Habitat Alteration

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Habitat Alteration

Introduction

Habitat alteration is a change in land use or land cover that has an impact on local ecosystems. Plants and animals live in specific places that have the conditions of climate and food resources needed for survival. Habitats vary from forest and grassland to urban areas, streams, ponds, and oceans. Over time, a habitat is subject to alteration, especially under the influence of human activities. Habitat alteration, which may lead to habitat loss, is the greatest current threat to living species.

Large areas of land and water are damaged by activities such as urbanization, agriculture, and overfishing. These cause fragmentation of habitats, which threatens those animals that need a large habitat for breeding and survival. Many species have already become extinct, particularly in tropical areas. This loss of biodiversity impacts food resources, such as fish stocks. Loss of trees can increase soil erosion and accelerate climate change. Sustainable use of land and water is therefore needed to minimize the impact of habitat alteration.

Historical Background and Scientific Foundations

Habitat is the term used to describe the location and the environmental conditions where an animal or plant lives. Another, more functional, term is niche, which describes not just the habitat of a particular species, but also its needs for food and how it relates to other species present. Niche includes important factors like salinity, temperature, and humidity, predator-prey relationships, and competition for resources such as food and light.

Some species, like rats and dogs, can occupy broad niches, and are found in many different geographical locations and under varying conditions. Others are endemic to a specific area or very dependent on a specific resource, such as the koala, which eats eucalyptus leaves. Plants and animals nearly always have to share their habitat with other species. Direct competition would cause one or more species to die out. Instead, they survive by using different parts of a niche. For instance, plant species can share nitrogen resources by getting it from different depths of soil, in different chemical forms and at different times of the year.

Both habitats and niches change naturally over time and species adapt, sometimes becoming new species in the process through natural selection. Human activities, however, have the capability of changing habitats far more rapidly than geological processes can. As the human population has grown, the need for food and housing has led to widespread habitat alteration, which is a change in land use or cover affecting ecosystems, around the world.

Perhaps the most obvious forms of habitat alteration are deforestation, which involves cutting of trees to free forest for agriculture and housing, and conversion of wild grassland to agriculture. Deforestation has led to the loss of about half of Earth’s forest over the last three centuries, with destruction being most intense in tropical areas, where pressures to provide food and housing are greatest. Overgrazing and poor farming practices can easily lead to soil erosion, resulting in desertification, where agricultural land is converted to desert.

Extracting materials from land or water can also lead to habitat alteration. Mining for coal and metals strips the land directly and removes everything growing or living there. The resulting debris also often chokes up nearby waterways. Additionally, the oceans are overfished, and populations of many larger fish are now stressed, which in turn, has a direct effect upon marine ecosystems and food webs there. Fishing causes habitat alteration in other ways. Bottom trawling is a fishing method used by commercial operators in which large bag-shaped nets are dragged along the sea or ocean floor in the search for shrimp, cod, and other species that dwell on the bottom. The use of bottom trawlers has spread so they are used in complex ecosystems like deep-sea coral forest and areas covered with boulders or rocks. Often many non-target marine species are swept up by the trawler.

On land, roads are a major contributor to habitat alteration. Most human activities require roads to support them. Therefore roads are built through habitats as part of a building program, or to support logging, mining, or agriculture. The construction of dirt roads during deforestation and mining often damages habitats through soil erosion and landslides. Paved roads will encourage run off so that polluted water contaminates local land and water supplies. And any new road will bring an increase in traffic which poses a direct threat to any species in that habitat, leading to an increase in “road kill” and damage to plants.

Human activities often impact on aquatic habitats. The need to increase scarce water supplies leads to the diversion of streams and rivers to build dams or for irrigation. Waterways are dredged to make them deeper or to create a harbor area. These activities may flood neighboring land and aquatic habitat, altering salinity and nutrient levels, and causing deposition of silt. Meanwhile, wetlands, which are often very rich ecosystems, may be drained to use for development.

Recreational activities may also lead to habitat alteration. Fishing, hiking, sailing, and adventure sports, also

WORDS TO KNOW

BIODIVERSITY: Literally, “life diversity”: the wide range of plants and animals that exist within any given geographical region.

BOTTOM TRAWLING: An industrial fishing practice in which large, heavy nets are dragged across the sea floor.

DESERTIFICATION: Transformation of arid or semiarid productive land into desert.

DREDGING: The excavation of sediment from the bottom of a body of water.

SUSTAINABLE: Capable of being sustained or continued for an indefinite period without exhausting necessary resources or otherwise self-destructing: often applied to human activities such as farming, energy generation, or the maintenance of a society as a whole.

take their toll on habitats. Off-road vehicles provide access to remote areas where wildlife is disturbed and may be either deliberately or accidentally killed. Meanwhile, cruise ships may also cause considerable environmental damage. They tend to have huge anchors, which, with their associated chains, may land on sensitive

areas of the sea floor. Smaller boats are damaging as well, as they have been known to run aground on coral reefs.

All of the aforementioned changes in land use tend to lead to fragmentation of a habitat. This is defined as breaking up a habitat into smaller areas as patches within the original area are converted to new land uses, such as house building. The transition zones between the old and new areas often expose species in the original habitat to factors such as strong wind and competition from pests and species from other habitats.

Finally, pollution and climate change are significant factors in habitat alteration. They occur as a result of many of the human activities described earlier. Pollutants include heavy metals, acid rain, and agricultural runoff, particularly nitrogen and phosphorus. Climate change driven by increased carbon emissions increases temperatures and alters precipitation levels as well as causing a rise in sea levels. All of this changes habitats faster than species can either migrate or adapt.

Impacts and Issues

Species rarely thrive as a result of habitat alteration, although dogs, rats, and cockroaches tend to benefit as a result of human activities. Many species are put at risk through habitat alteration, which all too often leads to outright habitat loss. Large animals like apes and monkeys are always more vulnerable because they require large areas for establishing a breeding population, so they are disadvantaged when fragmentation occurs. Many freshwater fish face extinction because of pollution and as many of 83% of listed endangered plants are at risk from factors like eutrophication, which favors taller weedy plants.

Habitat alteration and destruction is a major factor in the decline and loss of amphibian species. These animals, which can live on land or water, have been in existence for 300 million years. Currently, there are 6,136 species of amphibians of which 43% are in decline and one third are endangered, while 168 species have actually gone extinct. Amphibians suffer particularly when wetlands are drained or when livestock trample on the banks of streams.

Loss of biodiversity through habitat alteration has a number of adverse effects. It reduces resources of food and materials and upsets the complex balance of ecosystems, the long-term impact of which is unknown. Deforestation and loss of vegetation contribute to climate change. People are increasingly aware of the importance of habitat conservation, however. And habitats resulting from human activities are not, of course, devoid of life, but they do tend to attract weedy species of plant and their ecosystems are not, in general, as bio-diverse as those they replace. However, conservation efforts must be made with understanding of how ecosystems work. Reforestation is widespread, but the original ecosystems will not return, for birds like the Northern Spotted Owl that require the tree canopy that can only be provided by old growth forests. Large mammals need large habitats, which is why conservation programs creating complimentary nature reserves joined by corridors are most successful.

BIBLIOGRAPHY

Books

Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.

Web sites

Amphibia Web. “Habitat Destruction, Alteration, and Fragmentation.” September 23, 2003. http://amphibiaweb.org/declines/HabFrag.html (accessed April 13, 2008).

See-the-Sea. “Habitat Alteration.” http://see-the-sea.org/topics/habitat/habitat_alteration.htm (accessed April 13, 2008).

Susan Aldridge

Environmental Science: In Context