result of a food web disruption, such as when an environmental change suddenly favors the propagation of the predator/herbivore without offering such a benefit to the prey. Similarly, when two species occupy the same environmental niche, meaning that they prefer similar habitats and food types, they can be in direct competition for limited resources. If the competing species is better able to inhabit that niche, by gaining more territory and consuming more food, it can drive its competitors to extinction. Humans sometimes hunt animals for sport. This is not exactly predation or competition, but it can also result in the death of an entire species, for example, the dodo bird.

Environmental alteration can also lead to extinction. This may be part of a global climate change, one of Earth's natural climate fluctuations. In this case, one might expect to find a high number of global extinctions, especially among animals in regions greatly affected by the temperature and weather pattern differences. The microenvironment also undergoes change over time. For example, a new mountain range may form along a plain, an island may sink into the ocean, and a river may divert its course. Each of these examples would strongly affect the organisms that depend on those particular microenvironments for sustenance. In some cases, this local regional change can be called habitat loss. This term describes the destruction of a particular type of habitat, such as decline of biodiversity in the North American natural plains, beginning in the nineteenth century.

Evolution can also be considered a direct mode of extinction. Taxonomic boundaries are sometimes very arbitrary, meaning that the distinction between groups is unclear. The lines between groups are drawn using evidence about the organism's habitat, body shape (or morphology), and living habits; most often, however, only the morphology can be relied upon. Some species in the fossil record show gradual change in morphology over evolutionary time. Paleontologists, scientists who research extinct organisms, must sometimes examine a progression from one body type to another and determine at what point the original species can be called a new species. Nonetheless, the original species can no longer exist when the new species begins, and this in itself is a kind of extinction.

Factors that Can Contribute to Extinction

Several factors influence the likelihood that an organism will go extinct. Some species have a very small range, so that they are very susceptible to small-scale changes in the environment. This is an extremely common explanation for the extinction of subspecies. Subspecies sometimes evolve when isolated populations of a species living at the boundaries of its natural range develop distinct behaviors and appearances that distinguish them from the parent species. Given a long enough period of separation from the parent species, these subspecies would develop into an entirely novel organism. Despite this, they often are subject to extinction because their small population size and range cause them to either die out or to be reassimilated into the parent species.

A particular type of feeding pattern is another extinction factor. Whereas generalist feeders, which rely on many sources of nutrition, can switch to a different diet if their food source were to disappear, specialist feeders, which consume only one particular food source, cannot and are therefore highly susceptible to extinction.

Some organisms can be said to have a very delicate niche dynamic for a combination of the above reasons. For example, olive ridley sea turtles return to the same beach year after year to lay eggs. The eggs are highly predated by local animals, and the beaches are often brightly lit and filled with human tourists, factors that decrease the fitness of the mothers and the young. The turtles, however, seem physiologically incapable of choosing other sites for nesting because of their dependence on a particular sort of fine-grained sand and on particular temperature patterns at the chosen beaches.

Once species numbers decline to a small amount of individuals, extinction speed is increased by a factor known as inbreeding depression. This occurs when so few individuals remain in a species that they are forced to mate with members of their own families out of necessity. The smaller gene pool causes increased incidence of genetic disorders, general poor health, and increased susceptibility to disease, all of which increase the species' decline.

Difficulties Determining When an Extinction Has Occurred

Extinctions are not always easy to pinpoint or determine. When a fossil organism stops appearing in the fossil record, this may be explained by a variety of reasons. It may be that environmental conditions during the following period did not favor fossil formation, so that the species survived but was no longer fossilized. Another possibility is that only a small population of a widely dispersed species may live in the region that allows fossilization; if this local population goes extinct, the entire species will disappear from the fossil record, but that does not necessarily mean that the entire species has also gone extinct. Another explanation is that the species may have merely migrated from a fossil-friendly environment to a fossil-unfriendly habitat, or perhaps to a region for which the fossil bed has not yet been found. A confounding factor in determining extinction can be as simple as researcher bias. Organisms seem to go extinct at the boundaries between geological periods of Earth's history, but this may be due to a quirk of the field of paleontology: Paleontologists often study only one period in the rock strata. They categorize all organisms that appear at the beginning of their period, and may not realize that the scientists researching an earlier period have already named the same species. Thus although it appears that an organism has become extinct, in reality it has just been mistakenly renamed.

Paleontologists try to account for all of these sources of error through thorough study and the use of probability equations, but the fossil record is irregular and difficult to interpret. For example, in the mid-twentieth century, the coelacanth, a species of bony fish that scientists had falsely declared extinct, was rediscovered as a living species. Coelacanths were thought to have gone extinct 70 million years ago because they disappeared from the fossil record. This mistake was remedied in 1938 when a fisherman caught a living coelacanth off the coast of southeastern Africa.

Reasons for Preventing Extinctions

Although extinction is a natural occurrence, there are many reasons to actively protect existing species from extinction. More and more, ecosystems are viewed as integrated modules, so that the extinction of one species in the ecosystem can disrupt all of the other species' population dynamics. If humans ignore the extinction of a seemingly uninteresting species, this could cause widespread extinctions in many other organisms. In general, biodiversity (the presence of a high number of species in an environment) is equated with a healthy ecosystem, and high extinction rates decrease biodiversity. Often, an environment of low biodiversity reveals what humans consider to be pests. These are merely organisms that are able to survive in an ecosystem reduced and dominated by humans. People naturally prefer a healthier ecosystem, with clean breathing air, green surroundings, and a wide variety of species. There is a value to the beauty of the environment, which is damaged by high extinctions. Furthermore, humans directly depend on some species for medical benefits, such as medicine, tissue and organ donations, and animal models of human disease. Organisms that go extinct can no longer be researched as possible cures for human ailments or be studied by engineers as models for building computers, machinery, and vehicles.

Preventing extinction is a political as well as a biological priority. The needs of humans often overshadow the decline of endangered species. For example, the economy of many Third World countries depends on agriculture. If natural ecosystems such as rain forests must be destroyed to support the agricultural needs of these nations, many species are put at risk and forced into extinction. Without that source of income, however, the peoples of these countries could languish and starve. A balance between preserving the rain forest habitat and ensuring the well-being of the humans must be sought.

Methods of Preventing Extinction

There are several contemporary means of preserving a particular species. Active breeding programs at nature reserves and at zoos attempt to maintain a sizable population of individuals to avoid inbreeding depression. When animals such as cheetahs are the victims of inbreeding depression, specialized veterinarians and behaviorists monitor their health and attempt to circumvent the danger of disease. Hunting and fishing regulations attempt to predict population flux and disallow overharvesting of game animals. Specialized herbariums and eco-landscaping firms are working to restore lost habitats within depleted landscapes.

Despite these endeavors, there is still an acute danger that many of the world's habitats will be thrown into disarray by human intervention. The shrinkage and parceling of natural habitats is especially damaging to animals with large ranges, such as Siberian tigers, bald eagles, and buffalo. Nature preserves are under constant study to improve the efficiency of the surroundings in order to prevent extinction. span>

see also Feeding Strategies; Fossil Record; Habitat Loss; Habitat Restoration; Hunting; Paleontology; Zoological Parks.

Rebecca M. Steinberg

Bibliography

Courtillot, Vincent. Evolutionary Catastrophes: The Science of Mass Extinction. Cambridge, U.K.: Cambridge University Press, 1999.

Glen, William, ed. The Mass-Extinction Debates: How Science Works in a Crisis. Stanford, CA: Stanford University Press, 1994.

Hallam, Anthony, and Paul B. Wignall. Mass Extinctions and Their Aftermath. Oxford,

U.K.: Oxford University Press, 1997.

Raup, David M. Extinction: Bad Genes or Bad Luck? New York: Norton, 1992.

Schneiderman, Jill S. The Earth Around Us: Maintaining a Livable Planet. New York:

W. H. Freeman, 2000.

Snyder, Noel F., and Helen Snyder. The California Condor: A Saga of Natural History and Conservation. San Diego: Academic Press, 2000.

Weinberg, Samantha. A Fish Caught in Time: The Search for the Coelacanth. New York:

Harper Collins, 2000.

CALIFORNIA CONDOR

California condors are the largest birds in North America, weighing up to 11.5 kilograms (25 pounds), with a wingspan of more than 2.7 meters (9 feet). In 1967 condors had declined to approximately thirty individuals. In 2000, eighty-seven condors were living, but only three were in the wild. While captive breeding programs have improved the outlook for condors, their future now depends entirely on human intervention.