The greatest loss with the longest-lasting effects from the ongoing destruction of wilderness will be the mass extinction of species that provide Earth with biodiversity. Although great extinctions have occurred in the past, none has occurred as rapidly or has been so much the result of the actions of a single species. The extinction rate of today may be 1,000 to 10,000 times the biological normal, or background, extinction rate of 1-10 species extinctions per year.

So far there is little evidence for the massive species extinctions predicted by the species-area curve in the chart below. However, many biologists believe that species extinction, like global warming, has a time lag, and the loss of forest species due to forest clearing in the past may not be apparent yet today. Ward (1997) uses the term "extinction debt" to describe such extinction of species and populations long after habitat alteration:

Decades or centuries after a habitat perturbation, extinction related to the perturbation may still be taking place. This is perhaps the least understood and most insidious aspect of habitat destruction. We can clear-cut a forest and then point out that the attendant extinctions are low, when in reality a larger number of extinctions will take place in the future. We will have produced an extinction debt that has to be paid... We might curtail our hunting practices when some given population falls to very low numbers and think that we have succeeded in "saving" the species in question, when in reality we have produced an extinction debt that ultimately must be paid in full... Extinction debts are bad debts, and when they are eventually paid, the world is a poorer place.

For example, the disappearance of crucial pollinators will not cause the immediate extinction of tree species with life cycles measured in centuries. Similarly, a study of West African primates found an extinction debt of over 30 percent of the total primate fauna as a result of historic deforestation. This suggests that protection of remaining forests in these areas might not be enough to prevent extinctions caused by past habitat loss. While we may be able to predict the effects of the loss of some species, we know too little about the vast majority of species to make reasonable projections. The unanticipated loss of unknown species will have a magnified effect over time.

The process of extinction is enormously complex, resulting from perhaps hundreds or even thousands of factors, many of which scientists (let alone lay people) fail to grasp. The extinction of small populations, either endangered or isolated from the larger gene pool by fragmentation or natural barriers like water or mountain ranges, is the best modeled and understood form of extinction. Since the standard was set by MacArthur and Wilson in The Theory of Island Biogeography (1967), much work has been done modeling the effects of population size and land area on the survival of species.

The number of individuals in a given population is always fluctuating due to numerous influences, from extrinsic changes in the surrounding environment to intrinsic forces within a species' own genes. This population fluctuation is especially a problem for populations in isolated forest fragments and species that are critically endangered throughout their range. When a population falls below a certain number, known as the minimum viable population (MVP), it is unlikely to recover. Thus the minimum viable population is often considered the extinction threshold for a population or species. There are three common forces that can drive a species with a population under MVP to extinction: demographic stochasticity, environmental stochasticity, and reduced genetic diversity.

Demographic stochasticity involves birth and death rates of the individuals within a species. As the population size decreases, random quirks in mating, reproduction, and survival of young can have a significant outcome for a species. This is especially true in species with low birth rates (i.e. some primates, birds of prey, elephants), since their populations take a longer time to recover. Social dysfunction also plays an important role in a population's survival or demise. Once a population's size falls below a critical number, the social structure of a species may no longer function. For example many gregarious species live in herds or packs which enable the species to defend themselves from predators, find food, or choose mates. In these species, once the population is too small to sustain an effective herd or pack, the population may crash. Among species that are widely dispersed like large cats, finding a mate may be impossible once the population density falls below a certain point. Many insect species use chemical odors or pheromeres to communicate and attract mates. As population density falls, there is less probability that an individual's chemical message will reach a potential mate, and reproductive rates may decrease. Similarly, as plant species become rarer and more widely scattered, the distance between plants increases and pollination becomes less likely.

Environmental stochasticity is caused by randomly occurring changes in weather and food supply, and natural disasters like fire, flood, and drought. In populations confined to a small area, a single drought, bad winter, or fire can eliminate all individuals.

Reduced genetic diversity is a substantial obstacle blocking the recovery of small populations. Small populations have a smaller genetic base than larger populations. Without the influx of individuals from other populations, a population's genome stagnates and loses the genetic variability to adapt to changing conditions. Small populations are also prone to genetic drift where rare traits have a high probability of being lost with each successive generation.

The smaller the population, the more vulnerable it is to demographic stochasticity, environmental stochasticity, and reduced genetic diversity. These factors, often working in concert, tend to further reduce population size and drive the species toward extinction. This trend is known as the extinction vortex. See the box on the right for an example of an extinction vortex.

Some mathematical ecologists have suggested that population fluctuations may be governed by properties of chaos making the behavior of the system (the fluctuation of a species's population size) nearly impossible to predict due to the complex dynamics within a given ecosystem.

Tropical species are not only threatened directly by deforestation, but also by global climate change. Even if species survive in protected reserves, they may perish as a result of rising ocean levels and climactic changes. Many tropical species are used to constant, year-round conditions of temperature and humidity. They are not adapted to climate change even if it is as small as 1.8F (1C). Changes in seasonal length, precipitation, and intensity and frequency of extreme events that could occur should the Earth warm may strongly impact biodiversity in seasonal tropical forests and cloud forests. Studies show that unusual weather conditions—such as those under el Niño and la Niña—can cause population fluctuations of many forest animals. Should the frequency and intensity of such extreme events reach the level where whole populations are unable to recover to their normal level between events we could see localized extinctions and serious changes in the ecosystem. Climate changes could especially impact some sensitive ecosystems like cloud forests, which would be drastically affected by any lifting of the cloud cap. One often-overlooked consequence of increased temperatures is the spread of disease among wild animals. For example, there is a good chance that avian malaria and bird pox will be spread to Hawaiian upland forests by mosquitoes currently limited to elevations below 4,800 feet (1,500 m) due to temperature constraints. The spread of these diseases to upland forest would probably mean the extinction of several endangered bird species.

Many forest communities have survived global climate change in the past by "migrating" north or southward. However, today, because of fragmentation and human development, there are few corridors of wild territory for migration. Highways, parking lots, plantations, housing developments, and farms impede the slow, but steady movement necessary for many communities to survive changing climate conditions. Unable to escape the changes, many species within these communities will have to cope or face extinction. One of the contributing factors to the worldwide decline in amphibian populations may be the gradual climate change over the past 100 years, which when coupled with the increase in UV-B radiation, may have weakened their defense to a previously harmless fungal infection. This fungus has been detected on dead or dying frogs in locations around the world.

Global climate change may have had an impact on the extinction of North American megafauna at the end of the ice age some 10,000 years ago. One of the leading theories for the demise of these mammals—which included such wild beasts as giant sloths, mammoths, sabertooth cats, and oversized horses and rhinos—is that habitat fragmentation, caused by global climate change, split species into small populations, making them more vulnerable to extinction. As the last glacial interval came to a close and the great ice sheets receded, an additional factor came into play: the presence of hungry human hunters. Models (the Moisimann and Martin model of 1975, amended by Whittington and Dyke in 1989) suggest that by merely killing off 2 percent of the mammoth population every year, year after year, the entire species would be doomed to eventual extinction some three or four centuries down the road. These natural (climate change) and unnatural (human) influences working in concert surely condemned to extinction some of the most magnificent creatures ever seen by man. Today we are facing a similar situation, only this time we may be responsible for both factors, the global climate change and the overexploitation.

Extinction of a large number of species is highly likely because of the intricate relationships between species. David Quammen (1981) explains:

The educated guess is that each species of plant supports ten to thirty species of dependent animal. Eliminate just one species of insect and you may have destroyed the sole specific pollinator for a flowering plant; when that plant consequently vanishes, so may another twenty-nine species of insects that rely on it for food; each of those twenty-nine species might be an important parasite upon still another species of insect, a pest, which when left uncontrolled by parasitism will destroy further whole populations of trees, which themselves had been important because . . .

The complexity of the rainforest makes it impossible to anticipate when and what species will disappear.

Besides losing unique species that have lived on the planet for longer than we have and have every right to exist as we do, we are losing an incredible pool of genetic diversity which we could harness to help our own kind. As each species is lost, a unique combination of genes which has been produced over the course of millions of years, is lost and will not be replaced during our time. We head toward a future impoverished of the magnificent beasts that we remember learning about as children: ferocious tigers; armored rhinos; brilliant macaws; colorful frogs and toads. As these species vanish from the globe, the world is truly a poorer place.

Estimates of species loss each year range greatly as shown by this table.

Review questions:

• Why is there a "lag time" for species extinction?
• Why do small populations have a lower probability of survival?
• How might climate change impact global biodiversity?
• Why are frogs dying around the world?

Other versions of this page

spanish | french | portuguese | chinese | japanese



Continued / Next: How to Save Tropical Rainforests

Some good overviews of the current concerns over species extinction can be found in Pimm, S.L., Jones, H.L., and Diamond, J. "On the risk of evolution," American Naturalist, 132 (6) 757-785, 1988; Simberloff, D.S., "Are We on the Verge of a Mass Extinction in Tropical Rainforests?" in D.K. Elliot, ed. Dynamics of Extinction, New York: Wiley 1986; Wilson, E.O. "The current state of biological diversity." In BioDiversity, Wilson, E.O. and Peter, F.M., eds. National Academy Press, Washington D.C. 1988; Wilson, E.O. "Threats to Biodiversity," Scientific American, Sept, 1989; Wilson, E.O. "Wildlife-Legions of the Doomed," Time Magazine, 1991; Wilson, E.O., The Diversity of Life, Belknap Press, Cambridge, Mass. 1992.

May, E. M., Lawton, J.H., Stork, N.E. compare the estimated current extinction rate to the background extinction rate in "Assessing Extinction Rates" in Extinction Rates, Lawton and May, Eds., Oxford: Oxford University Press, 1995 in Biodiversity II.

T.C. Whitmore ("Tropical Forest Disturbance, Disappearance, and Species Loss," Tropical Forest Remnants: Ecology, Management, and Conservation of Fragmented Communities, W.F. Laurance and R.O. Bierregaard, Jr, Eds., Chicago: University of Chicago Press, 1997) notes that while there is little evidence of the mass extinctions predicted by the species-area curve, extinction probably has a time lag so species loss from habitat destruction in the past is not yet apparent.

In his book The Call of Distant Mammoths: Why the Ice Age Mammals Disappeared (Copernicus New York. 1997), P.D. Ward provides a popular account of the extinction of Ice Age megafauna. He explores the leading extinction theories and reviews terminology associated extinction such as "extinction debt." For more detailed examination of extinction debt, see McCarthy, M.A., Lindenmayer, D.B., and Drechsler, M. "Extinction debts and risks faced by abundant species." Conservation Biology Vol. 11 No. 1 (221-226), Feb. 1997 and Tilman, D. et al. "Habitat destruction and the extinction debt." Nature 371: 65-66. 1994. Recently Cowlishaw, G. ("Predicting the Pattern of Decline of African Primate Diversity: an Extinction Debt from Historical Deforestation." Conservation Biology, Pages 1183-1193. Vol. 13, No. 5, October 1999) examined extinction debts among west African primates, while Brooks, T.M., Pimm, S.L., and Oyugi, J.O. ("Time Lag between Deforestation and Bird Extinction in Tropical Forest Fragments." Conservation Biology, Pages 1140-1150. Vol. 13, No. 5, October 1999) surveyed the extinction debt-time lag among insular Southeast Asian bird species.

Comparing the occurrence of bird species in isolated forest fragments with the original avifauna Renjifo, L.M. ("Composition Changes in a Subandean Avifauna after Long-Term Forest Fragmentation." Conservation Biology, Pages 1124-1139. Vol. 13, No. 5, October 1999) found a reduction in diversity after fragmentation.

The species extinction table is derived from a similar table in Biodiversity II, Reaka-Kudla, Wilson, Wilson, eds.., Washington D.C.: Joseph Henry Press, 1997. The extinction estimates come from several sources including Raven, P.H. "Our Diminishing Tropical Forests," In BioDiversity, Wilson, E.O. and Peter, F.M., eds. Washington D.C.: National Academy Press, 1988; Wilson, E.O. "Threats to Biodiversity," Scientific American, Sept, 1989; May, R.M., "How Many Species Are There on Earth?" Science, 241: 1441-49, 1988; Wilson, E.O. The Diversity of Life, Cambridge, Mass.: Belknap Press, 1992; Reid, W.V. "How Many Species Will There Be?" In Tropical Deforestation and Species Extinction, Whitmore, T.C. and Sayer, J.A., eds., London: Chapman and Hall, 1992; and Mace, G.M.; 1994; "Classifying threatened species: means and ends," Phil. Trans. R. Soc. Lond. Bulletin 344, 91-97; Lovejoy, T. E. "A projection of species extinctions," in The Global 2000 Report to the President, G.O. Barney, Study Director, Entering the twenty-first century, vol. 2. Council on Environmental Quality,U.S. Government Printing Office, Washington D.C. 1980; From Biodiversity II. Reaka-Kudla, Wilson, Wilson, eds. Joseph Henry Press. Washington D.C. 1997; and Lovejoy, Thomas E. "Biodiversity: What is it?" In Biodiversity II, Reaka-Kudla, Wilson, Wilson, eds.., Washington D.C.: Joseph Henry Press, 1997.

The worldwide decline in amphibians is discussed by Lips ("Decline of a montane amphibian fauna," Conservation Biology Vol. 12 No. 1 (106-117), Feb. 1998.), Sessions et. al. (Sessions, S.K. Franssen, R.A., Horner, V.L., "Morphological Clues from Multilegged Frogs: Are Retinoids to Blame?" Science. 284 (5415) 1999), Tangley ("The Silence of the Frogs," U.S. World and News Report 8/3/98.), and Tuxill ("The Latest News on the Missing Frogs," World Watch, May/June 1998.).

Population sinks and sources are discussed in Merenlender, A., Kremen, C., Rakotondratsima, M., and Weiss, A., "Monitoring Impacts of Natural Resource Extraction on Lemurs of the Masoala Peninsula, Madagascar," Conservation Ecology Vol. 2(2): No. 5, 1998.

Suplee, C. reported the findings of the IUCN that roughly 12% of the world's flora can be classified as being threatened with extinction ("One in Eight Plants in Global Study Threatened," The Washington Post, 4/8/98).

In their The Theory of Island Biogeography (Princeton, New Jersey: Princeton University Press, 1967) R.H. MacArthur and E.O. Wilson discuss the geographic distribution and number of species of species on islands of varying sizes and vegetation types.

Mass extinctions are defined in Sepkoski, J.J. "Mass extinctions in the Phanaerozoic oceans: A review," Geological Society America, Special Paper 190, 1982, and Ward, P.D., On Methuselah's Trail: Living Fossils and the Great Extinctions, New York: W.H. Freeman and Company, 1992 and further explored in Raup, D., The Nemesis Affair, New York: W.W. Norton, 1986 and Martin, P.S. and Klein, R.G., eds., Quaternary Extinctions: A Prehistoric Revolution, Tucson: University of Arizonia. 1984.

The role of extra-terrestrial objects in past extinction events is evaluated by Alvarez et al. (Alvarez, L., Alvarez, W., Asaro, F., and Michel, H., "Extra-terrestrial cause for the Cretaceous-Tertiary extinction," Science 208: 1094-1108, 1980), Gore (Gore, R., 1989, "Extinctions," National Geographic, Vol. 175:6, p. 662-698), Raup (Raup, D., Extinction: Bad Genes or Bad Luck? New York: W.W. Norton; 1991), Sheehan (Sheehan, P.M., 1991, "Sudden extinction of the dinosaurs: latest Cretaceous, Upper Great Plains, U.S.A.," Science, v. 236, p. 835-839), and Hecht (Hecht, J., 1993, "Asteroidal bombardment wiped out the dinosaurs" New Scientist, v. 138, p. 14).

Forces (demographic stochasticity, environmental stochasticity, and reduced genetic diversity) that can drive a species with a population under MVP to extinction are explored in The Call of Distant Mammoths: Why the Ice Age Mammals Disappeared (New York: Copernicus, 1997) by P.D. Ward and in Conservation and Biodiversity, New York: Scientific American Library, 1996 by A. Dobson.

The concept of minimum viable populations is developed in Soulè, M.E. and Wilcox, B.A., eds., Conservation Biology: An Evolutionary-Ecological Perspective, Sunderland: Sinauer 1980; in Frankel, O. and Soulè, M.E. Conservation and Evolution, Cambridge: Cambridge University Press, 1981; and in Gilpin, M E. and Soule, M.E. (1986). "Minimum viable populations: the processes of species extinction," In Conservation Biology (pp. 19-34), Sunderland, MA.: Sinauer Associates, M. E. Soule (Ed.). Gilpin has continued to apply mathematical physics and operations research in his approach to examining island biogeography and population genetics in several books (including Restoration Ecology. (1987). J. Aber, M. Gilpin and W. Jordan, Eds. Cambridge University Press, London; Metapopulation Dynamics: Theoretical Models and Empirical Investigations. (1991). M. Gilpin and I. Hanski, Eds. Academic Press, New York; and Metapopulation Dynamics: Genetics, Evolution and Ecology. (1996). I. Hanski and M. Gilpin, Eds. Academic Press, New York).

The role of social dysfunction in population extinction is considered in Raup, D., Extinction: Bad Genes or Bad Luck? New York: W.W. Norton, 1991, and R.B. Primack, Essentials of Conservation Biology, Sunderland, MA.: Sinauer Associates, 1993.

Alfred Wallace's concerns over biodiversity loss in Indonesia during the late 19th century can be found in his classic Island Life (1881) (reprint edition (December 1997) Prometheus Books).

The complexity of ecosystem dynamics and population fluctuations is discussed in M. Gilpin and I. Hanski, Eds., Metapopulation Dynamics: Theoretical Models and Empirical Investigations (1991). Academic Press, New York; May, R. and Nowak, M. "Superinfection, metapopulation dynamics, and the volution of diversity" Journal of Theoretical Biology 170: 95-114, 1994; Leakey, R. and Lewin, R., The Sixth Extinction: Patterns of Life and the Future of Humankind. New York: Doubleday, 1995; I. Hanski and M. Gilpin, Eds. Metapopulation Dynamics: Genetics, Evolution and Ecology. (1996). Academic Press, New York; and Ward, P.D. The Call of Distant Mammoths: Why the Ice Age Mammals Disappeared (New York: Copernicus 1997).

Holdgate (Holdgate, M., "The Ecological Significance of Biological Diversity," Ambio Vol. 25 No. 6, Sept. 1996) notes that only 724 species have been recorded as going extinct since 1600, but explains actual extinction rates are acutally considerably higher given our relative ignorance of the number of species and inter-relationships between species.

Wilson (Wilson, E.O., The Diversity of Life, Cambridge, Mass.: Belknap Press 1992) and Erwin (Erwin, T. L. "Tropical Forests: Their Richness in Coleoptera and other arthropod species." Coleopterists Bulletin 36:74-75. 1982) estimate that roughly half the world's species dwell in rainforests.

Critics (see "Truth Almost Extinct in Tales of Imperiled Species," The Washington Times, September 19, 1984, and Simon, Julian, and Aaron Wildavsky (1994). Species Loss Revisited. Endangered Species Blueprint (National Wilderness Institute) 5, 1: 6-9.) have argued that the "extinction crisis [based on these theoretical projections] is alarmist and exaggerated" (Brooks, T., Pimm, S.L., Collar, N.J., "Deforestation predicts the number of threatened birds in insular southeast Asia," Conservation Biology Vol. 11 No. 2, April 1997).

Ward, P.D. (The Call of Distant Mammoths: Why the Ice Age Mammals Disappeared (New York: Copernicus 1997) provides commentary on the Moisimann and Martin model of 1975 and its later amendment by Whittington and Dyke in 1989.

In his article "The Big Goodbye" (Outside, November 1981) D. Quammen articulates that the intricate relationships between species may result in the extinction of a large number of species.

E.O. Wilson laments the loss of biological diversity in The Diversity of Life. (Cambridge, Mass.: Belknap Press, 1992) noting that as each species is lost, a unique combination of genes - which has been produced over the course of millions of year - also disappears.

Norman Myers popularized the subject of the current extinction wave in The Sinking Ark. A New Look at the Problem of Disappearing Species, New York: Pergamon, 1979.

In his book The Future Eaters (New York: Braziller 1995.) T.F. Flannery provides a fine overview of ancient man's impact on the ecology and environment of Australia. He holds mankind largely responsible for the extinction of Australia's megafauna.

Easter Island

In his article "Easter's End" (Discover. Vol. 16, No. 8, Aug 1995) Jared Diamond evinces that the social collpase of Easter Island may be tied to its ecological degradation and subsequent impoverishment. This interesting and very readable article provides the substance for the text box on "Historical Consequences of Deforestation: Easter Island."

Steadman, D. W. ("Extinction of Birds un Eastern Polynesia: A review of the record, and comparisons with other Pacific Island groups," Journal of Archaeological Sciences, 16:177-205, 1989. From Biodiversity II, Reaka-Kudla, Wilson, Wilson, eds., Washington D.C.: Joseph Henry Press 1997). notes that only one of the original 22 species of seabird still nests on Easter Island.

For a larger scale perspective than Easter Island, C. Runnels, "Environmental degradation in ancient Greece," Scientific American 272 (3): 72-75, 1995 and R. Adams, Heartland of Cities, Chicago: University of Chicago Press, 1981 link environmental degradation with the decline of civilization in ancient Greece and Mesopotania, respectively.