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Applying species-area curves to estimate extinction rates requires that we calculate the amount of natural landscape that has been lost in recent decades— a difficult task. Most studies have focused on tropical rainforests, which cover a broad swath of land roughly 20 degrees north and south of the equator. Tropical rainforests contain, by far, the greatest number of species of any biome, or major habitat type, on Earth. An early estimate of the rate of habitat destruction in the rainforest was made by biologist Edward O. Wilson, who calculated that about 1% of the tropical rainforest is converted to agricultural use every year. Conservatively estimating the number of species in the rain forest at 5 million, Wilson applied the generalized species-area curve and projected that nearly 20,000 to 30,000 species are lost each year due to rainforest destruction.

More modern studies using images from satellites (Figure 14.6a) indicate that approximately 20,000 square kilometers (about 7,722 square miles, an area the size of the state of Massachusetts) of rainforest in South America's Amazon River basin is cut each year. This is a rate of 2% per year, or twice the rate Wilson estimated. At this rate of habitat destruction, tropical rainforests will be reduced to 10% of their original size within about 35 years. If we apply the species-area curve, the habitat loss translates into the extinction of about 50% of the species that call Amazonian rainforests home. The extinct species in the rainforest would include about 50,000 of all the known 250,000 species of plants, about 1,800 of the known 9,000 species of birds, and about 900 of the 4,500 kinds of mammals in the world.

Of course, habitat destruction is not limited to tropical rainforests. When all of Earth's biomes are evaluated, freshwater lakes and streams, grasslands, and temperate forests are also experiencing high levels of modification. According to the IUCN, if habitat destruction around the world continues at present rates, nearly 4 of all living species will be lost within the next 50 years.

Some critics have argued that these estimates of future extinction are too high, because not all groups of species are as sensitive to habitat area as the curve in Figure 14.5b suggests, and many species may still survive and even thrive in human-modified landscapes. Other biologists counter that there are also other threats to species, therefore the rate of species loss should be even higher than these estimates.

Other threats to biodiversity by humans include habitat fragmentation, the introduction of exotic species, overexploitation, and pollution (Figure 14.6b-e). Habitat fragmentation occurs when large areas of intact natural habitat are subdivided by human activities. The species remaining in the habitat fragments are more susceptible to extinction because it is more difficult for individuals to move across modified landscapes than across natural landscapes. Habitat fragmentation thus makes it impossible for species to move from an area that is becoming unsuitable because of natural environmental changes to areas that are suitable. Individual organisms within habitat fragments are also more susceptible to being killed or disrupted by humans and human-adapted species (such as domestic cats), because they are in closer proximity to these threats than individuals in unfragmented habitats. For example, grizzly bears need from 200-2,000 square kilometers of habitat to survive a Canadian winter, but the Canadian wilderness is increasingly bisected by roads built for tree harvesting. Every interaction between grizzly bears and humans represents a greater danger to the bears than to humans—for example, of the 136 grizzlies that died in Canada's national parks between 1970 and 1995, 119 bears were killed by humans. Exotic species include domestic cats and organisms introduced by human activity to a region where they had never been found. Exotic species are often dangerous to native species because they have not evolved together—for instance, many birds on oceanic islands such as Hawaii and New Zealand are unable to defend themselves from introduced ground hunters, such as rats. Overexploitation encompasses overhunting and over-harvesting and occurs when the rate of human destruction or use of a species

(a) Habitat destruction

(b) Habitat fragmentation

(a) Habitat destruction

(b) Habitat fragmentation

Humans are rapidly modifying tropical rainforests. This 1999 satellite photo illustrates the extent of modification in an area of Brazilian rainforest, that, until 30 years ago, contained no agricultural lands. The lighter parts of the photo are agricultural fields; the darker regions are unmodified forest.

The introduced brown tree snake is responsible for the extinction of dozens of native bird species on the Pacific island of Guam.

(e) Pollution

Pollution from herbicides appears to be responsible for the increase of deformities in frogs in the midwestern United States and may partially explain the worldwide decline in frog species.

These "islands" of tropical forest were created when the surrounding forest was logged. Scientists have documented hundreds of localized extinctions whithin these fragments.

(d) Overexploitation of species

(d) Overexploitation of species

These tiger skins represent a small fraction of the illegal harvest of tigers in Asia, primarily for the Chinese market.

Figure 14.6 The causes of extinction.

(c) Introduction of exotic species

(c) Introduction of exotic species

outpaces its ability to reproduce. Humans overexploited passenger pigeons to the point of extinction and decimated populations of gray wolves, sea turtles, and many whale species as well. Pollution, the release of poisons, excess nutrients, and other wastes into the environment, poses an additional threat to biodiversity. The massive fish kills in Upper Klamath Lake were caused by fertilizer pollution, and as we discuss in Essay 14.2, global climate change caused by pollution may be the most serious threat to biodiversity yet.

All of these threats indicate that ESA critics who describe modern species extinction rates as "natural" are most likely incorrect. Over the past 400 years, humans have caused the extinction of species at a rate that appears to far exceed past rates, and it is clear that human activities continue to threaten thousands of additional species around the world. In fact, many scientists argue that the Earth is on the brink of a mass extinction—a loss of species that is global in scale, affects large numbers of species, and is dramatic in its impact. Earth has experienced five episodes of mass extinction, in which 50-90% of all living species were lost over the course of a few thousand to a few hundred-thousand years (Figure 14.7). Past mass extinctions were probably caused by massive global changes—for instance, changes in sea level brought about by climate fluctuations, or changes in ocean and landform caused by movements of Earth's tectonic plates. Many scientists argue that we are now seeing biodi-versity's sixth great mass extinction—and the pervasive global change causing this extinction is human activity.

After previous mass extinctions, biological diversity did not reach pre-extinction levels for 5-10 million years. The species that replaced those lost in the mass extinction were also very different. We cannot predict what biodiversity will look like after another mass extinction. Many people who feel a moral responsibility to minimize the human impact on other species and preserve the majority of biological diversity for future generations continue to support actions that preserve species, despite the cost. However, in addition to supporting the ideals of rights for nonhuman species and preservation for the sake of our children and grandchildren, there is a practical reason to prevent the sixth extinction from occurring—the loss of nonhuman species can cause human suffering as well.

Figure 14.7 Mass extinction. This graph illustrates the general rise in biodiversity over the past 600 million years, as indicated by an increase in the number of marine families present in the fossil record. However, this rise has been punctuated by five mass extinctions, marked with black dots, which resulted in a global decline in biodiversity. The number of species lost during these mass extinction events appears to be even greater than the number of families lost, because families containing many species died out.

Figure 14.7 Mass extinction. This graph illustrates the general rise in biodiversity over the past 600 million years, as indicated by an increase in the number of marine families present in the fossil record. However, this rise has been punctuated by five mass extinctions, marked with black dots, which resulted in a global decline in biodiversity. The number of species lost during these mass extinction events appears to be even greater than the number of families lost, because families containing many species died out.

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