Habitat Loss

Many biologists consider habitat loss, habitat degradation, and habitat fragmentation the primary threats to species survival. Habitat is the place or kind of place where an organism or a community of organisms lives and thrives. Habitat loss occurs when habitat is converted to other uses, such as when a wetland is filled or a prairie is covered by housing developments. Habitat degradation occurs when the habitat is so diminished in quality that species are no longer able to survive. Urban development can degrade a habitat because plants and soil are replaced with asphalt and concrete. Water runs off instead of soaking in. Average temperature goes up because the asphalt and concrete absorbs more solar energy. Fragmentation occurs when terrestrial habitats are separated into small, isolated fragments. Even when the total acreage of habitat appears to be sufficient, the fragmentation prevents species from surviving.

Some species have a very limited habitat. For example, as its name implies, the habitat of the creosote bush grasshopper, Bootettix argentatus, is the creosote bush, Larrea tridentata. It is found nowhere else. Most animals avoid the creosote bush and find the leaves distasteful or even toxic, whereas this small grasshopper thrives on the leaves. If the creosote bush were to become extinct, so would the little grasshopper.

Fortunately for Bootettix argentatus, there are plenty of creosote bushes and neither the plant nor the grasshopper is in danger. This is not true for many other animals. The Texas horned lizard, Phrynosoma cornatum, has become so rare in much of its original range that it has been listed as a threatened species. Many people who grew up in Texas and Oklahoma in the 1940s and 1950s remember playing with this docile and easily caught lizard commonly called "horny toad" or "horned frog." The lizard's habitat is open dry country with loose soil (for burrows), supporting grass, mesquite, cactus, and plenty of the large ants they prefer to eat.

Several factors have caused the decline in population of Texas horned lizards (and the other horned lizard species as well). Many are taken for the pet trade. This is unfortunate, as they do not make good pets and most starve to death within a few months. Another factor contributing to the decline is the displacement of native ants by imported fire ants, Solenopsis invicta. Texas horned lizards feed almost exclusively on a few species of large red ants, such as harvester ants. Imported fire ants drive harvester ants and other ants out of their range, thus depriving the horned lizards of their primary food supply.

Many biologists believe that the major cause of the decline of the Texas horned lizard is the loss of habitat. The open areas with prickly pear, sparse grass, and mesquite are being converted into farmland and housing subdivisions. Of course, the first thing many homeowners do when moving into a new subdivision is to start eradicating the ants. People do require living space, clean water, food, and a safe environment, but there are many things that humans can do to reduce or prevent loss of habitat.

Causes

Many human activities can cause habitat loss, degradation, or fragmentation. In addition to urbanization, industrial agriculture, improper forest management, overgrazing, poorly managed mining, water development projects, pollution, the introduction of non-native species, and fire suppression all degrade habitat.

Urbanization.

Industrial agriculture is the main cause of habitat loss while urbanization is the major hindrance to species recovery. Urbanization is a complex process that involves a progressive increase of the percentage of a population that lives in an urban area and a corresponding decrease in the percentage of people living in rural areas. Urbanization is often accompanied by urban sprawl as the city expands to accommodate an ever-increasing population. Managing urban sprawl is an enormously difficult and political process.

Many urban and suburban developments try to preserve "green belts" for aesthetic and other reasons, but habitat fragmentation still occurs. For example, by the early 1990s urban sprawl in California had reduced the indigenous coastal sage scrub ecosystem by more than 90 percent. Coastal sage is the habitat of the threatened California gnatcatcher. The remaining 10 percent of sage scrub is broken up into small fragments. Instead of four or five large patches containing thousands of hectares, there are now hundreds of widely separated tiny patches of a few hectares each. These patches are too small to support healthy populations of California gnatcatchers.

Industrial agriculture.

Nearly half of the land area in the United States is devoted to agriculture. There are 472 million acres in cropland and 587 million acres in range or pasture. According to ecologist Curtis Flather and others, massive single crop industrial agriculture is the leading cause of habitat destruction in the United States, substantially affecting our forests, rangelands, and wetlands. Nearly 90 percent of recent wetland losses are due to agricultural practices.

Deforestation.

In many parts of the world, logging, grazing, and mining are the major threats to endangered ecosystems and species. Deforestation occurs when trees are removed at a rate faster than they can be replanted. Habitat degradation occurs when the largest and oldest trees are removed, leaving behind scrubby stands of small and immature trees. Worldwide, deforestation is decimating tropical rain forests with enormous habitat loss. Logging activities can have devastating impacts on habitats and the wildlife in those habitats. In the United States 260 threatened and endangered species live in our national forests. Poorly planned clear-cutting and building of logging roads degrade habitat by removing large stands of trees and fragmenting the remainder. Logging and the construction of timber roads also cause erosion that can clog streams with silt.

Grazing.

Livestock grazing is the most widespread of the federally subsidized, private commercial practices operating on public lands. Commercial livestock grazing is allowed on 270 million acres of land managed by the federal government. Poorly managed livestock grazing (including overgrazing) can severely damage wildlife habitat by changing the species composition of native ecological communities. In addition to directly destroying habitat, overgrazing has a number of indirect impacts. For example, land users often try to kill predators or species that may compete with livestock for food.

Mining.

Mining is the extraction of useful materials from the ground. Surface mining strips away overlying soil and rock, removing the useful material (usually coal) and then replacing the rock and soil. Properly done, surface mining can leave some habitats in good condition. Improperly managed mining significantly degrades ecosystems by degrading habitat and by polluting and degrading streams and waterways. Even well-managed mining increases road building. Poorly managed surface mining can destroy the surface ecosystem. In addition, mining requires a large amount of underground material to be brought to the surface. These materials, when exposed to rain, can create runoff that is highly acidic or has high concentrations of metal ore, both of which are highly toxic to aquatic species.

Water development projects.

Water development projects include dams, dredging, stream channelization, flood control structures, and canals. These projects adversely affect species in a number of ways. The natural flow of rivers and streams may be disrupted. Riparian (stream bank) habitat may be destroyed, fragmented, or degraded. Because riparian habitat is often unique to a region, water development projects have the potential to destroy a habitat entirely. Water projects also alter water flow, which may change wetlands, marshes, and other downstream habitat. For example, in the portion of the Colorado River that flows through the Grand Canyon, the river habitat has been completely changed by the construction of Lake Powell and the Glen Canyon dam. The red, silt-laden Colorado River with its frequent floods has been replaced by a cold, clear river that never floods. Native species of fish cannot tolerate the cold water, although imported species such as rainbow trout do well. The riparian habitat has been completely changed as well. When the Colorado River flooded, it stripped vegetation from the banks and built large sandbars. Now the banks are covered with another imported species, tamarisk or salt cedar, and the sandbars are disappearing.

Introduction of non-native species.

After outright habitat destruction, many biologists consider the introduction of exotic species to be the primary threat to rare and native species and even to complete ecosystems. Non-native species change the vegetation, compete with native species, and prey on native species. Hawaii, California, and Florida face particularly severe problems with exotic species. In Hawaii introduced species are now considered to be the single greatest cause of extinction of the state's native fauna and flora. For example, the introduction of cattle to the state has destroyed many plant communities. Many species in Hawaii, such as the hau hele 'ula (Hawaiian tree cotton), have been placed on the threatened or endangered species lists.

Pollution.

Pollution damages and degrades ecosystems in many ways. Airborne pollutants such as acid precipitation often affect natural communities miles away from the source. Acid rain and acid fog destroy northern forests, lakes, and streams hundreds or thousands of kilometers from the source of the pollutants. Acid precipitation can lower the pH of streams and lakes to the point that some fish are unable to reproduce and some die. Acid rain can cause chemical reactions in the soil that release metallic elements, such as aluminum. These elements can enter water supplies, damaging fish or other organisms. More than a billion pounds of toxic chemicals, including mercury and lead, were discharged directly into America's waters between 1990 and 1994, according to the Environmental Protection Agency. Thirty million pounds of these chemicals were known to cause cancer.

Fire suppression.

For decades in the United States, we have assumed that suppression of fire was a good thing. Fire kills wildlife, destroys trees and grasslands, and damages property. Now we realize that many ecosystems depend on fire for their survival. Fire suppression allows other species to flourish, changing the species composition. For example, in Central Texas the hills are covered with Ashe juniper, a small native tree commonly called cedar. When Europeans first saw these hills, they were covered by thick, tall grass with only a few stands of cedar. Frequent wildfires swept over the hills, burning both grass and trees. However, the grass quickly recovered after the fire, whereas many trees were permanently removed. Thus the fire helped to maintain a balance between grass and trees. The suppression of fire and overgrazing disrupted that balance. Now the trees dominate and open grassy areas are rare. The distribution of animal species also changed.

Fire is an integral part of many ecosystems, maintaining the ecosystem's natural vegetation. There are many plant species that require fire to trigger the release of their seeds. Fire also clears out the underbrush in forests, and the prevention of all forest fires actually leads to fires that burn hotter and longer because of the accumulation of underbrush. Thus the suppression of all fires leads to habitat destruction and degradation. If handled judiciously to protect life and property, fires can restore an ecosystem's natural balance.

Recreation.

Sometimes we love our natural areas to death. Recreation takes a great toll on wildlife and habitats, especially inappropriate recreational uses of open land. Probably the most destructive form of outdoor recreation is the improper use of off-road vehicles. These vehicles can provide access to remote areas otherwise unreachable. Improper operation of these vehicles can result in the harassment and inadvertent killing of wildlife. For example, vehicles can crush desert tortoises or the eggs of sea turtles and piping plover on beaches. These vehicles can also cause acceleration of soil compaction and erosion, pollution of water and air, and destruction of vegetation. Other forms of recreation, such as hiking and backpacking, are generally less harmful, but all recreational activities involve some harm to the environment.

Effects of Habitat Loss on Animal Species

Ecologists Curtis Flather, Linda Joyce, and Carol Bloomgarden studied the pattern of endangered species in the United States for the National Forest Service. In a report published in 1994, they concluded that habitat destruction was the leading cause of species endangerment, threatening 80 percent or more of federally listed species. They also found that habitat destruction and degradation was at least part of the reason why more than 95 percent of species listed as endangered or threatened were imperiled. In a different study of taxpayer-subsidized resource extraction, researchers found that logging affects approximately 14 to 17 percent of listed species, grazing affects 19 to 22 percent, water development affects 29 to 33 percent, recreation affects 23 to 26 percent, and mining affects 14 to 21 percent.

Habitat loss and degradation is a factor in the decline of every category of species. The decline of nearly 40 percent of migrant bird populations is directly linked to habitat destruction. For amphibians, declining populations are linked to habitat destruction, introduction of exotic species, water pollution, and ozone depletion.

Habitat Protection

The 1973 Endangered Species Act (ESA) has been a great success. Many species, such as the American alligator, have been brought back from the brink of extinction to healthy populations. However, many biologists question the species focus of the ESA. Rather, a focus on preserving extensive habitats is thought to be the best way to prevent the loss of wild species. Preservation can be achieved through a worldwide system of reserves, parks, and other protected areas. The plan put forward by biologists is ambitious, with a goal of setting aside 10 percent of Earth's land area. These preserves would conserve and manage entire ecosystems. This approach would be cheaper and more cost-effective than managing species one by one and would require less human intervention to prevent extinction. Activities in the preserves could include research and education as well as limited commercial activities such as ecotourism.

Elliot Richmond

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RATES OF DESTRUCTION OF BRAZIL'S TROPICAL RAIN FORESTS

Brazil, a country that is 8,511,960 square kilometers (3,286,969 square miles), originally was home to 2,860,000 square kilometers of rainforest. In 2001, the coverage had been reduced to 1,800,000 square kilometers. The annual rate of deforestation, then, works out to be 2.3%, or 50,000 square kilometers. This is the equivalent of 1000 football fields every year in Brazil alone.

Worldwide, scientists estimate that nearly all the tropical rainforest ecosystems will be destroyed by the year 2030 if the current rate of deforestation continues.