EARTH SCIENCE

Are we currently experiencing the largest mass extinction in Earth's history?

Viewpoint: Yes, human impact is currently causing the greatest mass extinction in Earth's history.

Viewpoint: No, several measures indicate that the current mass extinction, while severe and alarming, is not the largest in Earth's history.

Of all the species that have lived on Earth over the last 3 billion years, only about one in 1,000 is alive today. The rest became extinct, typically within 10 million years of their first planetary appearance, an extinction rate that has contributed to the planet's current biodiversity level. Presumably, all the species alive today will experience the same fate within the next 10 million years or so, making way for our own successors.

Mass extinctions—catastrophic widespread perturbations in which 50% or more of species become extinct in a relatively short period compared with the background extinction rate—happen planet-wide and affect a broad range of species on land and in the sea. Paleontologists have identified five large-scale extinctions in the fossil record. Such extinctions seem to be caused by the catastrophic impacts of agents such as asteroid or meteorites; or terrestrial agents such as volcanic activity, sea level variations, global climate changes, and changing levels of ocean oxygen or salinity.

Earth today is experiencing a mass extinction—more than 11% of the 9,040 known bird species are endangered, 20% of known freshwater fish are extinct or endangered, and more than 680 of the 20,000 plant species in the United States are endangered, to cite some examples. What scientists do not know is whether extinction is a natural part of evolution, or a by-product of periodic catastrophes. Another fundamental question is whether the mass extinction now under way is the largest in Earth's history.

Those who believe it is blame a 6-billion-strong world population that consumes between 30 and 40% of the planet's net primary production, the energy passed on by plants for the use of other life-forms. People also consume, divert for their own uses, or pollute 50% or more of Earth's freshwater resources.

Calculations of the rate of extinction now underway are based on the 1.4 million species that scientists estimate exist on Earth, and on two interrelated principles—loss of species through rain-forest destruction; and forest fragmentation, the survival of species in relatively small, restricted patches of ecosystem.

In 1979, the British biologist Norman Myers estimated that 2% of the world's rain forests were being destroyed annually. Initial estimates were that destruction of this magnitude was causing the extinction of between 17,000 and 100,000 species a year; or 833,000 to 4.9 million by 2050. This translates into species loss through the formula biologists use to determine rates of extinction: S = CA^z. As the American biologist Edward O. Wilson explains in The Diversity of Life, S is the number of species, C is a constant, A is the area of the fragment, and z is an exponent whose value varies with the organism and its habitat requirements. Wilson used this formula to calculate that the current extinction rate was 27,000 species a year. At that rate, by 2020 Earth will have surpassed the percentage needed for a genuine mass extinction. Today, 6.5 times more species are becoming extinct in a time frame that is 263 times faster than the fastest estimates for the Permian mass extinction.

Those who do not think Earth is in the throes of history's largest mass extinction explain that the fossil record is used to measure the severity of mass extinctions in several ways. The length of time it takes the extinction event to represent itself in the fossil record indicates the causative event's severity and immediacy. The extinction's duration, how quickly the biosphere adapts to the new conditions, is another measure of severity, as is the event magnitude, or total number of species affected.

By several of these measures, the current mass extinction is clearly not the largest in Earth's history. It is not occurring as suddenly as one brought on by an impact, such as the Cretaceous-Tertiary (K-T) extinction, and it has not affected as much of the biosphere as did the Permian-Triassic (P-T) extinction.

History's largest mass extinction in the fossil record was the event that defines the boundary between the Permian and Triassic periods, the P-T extinction, which occurred 250 million years ago and whose cause is still unknown. Researchers measuring the abundance of different marine species in the fossil record note a 90 to 95% reduction in the total number of marine species at that time.

Observational biases exist in current measurements and events inferred from the fossil record. The fossil record is the accumulation of hard-bodied creatures preserved and mineralized in sediment, so measuring extinction events is largely based on counts of marine species and does not necessarily reflect effects on other forms of life, like soft-bodied organisms and land-dwelling creatures. The fossil record says almost nothing about microorganisms, the most abundant type of life on Earth.

In addition, researchers disagree about how to determine the number of species in the fossil record. A species is defined as a group of organisms that can interbreed freely under natural conditions, a difficult thing to test when examining mineralized shells. So researchers use morphological features such as shape and size to assign species.

The true result of a mass extinction—depletion of the planet's total biodiversity—carries implications for the severity of the current event. As the fossil record shows, ancient extinctions affected large numbers of species and dramatically reduced the total number of living organisms. The current extinction of species may be reducing total biodiversity, thus reducing the ability of the biosphere to adapt and recover.

—CHERYL PELLERIN

Viewpoint: Yes, human impact is currently causing the greatest mass extinction in Earth's history.

As the American biologist Edward O. Wilson put it: "It is possible that intelligence in the wrong kind of species was foreordained to be a fatal combination for the biosphere…. Perhaps a law of evolution is that intelligence usually extinguishes itself." In other words, what death and taxes are for individual humans, extinction is for species.

Under normal circumstances in nature, species become extinct as conditions change, and they are usually replaced by new species better adapted to the new conditions. However, since about 1800, the beginning of the exponential increase in the human population and its concomitant intrusion into and disruption of natural habitats around the world, the extinction of species has accelerated and spread. Today it is a worldwide phenomenon. The severity of the current extinction is a contentious issue, but this essay will show that our world is on the brink of a mass extinction of unprecedented proportions. The best way to document the severity of the current extinction crisis is to describe the normal process of extinction—what occurred during the greatest mass extinctions in the past—and compare that to what is happening today.

The Normal Extinction Processes

The normal rate of extinction, which occurs during times of relative stability, has been dubbed by University of Chicago paleontologist David Raup the background extinction rate. Raup has shown that this background rate of random extinction is generally very low. During the past 500 million years, the background extinction rate has been approximately one species every four years. However, this figure includes those species that disappeared during mass extinction events. If the rate is recalculated omitting mass extinctions, that is, including only those extinctions that occur during stable periods, the background extinction rate is even lower.

Research has shown that, over time, species diversity has remained quite stable. In fact, the history of life on Earth indicates that, in general and over time, the rate at which new species have evolved is slightly greater than the rate at which species disappear.

Prior Mass Extinctions

A mass extinction is defined as a catastrophic, widespread perturbation in which a large number of species become extinct in a relatively short period of time compared with the background extinction rate. Generally, mass extinctions are defined as those in which 50% or more of species disappear.

Earth has experienced five great mass extinctions (see table 1).

Permian Mass Extinction

The greatest extinction event the world has ever known was the event that defines the boundary between the Permian and Triassic periods, the P-T extinction, 245 million years ago. The exact causes of this catastrophe are unknown, though hotly debated. Some experts argue for shifting tectonic plates (which moved together to form the supercontinent, Pangaea), a devastating collision from an asteroid, or changes in ocean salinity. But many scientists now suggest that the Permian extinction was caused by rapid and catastrophicglobal warming. This hypothesis states that numerous volcanic eruptions spewed enormous quantities of carbon dioxide (CO²) into the atmosphere. This powerful greenhouse gas caused the climate to warm, which warmed ocean waters, which led to the release of (CO²) held in ocean sediments into ocean waters. This release led to the collapse of marine life.

Rates and Percentages.

The P-T extinction is believed to have been preceded by several small extinction events, culminating in a major pulse of extinction. Many scientists believe that the major event occurred extremely rapidly—over a period of only one million years—a geological instant.

During this one-million-year period, 70% of land species and 90% of marine species became extinct. To compare different extinction events more easily, it helps to analyze these figures for smaller time periods. Thus, assuming a linear rate of extinction, see table 2.

Some scientists contend that within the one-million-year-long major pulse of extinction, the majority of species may have met their end during only a few tens of thousands of years, perhaps in as little as 50,000 years. If this is true, the extinction rate during the most catastrophic period would be as shown in table 3.

The Sixth Mass Extinction

The sixth mass extinction—the one currently underway and the most catastrophic in history—exceeds the Permian extinction in both its rapidity and in the percentage of species lost.

People and Resources.

The current extinction event started when humans began to move into and dominate all parts of the planet. Wherever human populations settled, biological diversity decreased. Since 1800, the human population has been increasing exponentially. It took the human population until about 1800 to reach the one-billion mark. After that, it took only 130 years for it to reach two billion, 30 years more to hit four billion, a mere 15 years more to reach five billion, and a measly 12 years to top six billion (the six-billionth human baby was born in 1999).

Of course, people take up space, and they need food and water. At the current rate of population growth, human demands on Earth's basic resources are unsustainable. We also threaten the continued survival of other organisms. At a population of six billion, humans on Earth today consume between 30 and 40% of the total net primary production on the planet. Net primary production (NPP) is the amount of energy passed on by plants for other organisms to use (plants are at the base of nearly all food chains, they are the organisms on which all others depend for life). The 40% figure includes direct consumption (food, wood, fuel), as well as indirect consumption (land clearing and development, feed grown for livestock) of NPP by people. Humans have also consumed, or diverted for their own uses, or have polluted, or made unusable and thus unavailable to other organisms, a significant percentage of the world's nonglacial, freshwater resources.

All in all, these figures indicate that human actions have dramatic consequences for all other life on Earth. Furthermore, the percentages of these crucial resources appropriated for human use is expected, inevitably, to skyrocket as the human population races toward 10 or 11 billion, at which time it may (or may not) stabilize.

How Many Species?

Before it can be determined whether we are in the throes of another mass extinction, it is necessary to know the number of species that currently exist. Herein lies a problem, because all biologists agree that we have named only a small fraction of the organisms on Earth. To date, scientists have discovered, named, and described approximately 1.4 million species.

The vast majority of the world's species reside in its rain forests—the cradle of biodiversity on Earth. In fact, biologists agree that rain forests hold so many more as-yet-unnamed species that the total number of species on Earth is many times the number so far counted. This is especially true for the arthropods. For example, Terry Erwin, an entomologist at the Smithsonian Institution, has intensively studied insects living in the canopy of the Amazon rain forest in Peru and Brazil, and in rain forest in Panama. Erwin's surveys turned up so many previously unknown insect species, often numerous new species in a single tree, that he estimated there are at least 30 million species of insects in the tropics alone. Further research has led scientists to adjust this estimate downward, to about 10 million insect species. Despite our lack of knowledge of species resident on coral reefs—the "rain forests of the sea"—most scientists accept that the number of species is likely between 5 and 30 million. A working estimate of 10 million species is generally agreed upon, and will be used here to illustrate the current problem. It is important, however, to point out that mass extinctions are based not on the actual number of organisms that die, but on the percentage of species that go extinct.

Current Extinction Patterns and Rates

Instantaneous Extinction.

When a rare cloud-forest ecosystem in Ecuador was destroyed to make way for agriculture, 90 species of unique plants immediately became extinct. Instantaneous extinction is the rule when rare ecosystems, and their unique plants and animals, are destroyed. If, as described above and as many scientists believe, many rain-forest organisms have an extremely limited habitat (one tree, for example), instantaneous extinctions will contribute significantly to the overall extinction.

Rain-Forest Destruction and Fragmentation.

Rain forests contain the greatest number and diversity of species on Earth. Calculations of the rate of extinction currently underway are based primarily on two interrelated principles: first, the loss of species through rain-forest destruction; second, rain-forest fragmentation. These principles arose from the study of island biogeography, the survival of species in relatively small, restricted patches of ecosystem.

In 1979, the British biologist Norman Myers estimated that 2% of the world's rain forests were being destroyed annually. His estimate was termed "alarmist" by extinction skeptics. Today, satellite imagery shows that his estimate might well be called quite accurate. In the 1970s, satellite pictures showed an annual rain-forest loss of about 28,960 sq mi (75,000 sq km); by 1989, that figure had jumped to 54,830 sq mi (142,000 sq km), a loss of 1.8% per year. Initial estimates were that destruction of this magnitude was causing the extinction of between 17,000 and 100,000 species every year; or 833,000 to 4,900,000 species by 2050.

In some regions, patches of rain forest are spared and left standing to preserve the biodiversity they hold. These "preserve patches" vary in size, usually from 0.004 to 40 sq mi (0.010 to 1,000 sq km). Scientists have carefully studied the fate of species within different-sized preserves. They have found that the edge effect, the vulnerability of the boundaries to outside conditions, nullifies species protection near the preserve perimeters. Habitat deep within a large preserve may retain its essential characteristics, but perimeters bordered by cleared land are exposed to the deleterious effects of wind, low humidity, pesticides, and the intrusion of humans and other animals not native to the preserve. Scientific and satellite studies indicate that animals living within 0.5 mile (0.8 km) of a preserve perimeter are highly vulnerable to extinction.

Fragments of ecosystems are essentially islands of habitat surrounded, not by the sea, but by land appropriated by people. Like islands in the ocean, the larger they are, the more species they can support. Likewise, the smaller they are, the greater the rate of extinction of those species that had once lived in the undisturbed habitat. For example, two preserve patches in Brazil were studied for 100 years. In that time, 14% of the bird species in one patch of 5.4 sq mi (14 sq km) became extinct; in the other patch of 0.07 sq mi (0.2 sq km), 62% of bird species became extinct.

Another factor in forest fragmentation is loss of one or more key species. For example, peccaries, small pigs also known as javelinas, disappeared from one 0.40-sq-mi (1-sq-km) forest fragment in the Amazon. When the peccaries fled, presumably because the fragment was too small to support them, three species of frog became extinct—the frogs required the mud pools created by wallowing peccaries. So the loss of one key species often creates a cascade of extinction within a forest fragment. How does this translate into species loss?

Loss of Species.

In The Diversity of Life, Edward O. Wilson explains the formula biologists use to determine rates of extinction based on "island" (habitat remnant) size. The formula is S = CA^z, where S is the number of species, A is the area of the fragment, and z is an exponent whose value varies depending on the organism and its habitat requirements ( C is a constant). In nearly all cases, the value of z varies between 0.15 and 0.35. Because it is an exponent, the higher the value of z, the greater the reduction in the number of species.

Wilson used this formula to calculate the current extinction rate, including only the most conservative numbers. He did not factor in the depredations of overharvesting or the lethally disruptive effects of invasive species. In his sample calculation, Wilson plugged into the formula the lowest z value of 0.15, for a low estimate of 10 million rain-forest species. He assumed that, for the purposes of this trial calculation, all 10 million species had large geographical ranges (eliminating instantaneous extinction). Finally, he added the 1.8% per year loss of rain forest to the formula. The result—optimistic because of the several low estimates used—indicated that each year, 27,000 species become extinct. That means 74 species a day; 3 species an hour. Although this estimate is derived based on some assumptions, it is nevertheless a clear and alarming signal.

Now and Then

How does this rate compare with the mass extinction rate during the Permian? A die-off event is considered a mass extinction when 50% or more of species become extinct in a relatively short geological time span. If 27,000 species are lost per year in a world containing 10 million species, approximately 3 out of every 1,000 species (0.3%) are lost each year. The background extinction rate for previous eras, assuming 10 million species, has been calculated at between 1 out of every 1,000,000 species to 1 out of every 10,000,000 species. Thus, the current extinction rate may be as much as 30,000 times higher than background.

Based on Raup's background-extinction rate estimate of one species becoming extinct every four years, the current rate of extinction is 108,000 times higher than the background extinction rate. A rate of 27,000 species extinctions per year means that 0.27% of species are lost annually. It is not hard to figure out that, at that rate, and assuming a linear relationship, in less than 200 years we will have met and surpassed the percentage needed for a true mass extinction.

The extremely rapid extinction rate during the Permian pales in comparison with the current rate of extinction (see table 4).

As this astonishing (and alarming) comparison shows, 6.5 times more species today are becoming extinct in a time frame that is 27 times faster than the fastest estimates for the Permian mass extinction. If the Permian extinction occurred over a one-million-year period, the current extinction rate becomes even more catastrophic.

Of course, the 0.27% per year loss will not (hopefully) continue indefinitely until every last living thing on Earth is gone. But island biogeography has confirmed that for every 10-fold decrease in rain-forest habitat, 50% of its resident species will go extinct: some instantly, some over a period of time, perhaps decades or centuries. The process has already begun, and people show no inclination to stop it, as seen in the minuscule areas of rain forest preserved: 4% in Africa, 2% in Latin America, and 6% in Asia. Worse, the island biogeography estimates are conservative. Because of the extremely limited range of some rain forest species, when 90% of a rain forest is destroyed, the result is not simply a percentage reduction in the populations of resident species. The result is the immediate extinction of some localized species, and the gradual decline and eventual extinction of others.

Even among known species, documented extinctions today are occurring far more rapidly than in the past. In his study of bird extinction in the rain forests of Hawaii, Stuart Pimm, professor of ecology at the Center for Environmental Research and Conservation at Columbia University, reports an extinction rate of one bird species per year—four times the background rate. Of 135 native Hawaiian birds, only 11 species are thriving in numbers that ensure their survival to 2100. Globally, in addition to those already extinct, at least 11% of all bird species are now critically endangered.

Some experts predict that, at current rates of destruction, 90% of the world's rain forests will be gone in a century; the remainder will likely be patches incapable of supporting diverse species. The combination of total rain-forest destruction and extinction due to edge effect and fragmentation will, many scientists believe, result in the extinction of about 50% of Earth's species in the next 100 years.

Today, one in eight plant species is at risk of disappearing, and some are so far gone on the road to extinction that they are not expected to recover. These plant extinctions are particularly worrying. The Cretaceous mass extinction may have wiped out the dinosaurs, but most plant species were spared. The sixth extinction is taking both plants and animals.

Although habitat in temperate zones is also being destroyed—gobbled up by development of one sort or another—the devastation of tropical rain forests puts us squarely in the midst of the greatest and most rapid mass extinction ever seen on Earth. As Stuart Pimm has said, "The sixth extinction is not happening because of some external force. It is happening because of us…. We must ask ourselves if this is really what we want to do…."

—NATALIE GOLDSTEIN

Viewpoint: No, several measures indicate that the current mass extinction, while severe and alarming, is not the largest in Earth's history.

Compelling evidence exists for mass extinctions throughout Earth's history. At several points in the fossil record, researchers have observed severe reductions in the total numbers and diversity of species. However, the cause of these historical extinctions often remains unclear. Some appear to have been caused by meteorite impacts, others by extensive volcanism or dramatic changes in climate. Others are mysterious, with the cause completely unknown.

For one mass extinction, however, the cause is all too clear. From the time humans began to spread across the globe over 100,000 years ago, they began to fiercely compete with other species. In historical times, humans have accelerated their impact on the natural world so much so that many believe humans are causing the extinction of large numbers of species at an unprecedented rate. To examine that claim, scientists study the fossil record.

The fossil record indicates that the severity of mass extinctions can be measured in a number of ways. The time it takes for the extinction event to represent itself in the fossil record indicates the severity, and immediacy, of the event that caused it. For example, a mass extinction caused by the impact of a giant meteorite occurs much faster than one caused by climate change, and the biosphere's ability to adapt can be stressed if the extinction event is too rapid. The duration of the extinction event is also an indicator of severity. This duration is essentially a measure of how quickly the biosphere can adapt or evolve to the new conditions. Finally, the magnitude of the event, or the total number of species affected, also serves as a measure of severity.

By several of these measures, the current mass extinction, despite its intensity, is nowhere near the largest in Earth's history. It has not been brought on as suddenly as the Cretaceous-Tertiary, or K-T extinction ( Kriede is the German word for Cretaceous), nor has it affected as much of the biosphere as the Permian-Triassic, or P-T extinction. In addition, observational biases exist in both the current measurements and the events inferred from the fossil record. These biases reflect on the interpretation of the fossil record, and on the policies instituted in response to the current mass extinction. Finally, the true result of a mass extinction—the depletion of the total biodiversity of Earth—carries implications for the severity of the current event. As the fossil record shows, the ancient extinctions affected large numbers of species and dramatically reduced the number of living organisms. The current onslaught on Earth's species may be resulting in a total reduction of biodiversity, reducing the ability of the biosphere ability to adapt and recover.

Earth's Current Mass Extinction

Is Earth currently experiencing a mass extinction? Of this there is little doubt. According the International Council on Bird Preservation, over 11% of the 9,040 known bird species are endangered, some 20% of known freshwater fish in the world are extinct or seriously endangered, and, according the Center for Plant Conservation, over 680 of the 20,000 plant species in the United States are endangered. Considered along with an exploding human population, unprecedented resource exploitation, and the destruction of wild habitat, the picture becomes grim indeed.

However, these are just a few direct observations. Most of the species of flora and fauna on Earth are largely unknown. Estimates of global rain-forest destruction (over 190,000 sq mi, or 500,000 sq km, per year) indicate the rate of extinctions are 1,000 to 10,000 times the "normal," background rate of species extinction that occurs naturally, and the global extinction rate is greater than it has been for over 100,000 years.

However, scientists and conservation groups are still struggling to directly measure the effect of humans on the biosphere. Most species on Earth are unnamed and their habits poorly understood, making a direct assessment difficult. Also, actual extinction events are local, isolated events and receive little attention, except in the case of high profile species such as the passenger pigeon. These biases lead many policy makers to ignore the current extinction, or claim the event is "natural." One look at the fossil record, however, reveals what a natural event really looks like.

The Largest Mass Extinctions

As serious as the current extinction seems, it is not the largest. Examining the fossil record, two major extinctions overshadow all others. Approximately 65 million years ago, at the boundary of the Cretaceous and Tertiary periods (the K-T boundary), the fossil record indicates that over 85% of all species suddenly went extinct. This event destroyed the dinosaurs and cleared the evolutionary field for small mammals to eventually evolve into humans. Some of the most abundant species in the fossil record—clamlike brachiopods and mollusks, as well as echinoids such as sea stars and sea urchins—were severely affected. The K-T extinctions event altered the entire character of life on Earth.

The cause of the K-T extinction remained a mystery until 1980, when researchers Luis and Walter Alvarez, Frank Asaro, and Helen Michel at the University of California, Berkeley, located a small layer of iridium in layers of Earth's crust that had been deposited during the boundary (the time of the change from one period to another) between the Cretaceous period and the Tertiary period. Iridium, although uncommon in Earth's crust, is common in meteorites. The group advanced the theory that the boundary, along with the extinction event, resulted from an impact with a giant meteorite. This impact raised enough dust to cover Earth and blot out the Sun for several years, destroying many plants and the animals that depended on them. As Earth's climate recovered, severe weather patterns further stressed the biosphere. Additional studies supported this theory.

The K-T extinction event was severe in two ways. First, the nature of the impact caused an immediate change in the global conditions. The dust drifting into the atmosphere reduced the amount of sunlight, and therefore the energy, available to plants on land and in the ocean. Organisms relying on the plants died out, and the predators of those organisms were subsequently affected. This precipitated a collapse of the planet's food web, and a large, rapid extinction event. Second, in total numbers, the K-T event stands out as one of the largest in Earth's history.

But the K-T event is not the largest in the fossil record. That title goes to the extinction event that defines the boundary between the Permian and Triassic periods. The P-T extinction occurred approximately 250 million years ago. Researchers measuring the abundance of different marine species in the fossil record note a 90 to 95% reduction in the total number of marine species at this time. In sheer numbers, the P-T event is by far the largest extinction in the history of Earth.

The cause of the P-T extinction is for the most part unknown. It was not as sudden as the K-T event, indicating that something other than a giant impact caused the extinction. Several possibilities exist. Global glaciations caused by dramatic climate change could have forced many species to extinction. Glaciations may have been more localized to the poles, causing a reduction in the overall sea levels and reducing marine habitat. It is also possible that the area of the continental shelf, a favorite habitat of sea creatures, was greatly reduced during the formation of the supercontinent Pangaea. It has also been suggested that an increase in global volcanic events could have triggered a climate change and caused the extinction. Whatever the reason, the P-T extinction, while not as sudden as the K-T, was certainly more severe in terms of the number of species affected.

Measuring the Severity: Observational Biases

It is not a simple matter to measure the effects of humans on the current biosphere. Living creatures are difficult to track, and despite the best efforts of researchers, estimates of the current mass-extinction rate must be made by inferences from the destruction of habitats such as rain forests. But what are some of the unique biases in the fossil record?

The fossil record consists of the accumulation of organisms that have been preserved and mineralized in sediment. This means that creatures with hard shells and skeletons in marine or shallow-water environments are preferentially preserved. The measurement of extinction events is therefore based largely on counts of fossils of marine species, and does not necessarily reflect the effects of such events on plants, soft-bodied organisms, and land-dwelling creatures. Furthermore, the fossil record says almost nothing about microorganisms, by far the most abundant type of life on Earth.

In addition, researchers disagree on how to determine the number of species in the fossil record. A species is defined as a group of organisms able to interbreed freely under natural conditions, a difficult feature to test when examining a bunch of mineralized shells. Therefore, researchers use morphological features such as the shape and size of an organism to assign species.

Still, the fossil record has one distinct advantage over present-day methods for determining extinction rates. By studying the layered sediment of an sea floor, field researchers can distinguish when an extinction occurred. At one level, there exists abundant life, in the next, it is severely depleted. Only by taking careful records for hundreds of years could today's researchers record such an extinction event happening now, and by then it could be too late.

The Real Problem:The Question of Biodiversity

Whatever the biases in the fossil record, or the difficulty in determining the rate of current extinctions, the true measure of the severity of extinction is the change in biodiversity. Bio-diversity is the overall diversity of species in the biosphere. What were the effects of other mass extinctions on biodiversity? Again, ignoring the observational bias in the fossil record, it seems that biodiversity was seriously reduced by both the K-T and the P-T extinction events. In both cases, the elimination of species opened niches for subsequent explosions of new species. The brachiopods, marine animals that looked like clams, fared poorly in the P-T extinction, but other organisms subsequently prospered. Similarly, the K-T extinction made the evolution of mammals, including humans, possible. Mammals were present long before the K-T extinction, but the K-T event cleared the way for them to evolve into previously unoccupied niches. However, biodiversity took 100 million years to recover from the P-T extinction, and over 20 million years to recover from the K-T, certainly a very long time by human standards.

Naturalists and ecologists agree that fostering the biodiversity of Earth is important for a healthy ecosystem. However, human activity is taking biodiversity in the opposite direction, removing varieties of species and replacing them with more of the same types of species. For example, the diversity of agricultural products has declined as humans rely on more limited, higher-yield crops. This is in addition to the endangerment of large numbers of other wild plants and animals. Biodiversity is needed to provide the genetic diversity for evolution to prosper. The American biologist Edward O. Wilson, author of The Diversity of Life, suggests that the current level of extinction is removing species at such a rate that biodiversity on Earth is seriously affected. As Wilson indicates, if the recovery from the current mass extinctions takes as long as for other mass extinctions in the fossil record, humans as a species may not live to see it.

Clearly, regardless of the impact human activity has on biodiversity, the biosphere, given enough time, will recover. Life on Earth has survived volcanoes, advancing ice sheets, catastrophic impacts, and a host of other life-threatening events. Whatever the results of the human effects on the biosphere, life, eventually, will go on. However, as researchers like Wilson are quick to point out, the reduction in biodiversity may result in the extinction of at least one more very important species. Combating the effects of the current extinction thus becomes not only a fight for the biosphere, but for the survival of Homo sapiens.

—JOHN ARMSTRONG

Further Reading

Ehrlich, Paul. Extinction. New York: Random House, 1982.

Eldredge, Niles. Life in the Balance: Humanity and the Biodiversity Crisis. Princeton, N.J.: Princeton University Press, 1998.

Erwin, Douglas. The Great Paleozoic Crisis: Life and Death in the Permian. New York: Columbia University Press, 1993.

——. "The Mother of Mass Extinctions." Scientific American (July 1996): 72-8.

Lawton, John. Extinction Rates. New York:Oxford University Press, 1995.

Leakey, Richard, and Roger Lewin. The Sixth Extinction: Patterns of Life and the Future of Humankind. New York: Doubleday, 1996.

Morell, Virginia. "The Sixth Extinction." National Geographic (February 1999): 42-59.

Ward, Peter D. On Methuselah's Trail: Living Fossils and the Great Extinctions. New York: W. H. Freeman and Company, 1993.

——. The End of Evolution: On Mass Extinctions and the Preservation of Biodiversity. New York: Bantam Books, 1994.

——. Rivers in Time: The Search for Clues to Earth's Mass Extinctions. New York: Columbia University Press, 2001.

Wilson, Edward O. The Diversity of Life. New York: W. W. Norton, 1999.

KEY TERMS

BACKGROUND EXTINCTION RATE:

Normal rate of species extinction during times of relative stability.

BIODIVERSITY:

The overall diversity of species in the biosphere. Many researchers believe biodiversity, and not the total number of species, is the true measure of the impact of an extinction event.

BIOSPHERE:

The sum total of all life on Earth.

CASCADE EFFECT:

Widening effect the removal of a key species has on other species that depend on it, or its effects on the ecosystem, for survival.

EDGE EFFECT:

Vulnerability to harmful outside effects of the perimeters of fragments of preserved ecosystems surrounded by land cleared by humans.

FRAGMENTATION:

Clearing of natural ecosystems leaving small remnants, or fragments, intact.

ISLAND BIOGEOGRAPHY:

Study of the survival of species in small islands, or remnants, of ecosystems.

MASS EXTINCTION:

Catastrophic, widespread disruption in which major groups of species, generally 50% or more of the total number of species, become extinct in a relatively short period of time compared with the background extinction rate.

MORPHOLOGY:

The shape, size, and other characteristic features of an organism.

OBSERVATIONAL BIAS:

Any effect that influences the types of samples or measurements in a given observation. For example, performing an opinion poll on a college campus would preferentially sample young, college-age people, and thereby create a bias in the results.

PRIOR MASS EXTINCTIONS

MASS EXTINCTION RATES

THE CRETACEOUS EXTINCTION

There is no question that the most "popular" mass extinction—the one that fires the human imagination—occurred during the Cretaceous period, when the dinosaurs breathed their last.

The Cretaceous extinction was the line that separated the "Age of the Dinosaurs" from the "Age of the Mammals." At the end of the Cretaceous period, and by the beginning of the Tertiary, 60% of the world's species disappeared. Although the dinosaurs were, by far, the most notable casualties, many more marine species became extinct during this event.

Experts have furiously debated the cause or causes of the Cretaceous extinction for decades. There are two major hypotheses.

INTRINSIC GRADUALISM

This hypothesis states that changes on Earth (intrinsic changes) caused the extinction. These changes would have taken place over millions of years. Volcanic eruptions, which increased greatly during the Cretaceous, may have put enough dust into the air to cause a global cooling of the climate. Also at this time, Earth's tectonic plates were in flux, causing oceans to recede from the land. As the oceans retreated, their mitigating effect on the climate would have been reduced and the climate would have become less mild. This too would have happened gradually, over millions of years.

EXTRINSIC CATASTROPHISM

This hypothesis, put forth by Luis and Walter Alvarez and others at the University of California, Berkeley, stipulates that an enormous extraterrestrial object—perhaps a meteor or a comet—collided with Earth at the end of the Cretaceous. The impact would have been great enough to send up into the atmosphere a huge cloud of dust, sufficient to cool the global climate for years. A huge crater discovered at Chicxulub, on the Yucatan Peninsula in Mexico, was found to have features that identified it as the most likely collision site. Other evidence supporting the extrinsic catastrophism hypothesis has emerged. Shocked quartz, which is formed during extremely violent earth tremors, has been found in rock of the Cretaceous period, as has a layer of soot in some rocks. The soot layer is indicative of widespread firestorms that would have raged over parts of Earth after an impact.

Proponents of each hypothesis are still discussing the subject; to date, and for the foreseeable future, no definitive answer has emerged. Too many questions remain unanswered by both hypotheses: Why did some species die out and others survive? Why did more marine species disappear than terrestrial species? Can climate change really account for the selectivity of the extinctions? Some scientists are now studying a complex of causes for the extinction, integrating one or more intrinsic events and the extrinsic impact event.

—Cheryl Pellerin