Experts believe that the increasing loss and decline of species cannot be attributed to natural processes, but results instead from the destructive effect of human activities. People hunt and collect wildlife. They destroy natural habitats by clearing trees and filling swamps for development. Aquatic habitats are altered or destroyed by the building of dams. Humans also poison habitats with polluting chemicals and industrial waste. Indeed, human activity may be causing changes in climate patterns on a global scale.
With each passing day, humans require more space and resources. According to the U.S. Census Bureau in Global Population at a Glance: 2002 and Beyond (March 2004), the human population totaled about one billion at the beginning of the nineteenth century. It surpassed the two billion mark in 1922 and began a period of rapid increase that saw the world population triple to six billion by the turn of the twenty-first century. According to the Census Bureau, the world population as of May 2006 was more than 6.5 billion people, and the population of the United States was expected to reach 300 million during October 2006. The large numbers of human beings puts tremendous pressure on other species.
Habitat destruction is probably the single most important factor leading to the endangerment of species. It plays a role in the decline of nearly all listed species and has had an impact on nearly every type of habitat and ecosystem.
Many types of human activity result in habitat destruction. Agriculture is a leading cause, with nearly half of the total land area in the United States used for farming. Besides causing the direct replacement of natural habitat with fields, agricultural activity also results in soil erosion, pollution from pesticides and fertilizers, and runoff into aquatic habitats. Agriculture has compromised forest, prairie, and wetland habitats in particular. Nearly 90% of wetland losses have resulted from drainage for agriculture. According to a study by Brian Czech, Paul R. Krausman, and Patrick K. Devers ("Economic Associations among Causes of Species Endangerment in the United States," Bioscience, vol. 50, no. 7, 2000), the role of agriculture in the endangerment of species is greatest in the Southeast and California. However, agriculture impacts threatened and endangered species throughout the country, contributing to endangerment in thirty-five states.
Urban expansion has destroyed wild habitat areas as well, and is a primary factor in the endangerment of many plant species. As with agriculture, urbanization leads to the direct replacement of natural habitat. It also results in the depletion of local resources, such as water, which are important to many species. According to Czech and his colleagues, urbanization contributes to the endangerment of species in thirty-one states. The greatest impact is in California, Florida, and Texas, the three states that are urbanizing most rapidly. In contrast, only two species are endangered by urbanization in Utah, Nevada, and Idaho. The authors argue that this is because a large proportion of land in these states is public land and therefore not available for private development.
Logging, particularly the practice of clear-cutting forests (removal of all trees in a designated area), destroys important habitat for numerous species. Clear-cutting or extensive logging can also lead to significant erosion, harming both soils and aquatic habitats, which become blocked with soil.
Numerous other forms of human activity result in habitat destruction and degradation. Grazing by domestic
livestock has a direct impact on numerous plant species, as well as animal species that compete with livestock. Mining destroys vegetation and soil, and also degrades habitat through pollution. Dams destroy aquatic habitats in rivers and streams. Finally, human recreational activity, particularly the use of off-road vehicles, results in the destruction of natural habitat. Czech, Krausman, and Devers reported that recreational activity has a particularly detrimental effect on species in California, Hawaii, Florida, as well as species in the Mojave Desert, which includes portions of Arizona, California, Nevada, and Utah.
Human land-use patterns often result in the fragmentation of natural habitat areas that are available to species. Studies have shown that habitat fragmentation is occurring in most habitat types. Habitat fragmentation can have significant effects on species. Small populations can become isolated, so that dispersal from one habitat patch to another is impossible. Smaller populations are also more likely to become extinct. Finally, because there are more "edges" when habitats are fragmented, there can be increased exposure to predators and increased vulnerability to disturbances associated with human activity.
Global warming is a phenomenon associated with the enhanced greenhouse effect. Gases such as carbon dioxide and methane in the atmosphere absorb and maintain heat in the same way that glass traps heat in a greenhouse. This natural greenhouse effect keeps Earth warm and habitable for life. (See Figure 1.6.)
An enhanced greenhouse effect refers to the possible increase in the temperature of Earth's surface due to the release of excessive amounts of greenhouse gases from the burning of fossil fuels. Figure 1.7 shows that the global average of carbon dioxide in the atmosphere has increased dramatically since the early 1980s. A global temperature increase has also been compellingly documented, as shown in Figure 1.8, and has already had important effects on ecosystems worldwide.
According to ecologist Chris D. Thomas in "Extinction Risk from Climate Change," (Nature, no. 427, January 2004), a study of habitats comprising 20% of the Earth's surface suggested that 15% to 37% of the world's species may be extinct by 2050 if recent warming trends continue. Summarizing his findings, Thomas said, "The midrange estimate is that 24% percent of plants and animals will be committed to extinction by 2050. We're not talking about the occasional extinc-tion—we're talking about 1.25 million species. It's a massive number."
FLGURE 1.6 The greenhouse effects "The Greenhouse Effect," in EPA Global Warming site: Climate, U.S. Environmental Protection agency, January 7,2000, http://yosemite.epa.gov/oar/globalwarming.msf/content/climate.html(accessed April 4, 2006)
Continued warming of the Earth would alter habitats drastically, with serious consequences for numerous species. In places like Siberia and the northernmost regions of Canada, habitats such as tundra—permanently frozen land supporting only low-growing plant life such as mosses and lichens—and taiga—expanses of evergreen forests located immediately south of the tundra—are shrinking. Deserts are expanding. Forests and grasslands are beginning to shift towards more appropriate climate regimes. Animal and plant species that cannot shift their ranges quickly enough, or have no habitat to shift into, are dying out. Some plants and animals that are found in precise, narrow bands of temperature and humidity, such as monarch butterflies or edelweiss, are likely to find their habitats wiped out entirely. Global warming is already endangering some of the most diverse ecosystems on Earth, such as coral reefs and tropical cloud forests. The impact on endangered species, which are already in a fragile state, may be particularly great.
Pollution is caused by the release of industrial and chemical wastes into the land, air, and water. It can damage habitats and kill or sicken animals and plants. Pollution comes from a wide variety of sources, including industrial operations, mining, automobiles, and agricultural products such as pesticides and fertilizers. Even animals that are not directly exposed to pollution can be affected, if other species they rely on die out. According to Czech and his colleagues, pollution currently affects a
FIGURE 1.7 Global trend in atmospheric carbon dioxide levels, 1981–2003 Adapted from "Carbon Dioxide Measurements," in Carbon Cycle Greenhouse Gases Figures, U.S. Department of Commerce, National Oceanic and Atmospheric Administration, Climate Monitoring and Diagnostics Laboratory, 2006, http://www.cmdl.noaa.gov/gallery/ccgg_figures/co2trend_global (accessed March 8, 2006)large number of species in the Southeast, particularly aquatic species such as fish or mussels.
Hunting and Trade
Humans have hunted numerous animal species to extinction, and hunting continues to be a major threat to some species. In the United States, gray wolves were nearly wiped out because they were considered a threat to livestock. The Caribbean monk seal was viewed as a competitor for fish, and exterminated. Other animals are hunted for the value of their hides, tusks, or horns, including elephants and rhinoceroses. Many exotic species, such as parrots and other tropical birds, are taken from their natural habitats for the pet trade.
Invasive species are those that have been introduced from their native habitat into a new, nonnative habitat, and which cause environmental harm. Most introductions of invasive species are accidental, resulting from "stowaways" on ships and planes. Invasive species harm native life forms by competing with them for food and other resources, or by preying on them or parasitizing them. As of 2005 approximately 50,000 species were believed to have been introduced into the United States alone. While there are sometimes beneficial effects from introducing nonnative species, most of the effects are harmful.
The introduction of invasive species can lead to genetic swamping. This is a condition that arises when large numbers of one species breed with a much smaller population of another related species. The genetic material of the invasive species becomes overwhelming, causing the resulting generations to lose many of the characteristics that made the smaller population a unique species in the first place.
Many species in peril are endangered partly or entirely because of invasive species. Similarly, the IUCN Red List suggested in 2000 that invasive species affect hundreds of species of threatened birds and plants. In fact, the International Union for the Conservation of Nature and Natural Resources found that the majority of bird extinctions since 1800 have been due to invasive species such as rats and snakes. In 2003 the IUCN reported that the unique flora and fauna of islands such as the Galapagos Islands, Hawaii, the Seychelles, the Falkland Islands, and the British Virgin Islands, have been devastated by invasive species. Human commensals—species that are used by and associated with humans—can be among the most destructive introduced species. In Hawaii, for example, grazing by feral pigs,
FIGURE 1.8 Global mean surface temperature anomalies, 1880–2005 "Figure 1a. Global-Mean Surface Temperature Anomaly (Degrees C), in "Global Temperature Trends: 2005 Summation, GISS Surface Temperature Analysis: 2005 Summation, National Aeronautics and Space Administration, Goddard Institute for Space Studies, January 12, 2006, http://data.giss.nasa.gov/gistemp/2005/2005cal_fig1.pdf (accessed March 10, 2006)goats, cattle, and sheep is responsible for the endanger-ment of numerous plants and birds.
The introduction of invasive species by humans has also taken a toll on mammalian wildlife. Australia is overrun with domestic cats whose ancestors were brought by settlers to the island continent 200 years ago. Stray domestic cats have driven indigenous species such as bandicoots, bettongs, numbats, wallabies, and dozens of other bird and mammal species, most of which are found nowhere else on Earth, towards extinction. Richard Evans, a member of the Australian Parliament, claims the feral cats are responsible for the extinction of at least thirty-nine species in Australia. He has called for total eradication of cats from the island by 2020, to be achieved by neutering pets and spreading feline diseases in the wild. The Australian National Parks and Wildlife Service reports that each house cat kills twenty-five native animals each year on average, and feral domestic cats kill as many as 1,000 per year.
Recognizing the threat posed by invasive species, President Bill Clinton signed Executive Order 13112 on Invasive Species in 1999. This order requires federal agencies to make every possible effort to control the spread of invasive species, and resulted in the formation of the Invasive Species Council, which drafted the first National Invasive Species Management Plan in January 2001. The plan emphasizes prevention of introduction of alien species, early detection of invasions, rapid response to them, and coordination of national and international efforts in management and control of these species.
In "Update on the Environmental and Economic Costs Associated with Alien-Invasive Species in the United States" (Ecological Economics, vol. 52, no. 3, February 15, 2005), researchers at Cornell University, including David Pimentel, reported that invasive species cause environmental damages and losses that cost the country nearly $120 billion per year. Invasive species are blamed, in part, for imperiling approximately 42% of the species on the list of threatened and endangered species in the United States.