Extinction and Endangered Species - Factors That Contribute To Species Endangerment
invasive habitat habitats human
Experts believe that the increasing loss and decline of species cannot be attributed to natural processes, but results
FIGURE 1.4 Contributing factors in species endangerment
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 is now causing changes in climate patterns on a global scale. In 1994, the Forest Service conducted a study of 667 listed species and identified the factors contributing to their endangerment. The breakdown is shown in Figure 1.4.
With each passing day, humans require more space and resources. The World Resources Institute has tracked the increase in global human population over the millennia. The human population has been growing with particular rapidity in recent centuries and passed the 6 billion mark in 1999. As of February 2004, the world population is estimated to be 6.4 billion. The large numbers of human beings puts tremendous pressure on other species. Americans, because they consume more energy and other resources than populations in other industrialized countries or in the developing world, have an even greater impact on the environment.
Habitat destruction is probably the single most important factor leading to the endangerment of species. It plays a role in the decline of some 95 percent of federally listed threatened and endangered species. Habitat destruction has impacted nearly every type of habitat and all ecosystems.
Many types of human activity result in habitat destruction. Agriculture is a leading cause, with about 45 percent of the total land area in the U.S. 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 impacted forest, prairie, and wetland habitats in particular. Nearly 90 percent 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, Krausman, and Devers, urbanization contributes to the endangerment of species in 31 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, 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 directly impacts 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 show 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 go 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.
The bulk of human energy requirements are obtained through the burning of fossil fuels. This results in the release of large amounts of carbon dioxide into the atmosphere. Increased levels of carbon dioxide create a "greenhouse effect," which results in warmer temperatures on Earth. A global temperature increase has been compellingly documented, and has already had important effects on ecosystems worldwide. Global warming is predicted to accelerate quickly if measures are not adopted to address it.
The 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 plants, mining, automobiles, and agricultural products such as pesticides and fertilizers. Even animals that are not directly exposed to pollution can be affected, as the species that they rely on for food, shelter, or other purposes die out. According to Czech, Krausman, and Devers, pollution currently impacts a 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
TABLE 1.3 Examples of invasive species, 2004
Autumn olive (Elaeagnus umbellata) Chinese tallow (Sapium sebiferum) Downy brome (Bromus tectorum) Garlic mustard (Alliaria petiolata) Japanese honeysuckle (Lonicera japonica) Japanese knotweed (Polygonum cuspidatum) Kudzu (Pueraria montana var. lobata) Leafy spurge (Euphorbia esula) Mile-a-minute weed (Polygonum perfoliatum) Multiflora rose (Rosa multiflora) Musk thistle (Carduus nutans) Russian knapweed (Acroptilon repens) Russian olive (Elaeagnus angustifolia) Saltcedar (Tamarix spp.) Scotch broom (Cytisus scoparius) Scotch thistle (Onopordum acanthium) Spotted knapweed (Centaurea maculosa) Tree of heaven (Ailanthus altissima) Yellow star thistle (Centaurea solstitialis)
Africanized honeybee (Apis mellifera scutellata) Asian long-horned beetle (Anoplophora glabripennis) Asian tiger mosquito (Aedes albopictus) Brown tree snake (Boiga irregularis) Cane toad (Bufo marinus) Cactus moth (Cactoblastis cactorum) Emerald ash borer (Agrilus planipennis) European gypsy moth (Lymantria dispar) European starling (Sturnus vulgaris) Formosan subterranean termite (Coptotermes formosanus) Glassy-winged sharpshooter (Homalodisca coagulata) Hemlock, woolly adelgid (Adelges tsugae) Red imported fire ant (Solenopsis invicta) Wild boar (Sus scrofa)
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 rare or 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, non-native habitat, and which cause environmental harm. Most introductions of invasive species are accidental, resulting from "stowaways" on ships and planes. Invasive species harm native species by competing with them for food and other resources, or by preying on them or parasitizing them. By 2000 about 50,000 species were believed to have been introduced into the United States alone. There are many types of invasive species, some of which are listed in Table 1.3. Figure 1.5 illustrates the effect of some invasive species on the habitats they have colonized. While there are sometimes beneficial effects from invasive species, most of the effects are harmful.
Some 35–46 percent of species listed by the U.S. Fish and Wildlife Service are endangered partly or entirely
because of invasive species. Similarly, the 2000 IUCN Red List of Threatened Species suggested that invasive species affect 350 species of threatened birds (30 percent) and 361 species of threatened plants (15 percent). In fact, the IUCN 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, goats, cattle, and sheep is responsible for the endangerment of numerous plants and birds.
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
FIGURE 1.5 Impact of invasive species
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 the year 2000 federal agencies spent $631.5 million dealing with damage caused by invasive species or attempting to control them.
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