Mass grading for the Panama Canal enlargement project will
entirely destroy this island. source: C.Michael Hogan Habitat destruction is the alteration of a natural habitat to the point that it is rendered unfit to support the species dependent upon it as their home territory. Many organisms previously using the area are displaced or destroyed, reducing biodiversity. Modifying habitats for agriculture is the chief cause of such habitat loss. Other causes of habitat destruction include surface mining, deforestation, slash-and-burn practices and urban development. Habitat destruction is presently ranked as the most significant cause of species extinction worldwide. Additional causes of habitat destruction include acid rain, water pollution, introduction of alien species, overgrazing and overfishing.
A closely related concept is that of habitat fragmentation, where a habitat is separated into fragments that lack effect ecological connectivity, reducing the viability of some of the resident species. The fundamental driver of habitat destruction has been the unprecented human population explosion, which has been a unique event of a single species dominating natural systems of the Earth within the short time span of 10,000 years. The waves of habitat destruction are closely correlated with the numerical expansion of the human population as well as settlement incursions such as the Maori in New Zealand and the Europeans to North America.
The chief proximate causes of habitat destruction are:
- Conversion of natural habitat for agricultural use including crops and grazing activity
- Pollution, especially chemical herbicide and pesticide use, water pollution, air pollution and acid rain
- Urban development and infrastructure, including roads, power plants, desert solar arrays, pipelines and transmission lines
- Timber harvesting and slash-and-burn practices leading to deforestation
- Introduction of alien species
Clearfelling of monoculture alien species conifer forest.
Aberdeenshire, Scotland. Source: C.Michael Hogan Starting in the mid-Holocene and continuing to the present time, agriculture has been the predominant cause of habitat destruction. Conversion of natural habitat to crop production as well as to grazing has eliminated the expanse of much of the Earth's original habitat. For example, in Europe, over 85 percent of all natural habitat has been destroyed, mostly for agricultural practices. In principle, grazing could be consistent with grassland conservation; however, widespread overgrazing practices have resulted in extensive loss of natural habitat.
Pollution compromises and destroys habitats in numerous ways. Acid rain alters the pH of both watercourses as well as soils, thus fundamentally transforming the abiotic integrity of natural habitats. The change in pH levels alters the metabolic capability of both plants and animals, leading to reduced numbers or complete loss of entire species within the affected area. Similarly water pollution can dramatically alter the survival of species within an aquatic habitat. Air pollution impacts include dispersal of oxides of ntirogen and sulfur dioxide, which among other gas contaminants can alter metabolism, fitness and mortality of flora and fauna.
Latvian monocultural agriculture displaced native grassland
and forest: beautiful but ecologically disastrous.
Source: C.Michael Hogan While urban development represents very visible evidence of habitat destruction, it accounts for far less of the net damage compared to agricultural and deforestation causes. One of the prominent effects of this type of destruction is the habitat fragmentation effects of long linear projects, especially roadways that create permanent barriers to habitat continuity.
Any type of deforestation represents habitat destruction; the most significant forms of this destruction are clearcutting and slash-and-burn agriculture. These two practices are responsible for massive habitat losses in such places as Madagascar, Indonesia and Brazil.
Perhaps the most subtle form of habitat destruction results from invasive species, flora or fauna which generally are introduced by humans and crowd out native species. This phenomenon can occur on such a massive scale and progress sufficiently slowly that a fundamental transformation may occur in the form of relatively modest annual steps. An example of this phenomenon is the destruction of most of the California coastal prairie, resulting from introduction of exotic European and Asian grasses when European settlement began in earnest in the mid 1800s.
The fundamental driver: human overpopulation
Hong Kong has displaced virtually all the original ecosystem
where it stands. Source: C.Michael Hogan While there are a number of clearly defined processes leading to destruction of habitat, the underlying cause of all these is the human population explosion. Ironically the majority of the human population growth is situated within the greatest biodiversity hotspots. Specific statistical analysis demonstrates that 87.9 percent of varation in species endangerment can be explained by the single variable of human population density. Some researchers like to further break down the pressure of human overpopulation into components of behavior; in one sense this is a distraction from the fundamental reality of the causation. These behaviors and attributes consist of such descriptors as: (a) lack of family planning; (b) lack of secure property rights; (c) famine; (d) poverty and (e) lax enforcement of environmental statutes.
Prominent consequences of habitat destruction may include local or global extinction of species and thus biodiversity loss. In a anthropocentric context a major consequence is reduction of ecosystem services or loss of economic value of the environment to humans. Specific elements of these losses include: (a) topsoil erosion; (b) reduction in sustainable yields of fisheries, forests and other biotic resources; (c) loss of pollinators; (d) reduction in water quality due to sedimentation; (e) loss of carbon storage; (f) reduction of surface water resources and (g) loss of genetic materials that provide medicinal value. Reduction of usable water resource is compounded by pollution degradation pollution along with reduced retention of freshwater resources as natural soils and detritus are replaced with less pervious soils and even pavement.
Without regard to the inestimable value of species lost and aesthetic degradation, the brute economic toll of habitat destruction is massive. Economic losses to fisheries and agricultural productivity equates easily to hundred of billions of dollars (US) per annum. More significantly, the uprooting of food security for hundreds of millions of people is an intrinsic consequence of the topsoil and pollinator losses. The loss of food security is occurring in the very places that habitat losses are currently greatest, and where population growth is the highest, implying a near certainty of increasing famine and warfare in those regions as food and water conflicts exacerbate.
By habitat type
Extensive loss of all major habitat types has already occurred. Vast percentages of forest, grassland, chaparral, wetlands, desert and tundra have been eliminated by the actions of man.Some of these losses such as forest clearly and wetland filling are quite visible, where other destruction such as grassland loss is much more subtle; a monocultural grain crop may replace a robust biodiverse native prairie, or overgrazing by livestock may remove native forbs and grasses by excessive erosion effects, as well as the frequent effect of importing alien grasses as weed seed within hay brought in from other regions.
Deforestation worldwide amounts to approximately one third of the original forest stands lost, giving full credit to secondary regrowth. In terms of virgin forests destroyed the percentage of deforestation is clearly much higher. Today's chief threat is to tropical forests, since most temperate regions are either lost or in a state of effective conservation. Devastation of tropical forests has been intense in recent decades in Indonesia, Phillipines and more recently Amazonia. While estimates of present loss rates vary, most developing countries in the tropics have an annual rate of loss of their rainforests ranging from 0.4 to 4.7 percent.
Grasslands are even more vulnerable than forests to habitat destruction, since their occurrence is often on easily farmed topography that is inviting for grain cultivation or livestock. In addition, grassland losses are harmful in that their inherent biodiversity is quite high. Replacement of natural grasslands with monoculture cereal crops totally transforms the landscape to effectively produce a biological desert in its lack of biodiversity. Tropical grasslands are generally derivatives of cleared forests, but their Holocene evolution has witnessed remarkable speciation and biodiversity gain as well. Tropical grasslands are endangered by pressures for overgrazing as well as grain production to feed the burgeoning tropical human population. While European grasslands have endured losses for centuries, it has been the last two hundred years that North American grasslands have been severely decimated.
Chaparral losses are also some of the least appreciated, since this biome is not universally respected for its natural beauty and biodiversity. In fact, humans often view these areas in the western USA, Southern Africa or Chilean Mattoral as landscapes not useful for any purpose other than housing developments; this misconstrued use often results in residential areas threatened by both wildfire as well as ensuing slope instability. Beginning in the 1980s in the western USA, the chaparral biome has begun to be understood and protected. Heathlands in the British Isles are European analogs of chaparral; these also have begun to gain understanding and respect within the United Kingdom habitat preservation planning scheme.
Beginning at least 7700 years before present in China and continuing to the present time, coastal wetlands have been under attack by humans. The diverse types of wetland destruction include filling for human habitation, pollution, harvesting of mangroves for charcoal and other types of agriculture, especially rice farming. In the last two centuries there has been extraordinary pressure on coastal wetlands for residential uses to house the human population explosion as well as tourism uses; this development has been intense in the last two centuries in such places as the California coast, Sea of Cortez in Mexico, Panama's Bocas del Toro coast, Antigua and other Caribbean Islands and the Mediterranean coast. The California coastal wetlands have shrunk by 90 percent in 150 years time.
The desert biome is a special case where the inhospitable nature of that landscape has limited human pressures in such disparate areas as Mongolia, the southwestern USA, the Kalahari Desert and the South African Karoo; however, a combination of the human population explosion and wealth have broken some of these barriers, notably in Southern California and Arizona. Besides wanton destruction by suburban sprawl and off-road vehicles, a more modern threat faces some of these deserts in the form of well-intentioned solar power. While superficially environmentally friendly, this form of large scale photovoltaic arrays can harm large tracts of desert lands, and thus is being debated more carefully at the present time. The great irony of the desert biome is the process of desertification, which one might think is leading to greater areas of desert in North Africa and China; however, desertification of drylands actually leads to a biologically depauperate landscape that lacks the inherent values of dryland, scrub or desert.
Some natural events such as volcanic eruptions, hurricanes, flooding, forest fires and other disturbances can cause habitat loss; however, these factors produce a very small percentage of the total habitat loss over the past 10,000 years. Furthermore, these natural events can be viewed as elements of ecological succession, that are part of the evolutionary fabric of speciation. More importantly, natural causes tend to produce relatively minor swaths of destruction compared to the systematic destruction of habitat by human activities. For example, volcanic eruptions from K?lauea, one of the world's most active volcanoes, has covered about four square kilometers of land per annum over the last 27 year period of intense eruption; moreover, much of the land covered three decades ago has been substantially recolonized by pioneer vegetation in the cycle that has built this island of Hawaii. By contrast in the central highlands of Madagascar, over a similar time span, slash-and-burn destruction of previous rainforests decimated over 60,000 square kilometers, with the destruction being irreversible, owing to the subsequent loss of topsoil and soil nutrients. Similarly hurricanes and flooding do not destroy a total habitat, but cause disruption which can be viewed as a natural cycle of nature, which has endured for hundreds of thousands of years within the context of ecosystems which have persisted over that time..
The outlook for halting habitat destruction is not favorable when viewed on a worldwide basis. A number of countries, such as the USA, Canada, Belize, Botswana, Israel, United Kingdom, Sweden, New Zealand and Australia, have advanced efforts for analysis of habitat values and national programs for protection of natural areas. Developing countries including China, Pakistan, Indonesia, Cambodia, Venezuela and most of Africa have substantial deficiencies in food production, and hence are under great pressure to exploit remaining natural areas for subsistence agriculture as well as cash crops. Approximately 98 percent of the usable agricultural area of the Earth has already been developed, so that enormous pressure will exist in the next four decades as the human population is expected to expand by antoher three billion people.
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