BRIA 25 1 Are We Headed for a Sixth Mass Extinction

Bill of Rights in Action
SUMMER 2009 (Volume 25, No. 1)

Environmental Issues

Are We Headed for a “Sixth Mass Extinction”?  |  The Columbian Exchange  |  What Caused Egypt’s Old Kingdom to Collapse?

Are We Headed for a ‘Sixth Mass Extinction’?

Earth has gone through five major mass extinctions of plant and animal species. Many scientists are convinced we are in the middle of a sixth mass extinction, caused mainly by human activity. If this is so, what can we do about it?

The extinction of plant and animal species has occurred on Earth since living organisms emerged, more than 3 billion years ago. Typically, a species will appear, thrive, and then die off over a span of millions of years. In fact, nearly all species that have ever lived on Earth are now extinct.

In the history of Earth, five rare but catastrophic events have caused “mass extinctions,” the disappearance of many species. These “big five” along with smaller extinctions lasted up to a few million years.

They had a number of natural causes, including widespread volcanic activity and large asteroid or comet impacts. These natural events projected dust into the atmosphere, causing periods of global cooling or warming. The dust also blocked plant photosynthesis, which eliminated the food supply of many animals. If plants and animals could not adapt to abrupt changes in the environment, they became extinct.

The last mass extinction, about 65 million years ago, ended the reign of the dinosaurs, and opened the way for the evolution of mammals. Scientists think that most mass exterminations took place when long-term stresses on the environment like global climate change combined with a “short-term shock,” like a large comet hit.

Species of plants and animals normally die off at about the same rate as new species arise. Biologists call this the “background rate” of extinction. When extinctions speed up sharply over a relatively short period, however, a mass extinction may be underway. Many biologists and other scientists are convinced that we are living right now in the middle of a sixth mass extinction.

Biologists have named about 1.8 million species of plants and animals. Probably tens of millions of species, mostly microorganisms, have yet to be discovered and studied.

The World Conservation Union maintains a worldwide “Red List” of at-risk species. The Red List now includes more than 16,000 known species in danger of extinction. Many unknown species, however, may become extinct before we even discover them.

The Red List includes 20 percent of the 6,000 mammal species, including large animals such as giant pandas, Asian elephants, and nearly half of all primates. Also on the Red List are 12 percent of the world’s bird species, and a third of amphibian species, such as certain frogs.

What disturbs biologists most, however, is the increasing rate of the extinctions taking place today. Species are dying off thousands of times faster than the normal “background rate.”

Harvard biologist Edward O. Wilson states that the extinction rate today is the highest ever and is accelerating. He predicts that if the extinction rate continues at its present pace, 20 percent of all plant and animal species are likely to be gone or near extinction by 2030. Half of all species, he warns, could be extinct by the end of the 21st century.

Aside from its unprecedented speed, extinction today has one other feature that makes it different from the “big five.” For the first time, the major short-term shock, triggering the current upsurge of extinctions, appears to be human activity.

Biodiversity and Extinction  

The opposite of species extinction is robust biological diversity (called biodiversity for short). Biodiversity means the widest possible variety of ecosystems like rainforests and deserts, species of plants and animals, and genes among members of a species.

Biodiversity promotes healthy organisms, enables the emergence of new species, and guards against disease and other threats to survival. When biodiversity collapses, as may be underway today, a mass extinction is the catastrophic result.

Biodiversity not only maintains species of plants and animals but humans, too. Stable and productive ecosystems with many healthy species provide what biologists call “ecosystem services” such as:

• regulating the world’s climate

• purifying air and water

• forming and enriching soil for farming

• eliminating toxic substances

• pollinating crops

• preventing erosion

About 100 cultivated plant species provide 90 percent of the world’s food supply for humans. In addition, scientists use the genetic diversity of wild plants to produce hybrids that improve food crop yields and resist pests and disease.

In the U.S., 40–50 percent of all prescription drugs contain substances first discovered in plants, animals, and microorganisms. The most famous is aspirin, originally developed from the bark of a willow tree.

Other examples include antibiotics, anti-malaria medicine, blood thinners, drugs to treat HIV-AIDS, and chemicals to stop organ transplant rejection. “Bioprospectors” discovered a powerful anti-cancer substance in the rosy periwinkle, a plant found only in Madagascar’s disappearing tropical rainforest.

Plant and animal species in a biodiverse ecosystem depend on each other for food, shelter, their reproductive cycle, and other needs. When one species becomes extinct, others may follow in an accelerating cascade. Eventually, entire ecosystems with their many species and gene pools are doomed to extinction. The quickening rate of species extinctions today puts biodiversity at risk throughout the world.

Humans began to cultivate crops and domesticate animals on a large scale about 8,000 years ago. People began to cut down forests and clear grasslands for farming. They commonly cleared wild areas by burning the plant cover, called “slash and burn” farming, which released carbon dioxide and other gases into the atmosphere. Scientists say the increase of such gases in the atmosphere turned around what should have been another ice age to a warming climate.

The invention of agriculture added greatly to the food supply and caused a boom in the human population. It also, however, eliminated some native plant and animal habitats, leading to more extinctions and less biodiversity.

The Industrial Revolution in Europe in the mid-1700s caused a rapid growth in the burning of fossil fuels like coal. As our energy demands have increased, scientists have observed a sharp increase in global warming. This climate change may cause the extinction of species unable to adapt.

Meanwhile, the human population exploded. Two-thousand years ago, there may have been 300 million humans on Earth. By 1800, there were about 1 billion. In 2000, the world’s population reached 6 billion, and it may rise to 10 billion by 2100.

The Human Impact Today

As the human population continues to grow, biodiversity is shrinking and the rate of plant and animal extinction is accelerating.

Habitat Damage and Loss

Today, farms, factories, cities, roads, dams, suburban housing, mines, and logging operations help to make up the “human footprint” on the natural environment. As this footprint gets bigger, the habitats of plants and animals get smaller. For example, the extent of the world’s forests peaked about 8,000 years ago, before the invention of agriculture. Only half of these forests remain today. Farmland, roads, and development fragment many of them into isolated “inland islands” too small to sustain some native species.

Invasive Species

Invasive species are plants and animals that enter an area where they did not naturally evolve. Invasives may take over the habitat and resources used by native species and eventually force them to extinction.

Humans themselves are the ultimate invasive species. But humans have also been the main carriers of invasive plants and animals, either on purpose or accidentally. Farmers introduced the Asian kudzu vine into U.S. Southern states to control erosion. The vine, which can grow a foot a day, was impossible to manage and took over the countryside, ruining the habitats of many native plant and animal species.


The damage to plants and animals caused by pollution has increased with the expansion of agriculture and industry. Some of most important sources of environmental pollution have been automobile exhaust, pesticides, farm fertilizer runoff, acid rain fallout from industry smokestacks, oil and sewage spills in the ocean, and fresh-water chemical dumping.

Some pollutants are long lasting in the environment. They may also move up the food chain, affecting the health and survival of many species.

Overharvesting and Poaching

Overharvesting involves hunting or fishing a species to extinction. This almost happened to the American bison in the late 1800s. Only a few hundred North American right whales, an overhunted marine mammal, survive today. Biologists count them among the “living dead” since there is little chance that they will avoid extinction.

Poaching is illegal hunting or fishing. In some parts of the world, people hunt endangered animals for “bushmeat” or for a valuable part of their body. Poachers have long killed elephants just for the ivory of their tusks. There is still a thriving market in Asian countries for powdered rhinoceros horn, which many people erroneously believe increases their sexual potency.

Global Warming

Most of the global warming (also called “climate change”) occurring today has resulted from the burning of fossil fuels like coal, oil, and natural gas. When these fuels burn, gases such as carbon dioxide, methane, and nitrous oxide escape into the atmosphere and trap heat from the sun like a glass greenhouse. Average annual global temperatures over the past decade are among the 12 warmest since 1850. They are also rising faster than previously predicted.

Although political debates surround global warming, nearly all climate scientists in the world agree that global warming today is mainly due to the greenhouse gas effect. The main culprit among the greenhouse gases is carbon dioxide (CO2), which the U.S. government recently classified as a form of pollution. CO2 makes up 56.6 percent of all greenhouse gas emissions and already exceeds what scientists think should be its maximum level in the atmosphere.

Global warming will force many heat sensitive plants and animals to seek a cooler climate northward or southward toward the poles or in higher altitudes. Some plants may not be able to migrate fast enough. Migrating animals may become invasive and replace native species. Other animals, like polar bears, will have nowhere to go.

The Intergovernmental Panel on Climate Change, an organization of world scientists, reports on the causes and projected impacts of global warming. The panel’s latest assessment report (2007), states that up to 30 percent of species will be at increasing risk of extinction if the global average temperature increases from 1–4 degrees Celsius (1.8–7.2 degrees Fahrenheit) relative to 1980–99. If the average temperature exceeds 40C, “significant extinctions around the globe” will occur.

‘Hot Spots’ and Endangered Species

“Hot Spots” are ecosystems of the world that contain the richest biodiversity and largest numbers of species found nowhere else. They are also under the greatest threats of species extinction.

On land, most hot spots are located in tropical rainforests, home to more than half the world’s plant and animal species. Thousands of square miles of rainforests along with their plant and animal habitats are lost each year to logging, farming, ranching, mining, and similar human activities.

The rainforest of the large island of Madagascar off the coast of East Africa had perhaps the most biodiverse environment anywhere in the world before humans arrived about 1,500 years ago. Today, Madagascar is a hot spot that has lost 80 percent of its rainforest, mainly to “slash and burn” farming.

Most people would probably never guess that biologists sometimes call lush tropical Hawaii “America’s extinction capital.” More than 70 species of native birds have gone extinct since humans first landed about A.D. 330. Today, this hot spot has more native bird species at risk of extinction than anywhere else in the U.S.

Hawaiian birds have especially suffered from invasive species of birds and mammals such as wild pigs, introduced into the islands by humans. Native birds are vulnerable to diseases carried by mosquitoes, another invasive species. To avoid the mosquitos, birds have moved to higher elevations. But global warming has allowed mosquitos to live in higher altitudes, forcing the native birds into smaller habitats at even higher altitudes where mosquitoes cannot live.

Coral reefs are the rainforests of the ocean. Corals are marine organisms that build the reefs, usually in tropical waters. Already, the world has lost about 20 percent of its coral reefs. The Coral Triangle, located in the ocean waters of Southeast Asia, is the world’s center of marine biodiversity and an endangered species hot spot. Covering an area half the size of the U.S., these reefs provide cover and feeding areas for 3,000 fish species plus whales, dolphins, sea turtles, and other marine animals.

Although tsunamis, overharvesting, fishing with explosives, pollution, and urban development have damaged or destroyed many coral reefs, the chief threat today is “coral bleaching.” This kills corals when the water temperature increases due to global warming.

In the Arctic, sea ice is a key part of the habitat of polar bears. Sea ice is rapidly shrinking each year because of global warming. Polar bears depend on the ice as a platform to hunt seals and other marine animals. The bears also use the ice to migrate to land for mating.

The loss of sea ice means polar bears will eventually starve and fail to reproduce. In 2007, the U.S. Geological Survey (USGS) projected that two-thirds of the world’s 25,000 polar bears, including all in Alaska, could be extinct by 2050. The USGS warned that by 2100 all polar bears may be extinct if greenhouse gas emissions continue at the current level.

Preventing the ‘Sixth Mass Extinction’

The Endangered Species Act (ESA), signed by President Richard Nixon in 1973, was one of the earliest efforts by the U.S. to save species in danger of extinction. The ESA lists species at risk as “endangered” or “threatened,” and identifies areas of “critical habitat.” The ESA prohibits the killing, harming, or damaging of listed species and habitats.

The ESA also requires recovery plans to increase the numbers of the endangered and threatened species so they no longer need to be on the list. As of August 2008, the government has delisted about 20 species, including the bald eagle and grizzly bear, due to their recovery. More than 1,300 other species remain on the list.

The U.S. is currently debating how best to slow down global warming. Some propose a carbon tax on emitters of C02 such as coal power plants. Others want to increase required miles-per-gallon standards for cars and trucks or boost taxes on the sale of gasoline to reduce driving.

Another approach involves a “cap and trade” system. Companies would be required to get government permits and credits, limiting them to specific amounts of greenhouse gas emissions (the “cap”). Those that exceed their cap would have to buy credits from other companies that emitted less than their cap (the “trade”).

Those calling themselves “green conservatives” reject taxes and cap and trade because they would make driving, electricity use, and many consumer products more expensive. Conservatives favor such approaches as developing clean coal technology, building more nuclear power plants, and granting tax credits to auto companies for the cost of switching to hybrids and hydrogen cars.

Biologist Edward O. Wilson argues that we should focus our efforts to avoid another mass extinction by preserving biodiversity. Doing this, he says, would force us to find workable ways for saving the world’s hot spots, protecting habitats, and slowing down global warming. For example, he suggests making conservation profitable to local economies by encouraging “ecotourism” that takes advantage of healthy biodiverse environments.

Preserving biodiversity and preventing plant and animal extinctions will require changes in how we currently use the environment. For example, the U.S. Forest Service issued a “Travel Management Rule” in 2005 that restricts the use of motorized vehicles to certain trails, roads, and areas in federal forests and grasslands.

The Forest Service rule regulates off-highway vehicles (OHVs), which may include dune buggies, motorcycles, ATVs (all-terrain vehicles), snowmobiles, and jet skis. The rule considers national forests and grasslands closed to motorized use unless posted notices permit specific types of vehicles. One of the reasons the Forest Service introduced this rule was because the increased popularity of off-roading was causing damage and pollution to natural areas and habitats.

The current loss of biodiversity and upsurge of extinctions is complex and affects every area of the world from rainforests and coral reefs to the Arctic and our own backyards. The question is: Do we have the will to make the difficult decisions necessary to save endangered species and ourselves along with them?

For Discussion and Writing

1.  What evidence do scientists present that Earth is headed for a sixth mass extinction?

2.  The U. S. Fish and Wildlife Service lists all endangered species by state at Go to this site and click on your state. A list of endangered (E) and threatened (T) species in your state will appear. Clicking on the scientific name of one of the species will bring up information about it. What information can you find about the status of this species in its Recovery Plan?

3.  What do you think is the most important action the U.S. should be taking right now to improve biodiversity and reduce plant and animal extinctions? Defend your choice.

For Further Reading

Walsh, Bryan. “The New Age of Extinction.” Time. 13 April 2009:43–50.

Wilson, Edward O. The Future of Life. New York: Vintage Books, 2002.


Restricting Vehicles and Bicycles on Public Land

Form small groups. In your groups, do the following:

A.  Discuss whether to support or oppose the following proposed rules to restrict the use of street vehicles, OHVs, and bicycles on public land. Public land includes national and state forests, grasslands, parks, deserts, marshlands, ocean beaches, and fresh water lakes and rivers. You may change a rule to make it more acceptable to the group.

1.  Hunters may not use motorized vehicles off-road to retrieve game.

2.  Motorized vehicles may not be used off-road for camping, fishing, or picnicking.

3.  OHVs are prohibited on beach or desert sand dunes.

4.  OHVs must be equipped with air pollution devices that meet the same standards as automobiles.

5.  Bicycle use is prohibited for cross-country travel or in areas likely to cause significant soil erosion, trail damage, or disturbance of wildlife or their habitats.

6.  Motor vehicles and bicycles may only be used on roads, trails, or other areas posted for their use.

B.  Write another rule to regulate the use of OHVs or bicycles on public land.

C.  Report and defend your conclusions on the six rules and the one written by your group to the rest of the class.



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