Jay Williams1 tells about an old woman who was living out the last days of her life. Surrounded by white walls, upon a white bed, in care of doctors and nurses, this dark-skinned relict fought off death with all her primitive vitality. She rebuked her attendants and intermittently broke forth in song and chants. But inevitably she collapsed onto her pillows and whispered, "Bury me behind the mountains." And so she died, but her skeleton was placed instead in a city museum, for she was the last of her kin. With her passing, the Tasmanian people became extinct.
Extinction is like that. It is the absolute terminus for a formerly recognized group of organisms. When mortality exceeds natality for a sufficient time to bring the total number of individuals of a species to zero or one (for those organisms which reproduce sexually), then extinction is pronounced.2
Since life began, many organisms have been lost from the biosphere through extinction. Some people feel that this is a normal expectation of life--a parade of passing species. It is to be expected that new "species" and higher taxa (levels of classification) will progressively evolve to displace existing or waning groups. On the other hand, some feel that this happening represents a net loss to the total biota. This latter group says no new "kinds" are being formed. Consequently, when populations of unique organisms die out, that genome is gone forever.
If extinction is part of the natural order, why save endangered species? The money and effort put into protecting, restoring, and sustaining a few minor species of plants and animals becomes a questionable national effort. Indeed, if we are a part of the evolving cosmos, we should look forward to the new order and help the evolutionary process! When we stop the parade, we slow down evolution (i.e., we are keeping critical niches occupied too long). The evolutionist requires extinction; it is like throwing out the slum dwellers for urban renewal.
Congress addressed these issues, and in their wisdom they concluded that endangered and threatened species of fish, wildlife and plants "are of aesthetic, ecological, educational, historical, and scientific value to the nation and its people."3 Perhaps protecting organisms is good stewardship of finite resources. Whatever one's view of the matter, the observation remains true that many more species are being lost in our time than are being formed, and this pace appears to be accelerating.
Extinction can occur as a result of any of three principal pressures: mortality/natality imbalance, loss of critical habitat, and minimum population density. Three examples are cited: Dodos in historical times were found only on a few small islands in the Indian Ocean. When man wiped them out of their tiny refuges, no replacements were available to rekindle the species--mortality exceeded natality. A pair of ivory-billed woodpeckers is said to require a territory of 5 km2 of primeval bottomland forest. A viable population probably needed 160 km2. Much of this type of appropriate habitat has gone and, to our knowledge, so has the bird. Theoretical limits appear to exist for some species in that when the numbers dwindle below that limit, genetic variability is lost to such an extent that ultimate extinction is assured. There is a failure to mate for social reasons below a certain population density. The whooping crane and California condor may have passed below this level--certainly the passenger pigeon did.
The record of extinctions in the United States is startling. Since the arrival of the settlers in America in the early 1600s, over 500 species and subspecies of native biota have passed on. Another 170 animals are designated "endangered" today. The rate of loss roughly follows the national population increase.
Paleontologically speaking, there are two chief classes of extinctions: those for which the paleontologist has a satisfactory explanation, and those for which he has none.4 There are extinctions in historical times which are due to depredations of man or stronger animals upon weaker ones. Before those times in the distant past there were even greater exterminations which apparently affected organisms globally but do not have obvious causes. In the geological record these are associated with the late Cambrian, Ordovician, Devonian, Permian and Cretaceous strata. Since these latter events were worldwide, they must have had worldwide causes. Two explanations can be offered: either catastrophes or subtle and insidious global reactions.
Newell5 says that man was the exterminator in historical times. A few species of large mammals dropped out of the fossil record in North America at the height of the last glaciation, and some of these (perhaps the giant sloth and saber-toothed cat in California) may have been due to over-hunting by man. The rate of extinction picked up rapidly when the climate became milder and glaciers began shrinking. Many large herbivores and carnivores existed worldwide through a great range in climate, yet they died out in only a few hundred years. In recent times about 75 percent of the North American herbivores have disappeared, and most of the ecological niches have not yet been filled. Glaciation was apparently not important for these extinctions.
Some investigators have suggested that the large mammals may have been hunted out of existence by pre-historic man using fire as a weapon. The wipe-out coincided with rapid growth in agriculture.
Correlating extinctions to ancient earth's history has been a scientific mystery according to Baker and Allen.6 The fossil record contains mass extinctions in many layers. As an example of mass extinction, two-thirds of the trilobite families disappeared at the end of the Cambrian Period. Nearly one-half of the then known animal species became extinct at the close of the Permian Period! A large extinction of reptiles also occurred at the end of the Triassic. The dinosaur extinction is the best known of all disappearances, and it occurred at the close of the Cretaceous Period in a relatively short period of time.
Many postulates have been put forward to account for mass extinctions of organisms. One such postulate is that a nearby supernova might have showered the earth with bursts of high-energy radiation. Since water is a good shield against radiation, we would expect to find the land organisms to be more affected than the marine ones. Yet, we know that for certain periods most of the extinctions occurred in the sea. Postulates on the change in the earth's magnetic field suffer the same limitations.
Another postulate is that a "biological drive" became exhausted. There is no direct evidence for any such force in nature or that extinction is a result of its exhaustion.
Mountain building has been suggested as a cause. Attempts to relate mountain building periods to times of mass extinction have led to conflicting conclusions. Further, the greatest periods of mass extinction are said to have occurred during geologically quiet times.
Climate change is the most popular explanation for the observed extinctions. It was assumed that sudden changes in climate would quickly eliminate vast numbers of organisms that could not adapt. Perhaps the dinosaurs were not able to handle cold-prevailing climates. But fossil plants do not show such changes to have occurred at the time this hypothesis would predict them.
Fluctuations in the sea level have been well documented. It is known that an increase or decrease of only a few feet would cover or expose large areas of land. The correlation is that diversification is greatest during times of flooding and extinctions greatly increased during withdrawals. However, the worldwide effect would imply massive continental coverage rather than coastal swamping.
Many hypotheses have been offered but none seem to hold a majority opinion. What do the data show over geologic time? Valentine7 has studied the states of the marine biosphere over time by examining the fossil record for various benthic taxa. He has minimized the bias of incomplete fossil records by using only well-skeletonized taxa. Further, the benthic shelf environment was selected because it is the best-represented major environment in the fossil record, and also the environment most resistant to major forces of change.
Notice that the greatest numbers of classes and orders occurred early in the geological record with phyla remaining essentially the same throughout. Families had a similar curve up to the Triassic strata, but the number has increased with time to the present as have the number of species. The curves show evidence of mass extinctions that speak of worldwide catastrophes especially associated with the Triassic. Valentine says, "in summary, the most likely causes of mass extinction appear to be those factors which are natural regulators of diversity under normal circumstances and which have effects that pervade the entire planet." His catastrophe is related to tectonic processes.
The creationist views these data as evidence supporting an early highly diverse set of higher taxa which are declining in numbers with time. Definite data in the geological record point to a worldwide catastrophe especially in the Triassic and Jurassic strata. Subsequent diversification of benthic families, genera, and species since then is probably due to horizontal variation within kinds.
When comparing the two models of extinctions, one would make predictions something like the following:
|Initial Number of Taxa
Number of Taxa With Time
Higher Taxa (phyla, class order)
Lower Taxa (family, genus, species)
Niche Replacement With "New" Taxa
Our present experience supports a rapid extinction of taxonomic groups with very few documented "new" species appearances. This may be just a declining period in earth history for biota, but man's impact on the biosphere appears to force us to project an extended period of losses under the human population increase and technological pressure. Many more organisms will probably join the Tasmanians before new organisms take their place--if they ever do.
- Williams, Jay, Fall of the Sparrow, Oxford, England: Oxford University Press, 1951, p. 158.
- Opler, Paul, "The Parade of Passing Species: A Survey of Extinction in the U.S.," The Science Teacher, V. 44, No. 1, 1977.
- United States Congress, "The Endangered Species Act of 1973," Public Law 93-205, U.S. Government Printing Office, 1973.
- Stokes, William, "Extinction and Replacement," Essentials of Earth History, Englewood Cliffs, New Jersey: Prentice-Hall, Inc., 1973, pp. 48-485.
- Newell, Norman, "Crises in the History of Life," Scientific American, V. 200, 1963, pp. 76- 94.
- Baker, Jeffrey and Garland Allen, "The Evolution of Animals," The Study of Biology, Reading, Mass.: Addison-Wesley Publishing Co., 1968, pp. 577-597.
- Valentine, James, "The Biosphere Level," Evolutionary Paleoecology of the Marine Biosphere, Englewood Cliffs, New Jersey: Prentice-Hall, Inc., 1973, pp. 373-408.
*Dr. Kenneth B. Cumming is Chairman of the Biology Department at Christian Heritage College and Research Associate in Bioscience for ICR. He has a Ph.D. in Biology from Harvard University, as well as extensive research, teaching and administrative experience with the federal government and in three universities.
Cite this article: Kenneth B. Cumming, Ph.D. 1980. Extinction. Acts & Facts. 9 (6).