Extinction, Causes of

Extinction, Causes of Richard B Primack, Boston University, Boston, MA, USA Rachel A Morrison, Institution of Oceanography, La Jolla, CA, USA Published by Elsevier Inc. This article is a revision of the previous edition article by Richard B. Primack, volume 2, pp 697–713, r 2001, Elsevier Inc.

Glossary

Habitat fragmentation Process by which a continuous area of habitat is divided into two or more fragments by roads, farms, fences, logging, and other human activities. Overexploitation Harvesting of a natural resource, such as fish or timber, at a rate more rapidly than it can be naturally replenished. Water pollution Lowering of water quality due to input of sewage, pesticides, agricultural runoff, and industrial wastes that can harm aquatic plants and animals.

Air pollution Lowering of air quality due to release of toxic materials by factories, automobiles, fires, and other human activities. Disease Infections by parasitic organisms that can cause weakness, decreased reproduction, and death. Exotic species Species that occurs outside its natural range owing directly or indirectly to human activity. Global climate change Current and predicted changes in global temperature, rainfall, and other aspects of climate due to increased human production of carbon dioxide and other greenhouse gases.

An Expanding Human Population

If species and natural communities are adapted to local environmental conditions, why should they be faced with extinction? Shouldn’t species and communities be able to persist in the same places that they have for thousands of years? Why are species going extinct now? The answers to these questions have become clear in recent decades: Massive disturbances caused by people have altered, degraded, and destroyed the natural landscape on a vast scale, drastically upsetting the balance between natural rates of speciation and extinction. Extinction rates have been increasing since 1650, and we are now in the midst of a sixth mass extinction episode, this one caused by human activities rather than by a natural disaster. The process of evolution will eventually create new species, but this will take thousands, if not millions, of years. And numerous unique species such as pandas, elephants, and cheetahs will be gone forever.

Mammals

The major human-induced threats to biological diversity are habitat destruction, habitat fragmentation, habitat degradation (including air and water pollution), overexploitation of species for human use, exotic species introductions, and increased spread of disease (Figure 1). Most threatened species face at least two or more of these threats, speeding their way toward extinction and hindering conservation efforts. Typically, these threats develop so rapidly and on such a large scale that species are not able to adapt genetically to the changes or to disperse to a more hospitable location. These threats will continue to increase as the human population grows, development and overexploitation continue, remaining natural habitats disappear, and as global climate continues to change. Amphibians

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Percent of threatened species affected Figure 1 Habitat loss and degradation are the greatest threats to the world’s species, followed by overexploitation. Groups of species face different threats; birds are more threatened by invasive species, whereas amphibians are more affected by disease and pollution. Percentages add up to more than 100% because species often face multiple threats. Reproduced with permission from Primack RB (2010) Essentials of Conservation Biology, 5th edn. Sunderland, MA: Sinauer Associates; based on data from the International Union for Conservation of Nature (IUCN), 2004.

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These threats to biological diversity are all caused by an everincreasing use of the world’s natural resources by an expanding human population. In Europe, only 15% of the land area remains unmodified by human activities, and the amount of many specific habitat types remaining is below 10%. The greatest destruction of biological communities has occurred during the last 150 years, during which the human population exploded from 1 billion in 1850 to 2 billion in 1930, and to 6 billion in 2001. World population will reach an estimated 7 billion by the end of 2011 and 10 billion by 2050 (Figure 2). Birth rates have remained high whereas mortality rates have declined, particularly during the last century, as a result of both medical discoveries (specifically, disease control) and more reliable food supplies. Population growth has slowed in industrialized countries but is still high in many areas of tropical Africa, Latin America, and Asia, where the greatest biological diversity is also found. The increase in human population is predicted to cause an additional 14% of bird and mammal species to be threatened with extinction by 2050. People use natural resources, such as fuelwood, wild meat, and wild plants, and convert vast amounts of natural habitat for agricultural and residential purposes. Agricultural systems now occupy one-fourth of the Earth’s land surface. Because some degree of resource use is inevitable, population growth is partially responsible for the loss of biological diversity. All else being equal, more people equals less biodiversity. Some scientists have argued strongly that controlling human population

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size is the key to protecting biological diversity; however, overconsumption of resources is also responsible for habitat destruction and species extinctions. The rise of industrial capitalism and materialistic modern societies has greatly accelerated demands for natural resources, particularly in developed countries. Inefficient and wasteful use and overconsumption of natural resources are major causes for the decline in biological diversity. For example, if fewer paper products were used and more paper was recycled, then there would be less need to cut down forests to manufacture new paper.

Unequal Use of Natural Resources In many countries there is extreme inequality in the distribution of wealth, with a small percentage of the population controlling, owning, and consuming much of the wealth and natural resources such as good farmland, livestock, and timber resources. In many developing countries, large landowners and business interests commonly force local farmers off their land, a pattern often reinforced by the government, the police, and the army. Political instability, lawlessness, and war also displace farmers into remote, undeveloped areas where they feel safer. Most practice shifting cultivation, a form of agriculture involving cutting down forest, burning the plant material, and planting crops in the nutrient-rich ash, because it is the simplest way to make a living when they may be obliged to move again within a short time. Landless farmers and their families often exploit natural resources in their surroundings just to stay alive; often these resources are components of species-rich biological communities. The unequal use of natural resources worldwide also drives the destruction of biological diversity in species-rich tropical areas. People in industrialized countries (and the wealthy minority in developing countries) consume a disproportionate share of the world’s energy, minerals, wood products, and food. Each year, the US, which has 5% of the world’s population, consumes roughly 25% of the world’s natural resources. The average US citizen annually uses 23 times more energy and 79 times more paper products than does the average citizen of India. The excessive consumption of wealthy individuals, cities, and countries thus leaves a large ecological footprint, meaning that a wide area of the world must supply their needs. This unsustainable resource consumption will cause massive environmental disruption if adopted by the expanding middle class in the developing world.

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Figure 2 The human population in 2011 is almost 7 billion. The World Resources Institute estimates current annual population growth at 1.1%, but even this modest growth rate will add more than 72 million people to the planet in the next year. This number will escalate each year as the increase is compounded. Reproduced from Primack RB (2010) Essentials of Conservation Biology, 5th edn. Sunderland, MA: Sinauer Associates; based on data from the US Census Bureau.

In many cases, the factors causing habitat destruction, particularly in the developing world, are the large industrial and commercial activities associated with a global economy – mining, cattle ranching, commercial fishing, forestry, plantation agriculture, manufacturing, and dam construction – and initiated with the goal of making a profit. Many of these projects are funded by national governments and international development banks and are touted as sources of jobs, commodities, and tax revenues. Others are initiated and funded by large multinational corporations. However, this exploitation of natural resources often is neither efficient nor cost-effective because

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the emphasis is on short-term gain, often at the expense of long-term sustainability and generally with little regard for the local people who depend on the resources.

Habitat Destruction Increasing human populations and their activities use even greater proportions of the world’s terrestrial and marine environments and associated natural resources, resulting in the inevitable destruction of genetic variation, species, habitats, and ecosystem processes.

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it is too soon to say how the recent civil war in the latter country has harmed its wildlife population. In the Mediterranean region which has been densely populated for thousands of years, only 10% of the original forest remains. Present rates of deforestation vary considerably among countries, with particularly high annual rates of 1.5–2% for tropical countries such as Vietnam, Coˆte d’lvoire, Mexico, Paraguay, and Costa Rica. For many important species, the majority of habitat in their original range have been destroyed, and a very little of their remaining habitat is protected. For certain Asian primates, such as the Javan gibbon, more than 95% of the original habitat has been destroyed. The orangutan, a great ape that lives in Sumatra and Borneo, has lost 63% of its habitat and is protected in only 2% of its range. Such habitat losses inevitably lead to extinctions.

Habitat Loss Habitat loss is the primary threat to the majority of vertebrate species currently facing extinction, a generalization that is certain to be true for threatened invertebrates, plants, and fungi as well. In many countries of the world, particularly on islands and in locations where human population density is high, most of the original habitat has been destroyed. Agriculture, commercial development, water projects, livestock grazing, pollution, infrastructure and roads, logging, and outdoor recreation threaten habitats of endangered species. More than 50% of the habitat has been destroyed in 49 of 61 Old World tropical countries. In tropical Asia, 65% of the primary forest habitat has been lost, with particularly high rates of destruction reported for Bangladesh (96%), Sri Lanka (86%), India (78%), and Vietnam (76%). Similarly, subSaharan Africa has lost about 65% of its forests, with losses most severe in Gambia (89%), Ghana (82%), and Rwanda (80%). Two biologically rich nations, Zimbabwe and the Democratic Republic of Congo (formerly Zaire), are relatively better off with about half of their forests remaining, although

Rain Forest Loss Tropical rain forests occupy just 7% of the Earth’s land surface but are estimated to contain over 50% of its species. Their destruction has therefore come to be synonymous with the loss of species. These evergreen to partly evergreen forests occur in frost-free areas below about 1800 m in altitude and experience at least 100 mm (4 in) of rain per month in most years. Rain forests are characterized by great species richness and complex species interactions and specializations unparalleled in any other community. Based on current patterns of rainfall and temperature, the original extent of tropical rain forests and related moist forests has been estimated at 16 million km2. Only 11.5 million km2 remained in 1990, and an additional 2.4% was lost between 2000 and 2005 (Figure 3). Most rain forest destruction results from small-scale agriculture and firewood collection, mostly by poor farmers who have moved to forest areas to practice shifting cultivation out of desperation and poverty. Other major causes include

Remaining tropical forest Cleared tropical forest Other areas designated as hotspots

Figure 3 This Fuller projection, which distorts the sizes and shapes of continents less than typical maps do, shows the current extent of tropical forests and areas that have been cleared. Note the extensive amount of deforested land in northern and southeastern South America, India, Southeast Asia, Madagascar, and western Africa. Reproduced from Primack RB (2010) Essentials of Conservation Biology, 5th edn. Sunderland, MA: Sinauer Associates, and Pimm SL and Jenkins C (2005) Sustaining the variety of life. Scientific American 293(33): 66–73, with permission from Sinauer Associates.

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clearing for commercial logging, cattle ranches, cash crop plantations (oil palm, cocoa, rubber, etc.), road building, and mining. At the current rate of destruction (approximately 140,000 km2 per year), there will be no large blocks of tropical forest left after the year 2040, except in the relatively small national parks and protected areas and a few remote areas of the Brazilian Amazon, central Africa, and the islands of Borneo and Papua New Guinea (Figure 4). The situation is actually grimmer than these projections indicate because the world’s population is still increasing, and poverty is rising in many developing tropical countries, putting ever-greater demands on the dwindling supply of rain forest.

Other Threatened Habitats The plight of the tropical rain forests is perhaps the most widely publicized case of habitat destruction, but other habitats are also in grave danger.

Tropical Deciduous Forests Tropical deciduous forests contain many species and in some places rival the diversity of the tropical rain forests. The land occupied by tropical deciduous forests is more suitable for agriculture and cattle ranching than the land occupied by tropical rain forests. Moderate seasonal rainfall, in the range of 250–2000 mm per year, allows nutrients to be retained in the soil, where they can be taken up by plants. These forests are also easier to clear and burn than rain forests. Consequently, human population density is five times greater in dry forest areas of Central America than in adjacent rain forests. Today, the Pacific coast of Central America has less than 2% of its original extent of deciduous dry forest remaining, and many species have been lost or are threatened with extinction.

Grasslands Temperate grasslands have also been almost completely destroyed by human activity, with a consequent loss of species. It is relatively easy to convert large areas of grassland to farmland and cattle ranches. Illinois and Indiana, for example, originally contained 15 million ha (37 million acres) of tall-grass prairie, but now only 1400 ha (3500 acres) of this habitat – one-ten-thousandth of the original area – remain undisturbed. The rest have been converted to farmland, and the remaining area of prairie is fragmented and widely scattered across the landscape. Though widespread efforts to restore prairies in many areas of the world are underway and should be encouraged, it is impossible to bring back species that have already been lost.

Wetlands and Aquatic Habitats Wetlands are critical habitats for fish, aquatic invertebrates, aquatic plants, and birds. They also provide services to humans such as flood control, drinking water, and power production. Wetlands are often filled in or drained for development, or they are altered by channelization of watercourses, dams, chemical pollution, and siltation. This habitat destruction increases extinction risk for aquatic species, especially one with limited distributions. During the last 200 years, over half of the wetlands in the US have been destroyed. The

Florida Everglades, a premiere wildlife refuge, is now on the verge of ecological collapse. Approximately 40–50% of the freshwater snail species in the southeastern US are either extinct or endangered. In California’s San Diego County, more than 97% of the vernal pools, which support a unique endemic biota, have been destroyed. Furthermore, massive development projects in other parts of the industrialized world, such as the Three Gorges Dam in China, are destroying aquatic ecosystems on an unprecedented scale.

Mangroves Mangrove forests are among the most important wetland communities in tropical areas. Composed of species that are among the few saltwater-tolerant woody plants, mangrove forests occupy coastal areas with saline or brackish water, typically where there are muddy bottoms. Such habitats are similar to salt marshes in the temperate zone. Mangroves are extremely important breeding grounds and feeding areas for shrimp and fish. They also reduce storm damage. Despite their great economic value, mangroves are often harvested for timber and charcoal production and cleared for development. In recent years, mangroves have been increasingly cleared for rice cultivation and commercial shrimp hatcheries, particularly in Southeast Asia, where as much as 15% of the mangrove area have been removed for aquaculture. Over 35% of the world’s mangrove ecosystems have been destroyed, and 40% of vertebrates endemic to mangroves are threatened with extinction.

Coral Reefs Tropical coral reefs contain an estimated one-third of the ocean’s fish species in only 1% of its surface area. Already 20% of all coral reefs have been destroyed, and an additional 20% have been degraded. The most severe destruction is taking place in the Philippines, where a staggering 90% of reefs are dead or dying. The main culprits are pollution, which either kills the coral directly or allows excessive growth of algae; sedimentation following the removal of forests; overharvesting of fish, clams, and other animals; climate change; invasive species; and, finally, fishermen blasting with dynamite and releasing cyanide and other poisons to collect the few remaining living creatures. Extensive loss of coral reefs is expected within the next 40 years in tropical East Asia, around Madagascar and East Africa, and throughout the Caribbean (Figure 4). In the Caribbean, a combination of overfishing, hurricane damage, water pollution, and disease is responsible for a dramatic decline of a large proportion of the coral reefs and their replacement by fleshy macroalgae. Elkhorn and staghorn corals, which were formerly common and gave structure to the community, have already become rare in many locations.

Desertification Many biological communities in seasonally dry climates are degraded into man-made deserts by human activities, a process known as desertification. These communities include tropical grasslands, scrub, and deciduous forests, as well as temperate shrublands, such as those found in the Mediterranean region, southwestern Australia, South Africa, central

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Stable Figure 4 Extensive areas of coral will be damaged or destroyed by human activity over the next 40 years unless conservation measures can be implemented. Reproduced from Primack RB (2010) Essentials of Conservation Biology, 5th edn. Sunderland, MA: Sinauer Associates, and Bryant D, Burke L, McManus J, and Spalding M (1998) Reefs at Risk: A Map-Based Indicator of Threats to the World’s Coral Reefs. Washington, DC: World Resources Institute, with permission from Sinauer Associates.

Chile, and California. Approximately 10–20% of these dry areas are degraded, with more than 25% of the productive capacity of their plant growth lost. Although these areas may initially support agriculture, repeated cultivation, especially during dry and windy years, often leads to erosion and loss of the soil’s capacity to retain water. Land may also be chronically overgrazed by domestic livestock, such as cattle, sheep, and goats, and woody plants may be cut down for fuel. The result is a progressive and largely irreversible degradation of the biological community and loss of soil cover. Ultimately, the original species are lost, and the region takes the appearance of a desert. The process of desertification is most severe in the Sahel region of Africa, which is estimated to have 2.5 times more people than the land can sustainably support.

Habitat Fragmentation In addition to outright destruction, habitats that formerly occupied wide, unbroken areas are now often divided into two or more fragments by roads, fields, farms, houses, industries, fences, power lines, and a broad range of other human constructs and activities. Fragmentation almost always occurs during a severe reduction in habitat area, but it can also occur when area is reduced to only a minor degree wherein the original habitat is divided by railroads, canals, fences, oil pipelines, fire lanes, or other barriers to the free movement of species. The island model of biogeography is applicable to this situation: the fragments, often isolated from one another by a highly modified or degraded landscape (Figure 5), may be considered as habitat islands in an inhospitable humandominated sea. Fragments differ from the original habitat in three important ways: (1) fragments have a greater amount of edge for the area of habitat, (2) the center of each habitat fragment is closer to an edge, and (3) a formerly continuous habitat hosting large populations is divided into pieces, with smaller populations.

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Forest Figure 5 The forests of tropical Asia have experienced massive deforestation and fragmentation in recent decades. Intense habitat destruction in Sumatra, a large island of Indonesia, has occurred over the past 100 years and was predicted to continue through 2010. Reproduced from Primack RB (2010) Essentials of Conservation Biology, 5th edn. Sunderland, MA: Sinauer Associates, and Bradshaw CJA, Sodhi NS, and Brook BW (2009) Tropical turmoil: A biodiversity tragedy in progress. Frontiers in Ecology and the Environment 7: 79–87, with permission from Sinauer Associates.

Barriers to Dispersal Fragmentation may limit a species’ potential for dispersal and colonization. Many bird, mammal, and insect species of the forest interior will not cross even very short distances of open area, partly due to the high risk of predation in edge and open habitats. Habitat fragmentation also creates barriers to normal dispersal and the colonization processes. In an undisturbed environment, seeds, spores, and animals move passively and actively across the landscape. When they arrive in a suitable but unoccupied area, a new population begins to develop at that site. Over time, population of a species may build up and go extinct on a local scale as the species disperses from one suitable site to another and the biological community undergoes succession. Because habitat fragmentation limits this dispersal, the species may gradually die out. Habitat fragmentation also reduces the foraging ability of individual animals. Many species, either as individuals or

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social groups, need to move freely across the landscape to feed on widely scattered resources. However, fences and other barriers may prevent the natural migration of animals, such as wildebeest or bison, forcing them to overgraze an unsuitable habitat, eventually leading to starvation and habitat degradation. Barriers to dispersal can restrict the ability of widely scattered species to find mates, causing a loss of reproductive potential for many animal species. Plants also may reduce seed production if butterflies and bees are less able to migrate among habitat fragments to pollinate flowers. Finally, dividing a formerly large population into several smaller ones means that the new, smaller populations will be more vulnerable to genetic and other problems associated with small populations, increasing their risk of extinction.

Edge Effects Habitat fragmentation often changes the microenvironment at the fragment edge, resulting in increased light levels, higher daytime temperatures, higher wind speeds, and lower humidity. Each of these edge effects can have a significant impact on the vitality and composition of the species in the fragment. Species that are sensitive to humidity such as amphibians, many insects, and herbaceous plants, will be eliminated from the forest fragments. Also, increased wind, lower humidity, and higher daytime temperatures make fires more likely in forest fragments. Fires may spread into habitat fragments from nearby agricultural fields that are being burned regularly, as in sugarcane harvesting, or from the irregular activities of farmers practicing shifting cultivation. In the process, many species will be eliminated.

Interspecific Interactions Habitat fragmentation also increases the vulnerability of the fragment to invasion by exotic and native pest species. Omnivorous native animals, such as raccoons, skunks, and blue jays, and introduced animals, such as rats, may increase in population size along forest edges, where they can eat food found in both undisturbed and disturbed habitats. These aggressive feeders may seek out the nests of interior forest birds, often preventing successful reproduction of many bird species hundreds of meters from the nearest forest edge. Domestic cats can also be important predators in settled areas with fragmented landscapes. Weedy plant species and exotic herbivores can eliminate native plant species along the edges of fragments, and disease can similarly spread into the interior of habitat fragments.

ground fires might not kill the mature trees, but the rich perennial wildflower community and insect fauna on the forest floor would gradually become impoverished. Keeping too many cattle in grassland communities gradually changes the biological community, often eliminating many native species and favoring exotic species that can tolerate grazing. Boat hulls, anchors, and divers’ flippers can crush fragile species on coral reefs. Trawling destroys 15 million km2 of seafloor communities each year, 150 times greater than the area of forest cleared annually. The subtlest form of environmental degradation is pollution, commonly caused by pesticides, sewage, fertilizer runoff from agricultural fields, industrial chemicals, and wastes, emissions from factories and cars, and sediment deposits from eroded hillsides. The general effects of pollution on water quality, air quality, and even global climate are causes for great concern, not only because of the threats to biological diversity, but also because of their effects on human health.

Pesticides The dangers of pesticides were brought to world’s attention in 1962 by Rachel Carson’s influential book Silent Spring. Carson described a process, now known as biomagnification, through which (dichlorodiphenyltrichloroethane DDT) and other organochlorine pesticides become concentrated as they ascend the food chain. These pesticides, at that time widely used to kill agricultural pests and mosquito larvae were harming wildlife populations, especially birds that ate large amounts of insects, fish, or other animals exposed to DDT and its byproducts. Birds with high levels of pesticides in their tissues, particularly raptors such as hawks and eagles, became weak and tended to lay eggs with abnormally thin shells that cracked during incubation. As a result of failure to raise young and the outright death of many adults, populations of these birds showed dramatic declines throughout the world. Recognition of this situation in the 1970s led many industrialized countries to ban the use of DDT and other chemically related pesticides. The ban eventually allowed the partial recovery of many bird populations, most notably peregrine falcons (Falco peregrinus), ospreys (Pandion haliaetus), and bald eagles (Haliaeetus leucocephalus). Nevertheless, the continuing massive use of pesticides and DDT in other countries is still a cause for concern, not only for endangered animal species, but also for the potential long-term effects on people, particularly the workers who handle these chemicals in the field and the consumers of agricultural products treated with these chemicals. These chemicals are widely dispersed in the air and water and are persistent in the environment even in countries where their use has been outlawed.

Habitat Degradation and Pollution Communities and species in a habitat can be profoundly affected by human activities, even if the habitat itself is not. Biological communities can be damaged and species driven locally or globally extinct by external factors that do not change the structure of dominant plants in the community, so that the damage is not immediately apparent. For example, in temperate deciduous forests, frequent and uncontrolled

Water Pollution Water pollution destroys important food sources and contaminates drinking water with chemicals that can cause immediate and long-term harm to human health. Water pollution also often severely damages aquatic ecosystems. Rivers, lakes, and oceans are used as open sewers for industrial and residential waste. Pesticides, herbicides, oil products, heavy

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metals (such as mercury, lead, and zinc), detergents, and industrial wastes can kill aquatic organisms outright or make the environment so inhospitable that species can no longer thrive. For instance, water pollution is a threat to 90% of the endangered fishes and freshwater mussels in the US. Unlike terrestrial dumps, whose effects are primarily local, toxic wastes in aquatic environments diffuse over a wide area. Many aquatic environments are naturally low in essential minerals, such as nitrates and phosphates, and aquatic species have adapted to the natural scarcity of minerals by developing the ability to process large volumes of water and to concentrate these minerals. When these species process polluted water, they concentrate toxic chemicals along with the essential minerals, which can eventually poison the plant or animal. Species that feed on these aquatic species then ingest these high concentrations of toxic chemicals. Essential minerals that are beneficial to plant and animal life can become harmful pollutants at higher levels. Human sewage, agricultural fertilizers, detergents, and industrial processes often release large amounts of nitrates and phosphates into aquatic systems, causing cultural eutrophication. For instance, humans release as much nitrate into the environment as do all natural processes, and this input is expected to increase in tandem with the increasing human population. Even small amounts of these nutrients can stimulate plant and animal growth, and high concentrations often result in thick blooms of algae at the surface of ponds, lakes, and coastal areas. These algal blooms may be so dense that they outcompete with other plankton species and shade out bottomdwelling plant species. As the algal mat becomes thicker, its lower layers die and sink. Bacteria and fungi then decompose the dying algae, absorbing all the oxygen in the water. Without oxygen, much of the remaining animal life dies off, sometimes visibly in the form of masses of dead fish floating on the water surface. The result is a greatly impoverished and simplified community consisting of only those species tolerant of polluted water and low oxygen levels. The spreading dead zone where the Mississippi River enters the Gulf of Mexico is an example of the direct consequences of water pollution. Such dead zones are increasing in number and size around the world as a result of human activities.

Air Pollution In the past, people assumed that the atmosphere was so vast that materials released into the air would be widely dispersed and their effects would be minimal. But today, several types of air pollution are so widespread that they damage whole ecosystems.

Acid Rain Acid rain is created when nitrates and sulfates released into the air by the burning of fossil fuels combine with atmospheric water to form acids that fall as rain. Acid rain lowers the pH of soil moisture and water bodies. Increased acidity alone damages many plant and animal species; for instance, acid rain has been blamed for the death of large number of trees in Europe and North America. As the acidity of water bodies increases, many fish either die or fail to spawn (Figure 6). Both

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increased acidity and water pollution are two likely causes of the dramatic decline in amphibian populations throughout the world.

Ozone Depletion and Ultraviolet Radiation As a result of human use of chlorofluorocarbons (CFCs) and other ozone-depleting chemicals, the atmospheric ozone layer has been significantly reduced. Ozone plays an important role in filtering out harmful ultraviolet radiation in sunlight. With less atmospheric ozone, more solar ultraviolet radiation reaches the Earth’s surface. In humans, exposure to this UV radiation increases the risk of skin cancer. This increased UV radiation will also possibly have a significant negative impact on animals and plants exposed to direct sunlight, such as amphibian eggs on the water surface.

Ozone and Smog Cars, power plants, and other industrial activities release hydrocarbons and nitrogen oxides as waste products. In the presence of sunlight, these chemicals react with the atmosphere to produce ozone and other secondary chemicals, collectively called photochemical smog. Although ozone in the upper atmosphere is important in filtering ultraviolet radiation, a high concentration of ozone at ground level damages plant tissues and makes them brittle, harming biological communities and reducing agricultural productivity. Ozone and smog are detrimental to both people and animals when inhaled, so controlling air pollution benefits both humans and biodiversity.

Effects on Lichens Even when communities are not destroyed by air pollution, species composition may be altered as more susceptible species are eliminated. Lichens, symbiotic organisms composed of fungi and algae that can survive in some of the harshest natural environments, are particularly susceptible to air pollution. Because each lichen species has a distinct tolerance to air pollution, the composition of the lichen community can be used as a biological indicator of the level of air pollution.

Global Climate Change Scientists are now intensively studying atmospheric carbon dioxide, methane, and other greenhouse gases that are

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transparent to light but that absorb heat. During the past century, global levels of carbon dioxide (CO2), methane, and other trace gases have been steadily increasing, primarily as a result of burning coal, oil, and natural gas. Clearing forests to create farmland and burning firewood for heating and cooking also contribute to the rising atmospheric concentration of CO2, which has increased from 290 to 387 parts per million (ppm) over the last 100 years and is projected to double in the latter half of this century. Humans currently release about 70 million tons of CO2 into the atmosphere every day.

Climate Change Predictions Many scientists agree that these increased levels of greenhouse gases have affected the world’s climate already, and that these effects will intensify in the future. The best evidence suggests that world climate has warmed by 0.61 Celsius (1C) over the last 100 years. Temperatures at high latitudes have increased even more, by 2–4 1C. Over the last 50 years, the Atlantic, Pacific, and Indian oceans have increased in temperature by 0.06 1C. Even with all of the available weather data, simulation models, and supercomputers, predicting future weather patterns is extremely complex and difficult. However, the consensus among leading scientists is that the global climate will increase in temperature by an additional 2–4 1C by 2100 as a result of increased levels of carbon dioxide and other gases in the atmosphere (Figure 7). The increase could be even greater if carbon dioxide levels rise faster than predicted, but it could also be slightly less if all countries agree to reduce emissions of greenhouse gases. The increase in temperature will be greatest at

high latitudes and over large continents. Many scientists also predict an increase in extreme weather events such as hurricanes, flooding, snowstorms, fires, and regional drought.

Extinctions and Climate Change As a result of global climate change, climatic regions in the northern and southern temperature zones will be shifted toward the poles, forcing species to migrate. It seems likely that many species will be unable to disperse rapidly enough to keep up with the changing climate. Habitat fragmentation caused by human activities may further slow or prevent many species from migrating to new habitats or escaping rising sea levels. Many species with limited distributions and poor dispersal ability will undoubtedly go extinct, with new communities favoring widely distributed and easily dispersed species. Endemic mammals that are restricted to isolated mountain peaks or fish species found in a single lake are examples of species that will not be able to easily cross inhospitable terrain to reach a new habitat. The best hope for many species will be to migrate higher on mountain slopes or to disperse along valleys, rivers, and coastlines that are aligned north to south. Warming waters and rising sea levels are already affecting marine species as well. Concerns about global climate change, as important as they are, should not divert our attention from the massive habitat destruction that is the principle current cause of species extinction. The preservation of intact communities and the restoration of degraded communities are the most important and immediate priorities for conservation.

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Figure 7 Computer models of global climate predict that temperatures will increase significantly when CO2 levels double in the mid- to late part of this century. Predicted temperature increases for 2080–2099 are shown, indicated as the amount of deviation (in 1C) from mean surface temperatures recorded for 1980–1999. Reproduced from Primack RB (2010) Essentials of Conservation Biology, 5th edn. Sunderland, MA: Sinauer Associates, and Intergovernmental Panel on Climate Change (IPCC) (2007) Climate change 2007: The physical science basis. In: Solomon S, Qin D, Manning M, et al. (eds.) Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press, with permission from IPCC.

Extinction, Causes of

Overexploitation Exploitation in Traditional Societies People have always hunted and harvested the food and other resources they need in order to survive. As long as human populations were small and the methods of collection simple, harvesting and hunting were sustainable. In traditional societies, restrictions were often imposed to prevent overexploitation of natural resources. For example, the rights to specific harvesting territories were rigidly controlled; hunting in certain areas was banned; there were often prohibitions against taking females, juveniles, and undersized individuals; certain seasons of the year and times of the day were closed for harvesting; and certain efficient methods of harvesting were not allowed. These restrictions, which allowed traditional societies to exploit communal resources on a long-term, sustainable basis, are almost identical to the rigid fishing restrictions imposed on and proposed for many fisheries in industrialized nations. Among the most highly developed restrictions were those of the traditional or artisanal societies of Micronesia and Polynesia. However, there are also numerous cases of large bird and mammal species being hunted to extinction by traditional people using simple methods of hunting.

Exploitation in Modern Societies As human populations have increased in number and size, their use of the environment has escalated, and harvesting methods have become dramatically more efficient. This has depleted large animals almost completely from many biological communities, leaving strangely empty habitats. In tropical rain forests and savannas, guns are now used instead of blowpipes, spears, or arrows. In the oceans, fish are caught by enormous factory ships and sold in the global market. Even small-scale local fishermen now have outboard motors on

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their canoes and boats, allowing them to harvest from a wider area more rapidly. In much of the world today, resources are exploited opportunistically. If a market exists for a product, local people will search their environment to find and sell it. Whether people are poor and hungry or rich and greedy, they will use whatever methods are available to secure that product. Sometimes traditional groups will sell the rights to a resource, such as a forest or mining area, for money to buy desired goods. In rural areas, traditional controls that strictly regulated the extraction of natural products have generally weakened, if not eliminated by substantial human migration, civil unrest, or war. As a result, species are more easily exploited to the point of extinction.

Trade in Wildlife The legal and illegal wildlife trade is responsible for the decline of many species. Worldwide trade in wildlife is valued at over $10 billion per year, not including edible fish. The fur trade has reduced the chinchilla (Chinchilla spp.), vicun˜a (Vicugna vicugna), giant otter (Pteronura brasiliensis), and numerous cat species to low numbers. Overharvesting of butterflies by insect collectors; of orchids, cacti, and other plants by horticulturists; of marine mollusks by shell collectors; and of tropical fish for aquarium hobbyists are other examples of targeting whole biological communities to supply an enormous international demand (Table 1). For instance, it has been estimated that 350 million tropical fish valued $1 billion are sold worldwide for the aquarium market, and many times that number are killed during collection and shipping. Besides a surprisingly large legal trade, billions of dollars are involved in the illegal trade of wildlife. A black market links poor local people, smugglers, corrupt customs officials, rogue dealers, and wealthy buyers who do not question the sources they buy from. This trade has many of the same

Major groups targeted in the Global Wildlife Trade

Group

Number traded annuallya

Comments

Primates

40,000

Mostly used for biomedical research; also for pets, zoos, circuses, and private collections.

Birds

4 million

Zoos and pets. Mostly perching birds, but also legal and illegal trade of about 80,000 parrots.

Reptiles

640,000

Zoos and pets; also 10–15 million raw skins. Reptile extracts are used in some 50 million manufactured products (mainly from the wild, but increasingly from farms).

Ornamental fishes

350 million

Most saltwater tropical fish come from the wild, caught using illegal methods that damage other wildlife and the surrounding coral reefs.

Reef corals

1000–2000 tons

Reefs are being destructively mined to provide aquarium decor and coral jewelry.

Orchids

9–10 million

Approximately 10% of internationally traded orchids are from the wild. These are sometimes deliberately mislabeled to avoid regulations.

Cacti

7–8 million

Approximately 15% of the traded cacti come from the wild, with smuggling a major problem.

a

Numbers refer to the number of individuals unless otherwise specified. Source: Based on data from World Resources Institute (WRI) (2005) World Rresources 2005: The Wealth of the Poor – Managing Ecosystems to Fight Poverty. Washington, DC: World Resources Institute; Karesh WB, Cook RA, Bennett EL, and Newcomb J (2005) Wildlife trade and global disease emergence. CDC Emerging Infectious Diseases 11: 100–1002.

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characteristics and practices, and sometimes the same criminal players, as the illegal trade in drugs and weapons. Confronting these illegal activities has become a dangerous job for international law enforcement agencies. Clearly, people involved in the illegal trade of wildlife are not concerned about species going extinct, unless it affects their profits.

Overfishing In the North Atlantic, one species after another has been overfished to the point of diminishing returns. The Atlantic bluefin tuna, for example, has experienced a 97% population decline since 1960. Similar grim scenarios can be recounted for other prized large fish, such as the swordfish (Xiphias gladius). One of the most dramatic cases of overexploitation in recent years is related to the booming demand for shark meat and shark fins, caused in large part by the popularity of shark fin soup in Asian restaurants. Shark fishing has become a lucrative alternative to targeting severely depleted commercial fish populations. But many shark populations are now declining dramatically as well, because most species have a relatively slow reproductive cycle. Heavily fished shark populations in the Atlantic, for instance, have declined by 40–99% over the last 20 years, and some species there and elsewhere may soon go extinct. Another striking example is the enormous increase in demand for seahorses (Hippocampus spp.) in China, which is tied to the nation’s economic development. The Chinese use dried seahorses in their traditional medicine because it resembles a dragon and is believed to have a variety of healing powers. Approximately 45 t of seahorses are consumed in China per year – roughly 16 million animals. Seahorse populations throughout the world are being decimated to supply this everincreasing demand, and international trade is now carefully regulated.

Exotic Species Humans have radically altered the patterns of species distributions by deliberately or accidentally transporting species throughout the world. The extent of this modern movement of human-transported species is unprecedented and has been described by Elton (1958) as ‘‘one of the great historical convulsions of the world’s flora and fauna.’’ Many areas of the world are strongly affected by exotic species. For instance, the US currently has over 20 species of exotic mammals, 53 species of exotic reptiles and amphibians, 88 species of exotic molluscs, 97 species of exotic birds, 138 species of exotic fish, 4500 species of exotic insects and other arthropods, and 5000 species of exotic plants. The great majority of exotic species do not become established or dominant because the new environment is not suitable. However, some species do establish themselves in their new homes, and many of these become abundant at the expense of native species. Exotic plants frequently displace native species because they are better suited to the new conditions created by people, such as increased fire and introduced grazing animals. Exotic perennials completely dominate many North American wetlands: purple loosestrife (Lythrum salicaria) from Europe overgrows marshes in eastern North

America, whereas Japanese honeysuckle (Lonicera japonica) forms dense tangles in bottomlands of the southeastern US. Exotic species are also often able to thrive because their populations are not held in check by any of the local parasites or predators. These exotic species may displace native species through competition for limited resources, they may kill and eat native species to the point of extinction, or may alter the habitat so that many natives are no longer able to persist. The effects of exotic insects on the native insect fauna can be devastating. At some localities in the southern US, insect species diversity has declined by 40% following the invasion of exotic fire ants, which attack, consume, or outcompete other insect species. Many bird species have dramatically declined after fire ants entered their habitat, because of direct attack or loss of insect prey. Exotic species are considered the most serious threat facing the biota of the national park system in the US. The effects of habitat degradation, fragmentation, and pollution can potentially be corrected and reversed in a matter of years or decades as long as the original species are present. It may be expensive, difficult, or even impossible, however, to eliminate exotic species that have become abundant, widely dispersed, or thoroughly integrated in the community.

Exotic Species on Islands Because they have evolved in the absence of mainland herbivores and predators, island species are particularly vulnerable to exotic species; the introduction of one exotic species to an island may cause local extinction of numerous native species. On Santa Catalina Island off the coast of California, 48 native plant species have been eliminated, primarily due to grazing by introduced goats, pigs, and deer. One-third of the plant species currently found on the island are exotics. Several native plant species have reappeared after goats were removed from part of the island. Birds of the Pacific are also vulnerable to exotic species. The brown tree snake (Boiga irregularis; Figure 8) has been introduced onto a number of Pacific islands, where it is devastating endemic bird populations by eating eggs, nestlings, and

Figure 8 The brown tree snake (Boiga irregularis) has been introduced onto many Pacific islands, where it devastates populations of endemic birds. This adult snake has just swallowed a bird. (Photograph by Julie Savidge).

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411

adults. On Guam alone, the brown tree snake has driven 10 of 13 endemic bird species extinct. Recent visitors have remarked on the absence of birdsong: ‘‘Between the silence and the cobwebs, the rain forests of Guam have taken on the aura of a tomb’’ (Jaffe, 1994).

Exotic Species in Aquatic Habitats Exotic species can have severe negative effects on vulnerable freshwater communities, particularly lakes and isolated stream systems. Shipping is a major cause of introductions, moving species that are attached to ship hulls or contained in ballast water. There is also a long history of introductions, many accidental, of exotic commercial and sport fish species into lakes. These exotic fish, often larger and more aggressive than the native fish fauna, may eventually drive the local fish species to extinction. Aquatic plants, invertebrates, and disease organisms can also become aggressive exotics outside their normal range. One of the most alarming recent invasions in North America was arrival of the Eurasian zebra mussel (Dreissena polymorpha) in the Great Lakes in 1988. Within two years, zebra mussels had reached densities of 700,000 individuals per square meter in parts of Lake Erie, choking out native mussel species. They have subsequently been found in the Detroit, Cumberland, and Tennessee Rivers. As it spreads south, this exotic species is causing enormous economic damage to fisheries, dams, power plants, water treatment facilities, and boats, as well as devastating the aquatic communities it encounters. Invasions also occur in marine and estuarine systems, with 84% of marine areas worldwide affected by at least one invasive species. In the Mediterranean, the invasive green alga Caulerpa taxifolia is outcompeting native algae species and reducing fish abundance. Because Caulerpa is used as a decorative plant in marine aquariums, this is an example of one negative human impact (the trade in aquarium and ornamental species) resulting in another. One solution would be to ban harmful invasive species from the aquarium trade and provide a list of noninvasive alternatives.

Disease Disease caused by internal parasites is a natural control mechanism that reduces populations when they reach high densities. However, levels of disease can often increase in populations as a result of human activity. When animals are confined to habitat fragments at abnormally high densities, disease may spread more easily among individuals. Also, animals that are stressed or weakened by living in a degraded or polluted environment may be more susceptible to disease. Furthermore, as habitats are fragmented by human activities, disease can spread more easily from domestic animals into wild populations. At Tanzania’s Serengeti National Park, at least 25% of the lions (Panthera leo) have recently been killed by canine distemper, a viral disease apparently contracted from one or more of the 30,000 domestic dogs living near the park. For endangered species, such outbreaks can do phenomenal harm: The last population of black-footed ferrets

Figure 9 Populations of flowering dogwood (Cornus florida) are declining in eastern North American forests due to anthracnose disease, which is caused by the introduced fungus Discula destructiva. (Photograph by Jonathan P. Evans).

(Mustela nigrepes) known to occur in the wild was destroyed by canine distemper virus. Ironically, some conservation efforts may also increase susceptibility to disease; for instance, species in captive breeding programs may contract diseases from other animals or even humans. Diseases transported by people to new parts of the world can also decimate species. North American chestnut trees (Castanea dentata), once common throughout the eastern US, have been virtually obliterated by an ascomycete fungi carried on Chinese chestnut trees imported to New York City. Introduced fungal diseases are also eliminating elm trees (Ulmus americana) and flowering dogwoods (Cornus florida) from North American forests (Figure 9). Introduced diseases have particularly powerful adverse effects on endemic island species. Many endemic Hawaiian bird populations have been decimated and even driven to extinction by introduced avian malaria protozoans spreading from introduced bird species through introduced mosquitoes. Increased rates of disease in animal populations can also negatively impact humans. The risk of diseases such as West Nile virus, Lyme disease, and bird flu being transmitted from animals to humans is increasing with increased movements of humans and domestic and wild animals around the world, as well as with warming temperatures that may be allowing disease-causing organisms to expand their ranges.

Multiple Factors A combination of factors acting simultaneously or sequentially can overwhelm a species, as illustrated by the case of the large freshwater mussel Margaritifera auricularia. This species was formerly known from Western Europe to Morocco, but now it occurs in only one river and its adjoining canals in Catalonia, Spain. Humans have used its attractive shell and pearls as ornaments since the Neolithic Age. The main reason for its decline, overharvesting, originally led to its disappearance from rivers in Central Europe in the fifteenth and sixteenth centuries. In more recent times, pollution, destruction of freshwater habitats, and overharvesting continue to reduce

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Extinction, Causes of

its range. The mussel is also affected by the loss of other species, because its larval stage needs to attach to certain species of fish to complete its life cycle. Unless strict conservation measures are implemented to prevent overharvesting, control water quality, maintain fish stocks, and protect mussel habitat, this culturally important species will soon be extinct. Such comprehensive conservation strategies are often needed to deal with the multiple threats to species. Threats to biological diversity come from a number of different directions, but their underlying cause is the same: the magnitude of destructive human activity. It is often easy to blame a group of poor, rural people, or a certain industry for the destruction of biological diversity, but the real challenge is to understand the local, national, and international linkages that promote the destruction and to find viable alternatives. These alternatives must include stabilizing the size of the human population, finding livelihoods for rural people that do not damage the environment, providing incentives and penalties that will convince industries to value the environment, and restricting trade in products that are obtained by damaging the environment. Yet an equally important part of the solution is to increase the willingness of wealthy and middle-class people in both developed and less-developed countries to reduce their consumption of the world’s resources and to pay fair prices for products that are produced in a sustainable, nondestructive manner.

See also: Air Pollution. Comparing Extinction Rates: Past, Present, and Future. Deforestation and Land Clearing. Desertification. Diseases, Conservation and. Environmental Impact, Concept and Measurement of. Extinction in the Fossil Record. Habitat Loss and Fragmentation. Human Impact on Biodiversity, Overview. Human

Impacts on Ecosystems: An Overview. Introduced Species, Impacts and Distribution of. Latent ExtinctionFThe Living Dead. Loss of Biodiversity, Overview. Mass Extinctions, Concept of. Mass Extinctions, Notable Examples of. Modern Examples of Extinctions. Natural Extinctions (not Human Influenced). Pesticides, Uses and Effects of

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