Habitat Loss

Introduction

All species need somewhere to live and access to food. This place is called the habitat of the species and covers locations as diverse as rain forest, deep ocean, urban parklands, and streams and ponds. Some plants and animals can thrive in many different habitats; others require a specialized habitat. Human activities often lead to habitat alteration and loss, which can threaten the well-being of species living there. Habitat loss is the greatest current threat to the natural world.

An area about the size of the United States will likely be damaged by human activities such as urbanization and agriculture over the next 30 years. Many species will therefore become extinct. Decreasing biodiversity through habitat loss has a number of impacts on human populations. Food resources, such as fish stocks, are expected to decline, and loss of trees could increase soil erosion and accelerate climate change. Finding and implementing methods of sustainable land and water use have, therefore, been identified as priorities among earth scientists in order to minimize habitat loss.

Historical Background and Scientific Foundations

A species’ habitat is composed of its location and the environmental conditions in which it lives. A more comprehensive term is “niche,” which describes not just habitat but also the requirements of the plant or animal in terms of food and energy, and its relationships with other species. Yale University Professor George Evelyn Hutchinson (1903–1991) defined a niche as the range of physical and chemical conditions a species can deal with as well as its various interactions with other plants and animals. Thus the term includes factors like salinity, termperature, humidity, and relationships that feature predator-prey issues and competition for resources.

Some species, like rats and dogs, occupy broad niches, and are found in many different geographical locations and under varying conditions. Others, like the island fox in Santa Catalina off the coast of Southern California, are found in only one location. Other species occupy a narrow niche because they live on specific foods, such as the giant panda, which only eats bamboo, and the koala, which can only live where there is eucalyptus.

Species rarely, if ever, live in complete isolation. They must share their habitat with others, and this means competition for limited resources. Direct competition is usually not a feasible survival strategy in the long-term. One of the species will die out, change habitat, or adapt to minimize the competition. This idea, known as the competition exclusion principle, was put forward by the Russian microbiologist Georgi Gause (1910–1986), using mathematical models to show that no two species will occupy exactly the same niche for long. To co-exist, they have to find ways of using different parts of the niche. For instance, swallows and bats can both eat insects in the same habitat, but the swallow will hunt by day, and the bat by night.

Habitats and niches are in a dynamic relationship with the species existing there. They change naturally over time, and species change with them, according to the principles of evolution. Such geological processes have led to speciation. Human activities are, however, tending to change habitats faster than many species can adapt. These changes are driven largely by an increase in both human population and the level of development around the world.

Thus, land that may formerly have been forest or wetland may be taken over for agriculture or urbanization. Roads are built through habitats as part of a building program, or to support agriculture or logging. The construction of dirt roads during deforestation and mining often cause soil erosion and landslides. Paved roads will encourage runoff so that polluted water contaminates local water supplies and land. 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. All these changes in land use lead to habitat loss through depletions, such as loss of trees, and an overall fragmentation of the area.

Deforestation, a reduction in forest area because of human activity, has led to the loss of about half of Earth’s forest over the last three centuries. Forests have been converted into croplands, cities, and roads. A related cause of habitat loss is reforestation, when new trees are planted. Ecosystems in reforested areas are often different from those in the original forest areas. Desertification, where soil erosion and overgrazing converts grassland into deserts, is also a significant cause of habitat loss, along with irrigation, mining, and wetland drainage. Surface mining strips land directly and removes everything growing there.

Human activities related to water also often lead to habitat losses. We interfere with the natural flow of rivers and streams by building dams to increase scarce freshwater supplies. Water courses are also diverted for irrigation and waterways are dredged. All of this may flood or otherwise damage neighboring land and aquatic habitat,

WORDS TO KNOW

DEFORESTATION: A reduction in the area of a forest resulting from human activity.

FRAGMENTATION: Break up of a continuous habitat into several smaller areas.

HABITAT: The location and accompanying conditions where a plant or animal lives.

NICHE: A term describing a species habitat, range of physical and biological conditions, and relationships within its ecosystem.

SPECIATION: The evolutionary development of new biological species.

possibly altering salinity and nutrient levels, and causing deposition of silt.

Most habitat loss occurs in the tropical areas of developing countries. Here populations are rising fast with accompanying demands on agriculture and development. Conservation efforts may be relatively weak compared to the pressure to use land and resources.

Changes in land use tend to lead to fragmentation of a habitat, rather than its complete elimination. Fragmentation involves 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 may create a new habitat, but also expose species in the original habitat to factors such as strong wind and competition from pests and species from other habitats.

Sometimes a species habitat is lost indirectly because of competition from alien species. Human activities such as increasing industrialization and international travel have introduced such species into new habitats where, if they can adapt well to it, they become invasive and competitive to those already there as well as damaging the habitat itself. For example, the zebra mussel came to the Great Lakes in the late 1980s and took up residence there, displacing other species. Left to flourish without predators, the zebra mussel now causes damage by clogging up water supplies coming from the lakes.

The Nile perch is a well-known example of the damage that an invasive species can do. It was introduced into Africa’s Lake Victoria for food and sport fishing and has now eaten its way through 200 native fish species. Meanwhile, the brown tree snake of Papua New Guinea arrived on the Pacific Island of Guam and has eliminated 12 out of 14 local bird species.

Pollution can also alter a habitat. A prime example is agricultural and sewage runoff, particularly into coastal habitats, which increase the nitrogen and phosphorus content of the water. This so called eutrophication, or enrichment, increases biomass by stimulating photosynthesis. Under these conditions, competition for light favors just a few, tall, fast-growing species and leads to the endangering of many others. Endangered plants are more abundant where biomass is relatively low and they are less stressed by competition.

Habitats are also damaged by sulfur (S) compounds, ground-level ozone (O₃), heavy metals, and pesticides. Acid rain results from airborne sulfur and often destroys forest and woodland habitats many miles from the source of the pollution. Many aquatic habitats are affected by pesticides and other industrial chemicals that act as environmental estrogens, interfering with the reproductive capacities of birds and fish. Human recreational activities, such as fishing, hiking, sailing, and adventure sports, also take their toll on habitats. A major issue is the use of off-road vehicles, which provide access to remote areas where wildlife is disturbed and may be either deliberately or accidentally killed. These activities also lead to pollution of air and water courses and destruction of vegetation.

Forest fires can cause severe damage to tropical habitats, removing vegetation and dependent species. However, periodic fires are also essential in some habitats. The burning of organic matter can lead to a spurt of plant growth. Some plant species, like the Lodgepole pine, require exposure to high temperatures, as provided by forest fires, to release their seeds. The suppression of fires eventually leads to breakout of fires that last longer because of an accumulation of organic matter, and has led to damage of certain habitats, like savannas and grasslands.

Finally, climate change is proving to be a major factor in accelerating habitat loss. Climate change increases temperatures and alters precipitation levels as well as causing a rise in sea levels. All of these elements will either cause outright habitat loss or will change habitats faster than species can either will migrate or adapt.

All habitats are at risk to a certain extent, but some are especially vulnerable. Coral reefs are a combination of animals called stony corals living in symbiosis with photosynthetic algae that give this habitat its characteristic vivid color. They are found in warm, shallow, clear waters where they provide a home for a diverse community of fish, worms, and crustaceans, protecting the small fish from larger predatory fish. It only requires a temperature increase of 1.8°F (1°C) to cause coral reefs to start bleaching. This is a process in which the algal component is expelled and the reef turns white. Without the algae, the reef begins to die and its dependent species lose their habitat.

Tropical sea temperatures have gone up by an average of 1.8°F (1°C) in the last decade, which in itself may not be a problem. What is troubling is that temperatures are predicted to continue to rise in this way and the effects on coral reefs are already being seen. Australia’s Great Barrier Reef is the largest coral reef in the world at over 1,200 mi (1,900 km) long. In 2002, 60% of the reef was noted to be affected by bleaching. In years to come, the whole reef will die along with hundreds of species dependent on it, unless global warming is slowed.

Mangrove swamps or forests are another important area near the tropical shores. Mangroves are salt-resistant evergreen trees that support fish or shrimp, helping to filter pollution from upstream waters. They also provide habitats to crocodiles, snakes, tigers, deer, otters, dolphins, and birds. They also provide shelter against erosion, cyclones, and wind. Around half of the world’s total mangrove is found in Indonesia, Australia, Brazil, Nigeria, and Mexico. The United Nations Food and Agriculture Organization (FAO) has warned that 20% of the world’s mangrove area has been destroyed since 1980.

The main causes of the destruction of mangrove swampland include human population pressure, conversion for shrimp and fish farming, agriculture, tourism, pollution, and natural disasters, the FAO said. Fortunately, countries now realize how important mangroves are and have started to ban aquaculture there, which is leading to better management and conservation. Still, the rate of mangrove loss is significantly higher than the loss of any other types of forests.

Impacts and Issues

The major impact of habitat loss is the accompanying loss of plants and animals that cannot migrate or adapt. Only a few species, such as rats, dogs, cockroaches, and finches, actually benefit and thrive from the human activities that lead to habitat loss. The main activities affecting endangered species are logging, grazing, water development, recreation, and mining.

Habitat loss has the obvious impact of endangering species or even rendering them extinct. When the last individual of a species dies, then that plant or animal is declared extinct. Species become endangered when there are only a few of them left that are capable of breeding. Extinctions lead to a loss of biodiversity in a habitat. Of course, changes in land use do create new habitats, but these are often less biodiverse than the original one.

Over 100 of our living primate species are likely to go extinct in the next 10 to 20 years because of habitat loss. Large animals like apes and monkeys are always more vulnerable because they require large areas for establishing a breeding population. As many as 83% of endangered plants are at risk from human activity. In migrant bird populations, a decrease of around 40% is directly linked with habitat destruction. Declining amphibian populations are linked to habitat loss and water pollution. Fish are also threatened, with a large percentage of freshwater fish facing extinction. Overfishing can often lead to destruction of niches as top predators are taken.

Climate change will only accelerate the problem of habitat loss. The United Nations Framework Convention on Climate (UNFCCC) change says that most of the world’s endangered species, which comprises

IN CONTEXT: HABITAT LOSS AND MITIGATION OF CLIMATE CHANGE

“Reducing both loss of natural habitat and deforestation can have significant biodiversity, soil and water conservation benefits, and can be implemented in a socially and economically sustainable manner.”

“Forestation and bioenergy plantations can lead to restoration of degraded land, manage water runoff, retain soil carbon and benefit rural economies, but could compete with land for food production and may be negative for biodiversity, if not properly designed.”

SOURCE: Metz, B., et al, eds. “IPCC, 2007: Summary for Policymakers.” In: Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

25% of mammals and 12% of birds, could become extinct in the next few decades as warmer conditions start to alter the forests, wetlands, and other habitats that they depend upon.

However, there is much that can be done to prevent habitat loss. Conservation efforts, such as national parks and sites of special scientific interest, work best if they can make natural habitats as big as possible, and join together the small ones with habitat corridors. This allows animals to move between isolated areas that keep their populations large and healthy. In some parts of the world, adjoining countries are connecting their national parks across international borders to make giant parks in a project called the Peace Parks Foundation.

The foundation is based in South Africa and one of its several projects is the Ai-Ais/Richtersveld Transfrontier Park, which spans some of the most spectacular scenery of the arid and desert environments in southern Africa. It consists of the Ai-Ais Hot Springs Game Park in Namibia and the Richtersveld National Park in South Africa and includes the Fish River Canyon and Ai-Ais Hot Springs. This arid zone is characterized by a unique and impressive variety of succulent plant species. Bisected by the Orange River, which forms the border between the two countries, it offers enormous potential for development as a viable conservation area.

Habitats can also be protected against invasive foreign species through very tight regulation on bringing plants and animals across international borders and through airports and shipping ports. New Zealanders are leading the world in ridding their country of pest species such as rats and cats, but it is a slow process, and preventing new arrivals is the cheapest and easiest way to do this.

Fishing, agriculture, and development activities need to be carried out sustainably to prevent habitat loss. Adventure tourism to places like the Andes and Antarctica can be a good way of raising public awareness of the natural world and its fragility. Such recreational activities, however, need to be enjoyed without any accompanying damage to these habitats.

See Also Ecological Competition; Extinction and Extirpation; Habitat Alteration; Human Impacts; Invasive Species; Land Use; Rain Forest Destruction; Sustainable Development; Wildlife Protection Policies and Legislation

BIBLIOGRAPHY

Books

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

Kaufmann, Robert, and Cutler Cleveland. Environmental Science. New York: McGraw-Hill, 2007.

Web Sites

Earth Restoration Service. “Habitat Loss.” http://www.earthrestorationservice.org/page/76/habitat-loss.htm (accessed April 20, 2008).

Guardian.co.uk. “Report Reveals ‘Alarming’ Rate of Mangrove Habitat Loss.” February 1, 2008. http://www.guardian.co.uk/environment/2008/feb/01/endangeredhabitats.conservation (accessed April 20, 2008).

Peace Parks Foundation. “Facilitating Peace Parks.” http://www.peaceparks.org (accessed April 20, 2008).

Susan Aldridge