Figure 25.29
Herds of bison also made up part of the tallgrass prairie community.
Another habitat that is being rapidly destroyed is forests, most significantly tropical rainforests, one of the two major ecosystems (or biomes) with the highest biodiversity on earth. The largest cause of deforestation today is slash-and-burn agriculture (Figure below), practiced by over 200 million people in tropical forests throughout the world. Depletion of the thin and nutrient-poor soil (even so, biodiversity here is high – can you guess why?) often results in people abandoning the forest within a few years, and subsequent erosion can lead to desertification (a process leading to production of a desert of formerly productive land [usually at least semi-arid]).
Half of the earth’s mature tropical forests are gone. At current rates of deforestation, all tropical forests will be gone by 2090. Poverty, inequitable land distribution, and overpopulation combine in many developing countries to add pressure to habitats which are already stressed. Use of firewood, charcoal and manure for cooking and other energy needs, and waste of crops further degrade environments, threatening biodiversity through habitat loss. Other causes of habitat destruction include poor fire management, invasion of pest and non-native species, overfishing, mining, pollution, and storm damage.
Why Habitat Destruction Threatens Species
Agriculture, forestry, mining, and urbanization have disturbed over half of the earth’s land. Inevitably, species disappear and biodiversity decreases. Habitat destruction is currently ranked as the most important cause of extinction of species worldwide.
The destruction of a species’ habitat may alter the landscape to such an extent that the species is no longer able to survive and becomes extinct. This may occur directly, such as the environment becoming toxic, or indirectly by limiting a species’ ability to compete effectively for diminished resources or with a new species.
Habitat destruction through pollution can kill off a species very rapidly, by killing all living members by contamination or sterilization. It can also occur over longer periods at lower toxicity levels, by affecting life span, reproductive capability, or competitiveness.
Habitat destruction can also occur physically by elimination of certain niches in a habitat. For example, elimination of dense tropical rainforest and replacement with open pastureland can affect certain species. Thus, a fern that depends on dense shade for protection from direct sunlight can no longer survive without trees to shelter it. Another example of this is the destruction of ocean floors by bottom trawling.
Fewer resources or introduction of new competitor species often accompany habitat destruction. Global warming has allowed some species to expand their ranges, sometimes into those of species that previously occupied that area. If these new competitors are predators, they may directly affect prey species, or they may compete with other species for limited resources. If such resources as water and food are limited during habitat destruction, then species can become extinct.
Another type of habitat that is being rapidly destroyed is the wetland. By the 1980s, over 80% of all historic wetlands in seven states of the U.S. were filled, at which time Congress acted to create a policy of “no net loss” of wetlands. In Europe, extensive loss of wetlands has resulted in loss of biodiversity. For example, many bogs in Scotland have been drained or developed because of human population expansion. Over half of the Portlethen Moss in Aberdeenshire, for example, has been lost and a number of species, such as the great crested newt, are no longer present.
Another example of species loss due to habitat destruction occurred on Madagascar’s central highland plateau. From 1970 to 2000, slash and burn agriculture eliminated about 10% of the country’s total native biomass and converted it to a barren wasteland. Adverse effects included widespread gully erosion that produced heavily silted rivers and eliminated a large amount of usable fresh water. Much of the riverine ecosystems of several large west-flowing rivers were also destroyed, several fish species have been driven to the edge of extinction, and some coral reef formations in the Indian Ocean are effectively lost.
Practices such as clear-cutting of old growth forests, strip mining (Figure below), and driftnet fishing can go beyond the harvesting of a single species or resource to degrade entire ecosystems. Overexploitation happens on the level of genes and ecosystems as well as individual species. Forest plantations, fish hatcheries and farms, and intensive agriculture reduce both species diversity and genetic diversity within species.
Figure 25.30
Strip coal mining, pictured here, has degraded the entire ecosystem.
Other Causes of Extinction
One of the primary causes of extinction (already mentioned briefly) is introduction of exotic species (alien or invasive species). Both intentionally and inadvertently, humans have introduced various species into habitats, which already have their own native species. As a result, these invasive species have often had very harmful effects on the native species.
As long ago as 3500 BC, ships from Polynesian times brought crop species and domesticated animals as well as stowaway rats and snakes. Recently cargo ships have transported zebra mussels, spiny waterfleas, and ruffe into the Great Lakes via ballast water ((Figure below). Europeans brought purple loosestrife and European buckthorn to North America to beautify their gardens.
Figure 25.31
These zebra mussels, an introduced species, colonize most man-made and natural surfaces, including native mussels. Here they have infested the walls of the Arthur V. Ormond Lock, on the Arkansas River. They have caused significant damage to American waterways, locks, and power plants.
Other invasive species have included the European starling, introduced by Shakespeare enthusiast Eugene Schieffelin to Central Park in the 1890s, because he thought Americans should experience every bird mentioned in the works of Shakespeare. This species is a hole-nesting species and has affected native species where it has been introduced (i.e. Australia, North America) because of competition for nest sites. Other examples of invasive species include the introduction of the cane toad, introduced to control the cane beetle, and the brown tree snake (Figure below).
Many of these exotic species, away from the predation or competition of their native habitats, have unexpected and negative effects in the new ecosystems. Introduced species can disrupt food chains, carry disease, prey on native species directly, and as we have already seen, out-compete natives for limited resources. All of these effects can lead to extinctions of the native species. In addition, some introduced species hybridize with native species, resulting in genetic pollution, which weakens natural adaptations.
Another major cause of extinction is global climate change. As we have already seen earlier in this chapter, our increasing reliance on fossil fuels in altering the earth’s atmosphere, and as a result, climate. This has many effects, some of which we have already discussed, but on a species level, these other effects, including changing air and water temperatures, rainfall patterns, and salinity threaten species adapted to pre-warming conditions and thus result in a decline of biodiversity globally.
Overpopulation (already mentioned previously), along with developments in technology, have added tremendous pressure to resource and land use and add to all of the previously mentioned threats to biodiversity. The highest rates of population growth are often in third world tropical countries where biodiversity is also highest. Therefore pressures from local populations as well as increased pressure from incoming tourists in some areas can produce enormous consequences for the local plant and animal ecosystems.
A final major cause of extinction is pollution, and mentioned earlier in this lesson. Pollution adds chemicals, noise, heat, or even light beyond the capacity of the environment to absorb them without major harmful effects on all kinds of organisms.
One good example of a toxic chemical affecting a species was the use of the pesticide, DDT. Use of this pesticide in the eastern United States resulted in the effect of biological
magnification (where many synthetic chemicals concentrate as they move through the food chain, so that toxic effects are multiplied), with the result of the disappearance of the peregrine falcon from this area. As a result, DDT was banned in the U.S.
Pollution continues to contribute to habitat destruction and decreasing biodiversity worldwide, especially in developing countries. Air pollution knows no boundaries and as we have already seen, its effects on acid rain, ozone depletion, and global warming all affect biodiversity.
Water pollution especially threatens vital freshwater and marine resources throughout the world. Specifically, industrial and agricultural chemicals, waste, acid rain, and global warming threaten waters, essential for all ecosystems. Finally, soil contamination, mostly from toxic industrial and municipal wastes (Figure below), salts from irrigation, and pesticides from agriculture all degrade soils, the foundation of terrestrial ecosystems and their biodiversity.
Figure 25.32
Soil contamination caused by underground storage tanks containing tar.
Outside the developed world, pollution controls often lag far behind those of the U.S. and Europe, and some developing nations, like China, are rapidly increasing their levels of pollution. Many pollution problems are also present in industrialized nations as well; industry and technology add nuclear wastes, oil spills (Figure below), thermal pollution from wastewater, acid rain, and more to the challenges facing the earth’s biodiversity (Figure below).
Figure 25.33
An oiled bird from an oil spill in San Francisco Bay. About 58,000 gallons of oil spilled from a South Korean-bound container ship when it struck a tower supporting the San Francisco-Oakland Bay Bridge in dense fog, 11/07.
Figure 25.34
A highly endangered Macquarie perch specimen was caught on a lure with barbless hooks in a high altitude upland river and was carefully released. This species is now extinct in most of its upland river habitats due to introduced trout species in the same habitats. Siltation from agricultural practices and flow regulation and thermal pollution by dams have also caused the extinction of this species in some upland rivers.
Importance of Biodiversity
Does it matter if we are losing thousands of species each year, when the earth holds millions and life has been through extinction before? The answer is yes; it matters even if we consider only direct benefits to humans. But there are also lots of indirect benefits, also known as ecosystem services, in addition to benefits to other species as well.
Biodiversity is important for a number of reasons. Economically, direct benefits include the potential to diversify our food supply; increase resources for clothing, shelter, energy, and medicines; a wealth of efficient designs which could inspire new technologies; models for medical research; and an early warning system for toxicity.
In our food supply, monocultures (large-scale cultivation of single varieties of single species) are very vulnerable to disease. As recently as 1970, blight affected the corn belt where 80% of maize grown in the U.S. was of a single type (Figure below). Contemporary breeders of various crop species increase the genetic diversity by producing hybrids of crop species with wild species adapted to local climate and disease.
Figure 25.35
In order to increase the genetic diversity of corn, these unusually colored and shaped Latin American maize are bred with domestic corn lines. Such hybrids have the potential for increased productivity, nutritional value, adaptation to local climates, and resistance to local diseases.
As many as 40,000 species of fungi, plants, and animals provide us with many varied types of clothing, shelter, and other products. These include poisons, timber, fibers, fragrances, papers, silks, dyes, adhesives, rubber, resins, skins, furs, and more. In addition to these above raw materials for industry, we use animals for energy and transportation, and biomass for heat and other fuels.
According to one survey, 57% of the most important prescription drugs come from nature (bacteria, fungi, plants, and animals) (Figure below), yet only a fraction of species with medicinal properties have been examined. Bionics, also known as biomimetics or biomimicry, uses organisms as models for engineering inspiration. For example, rattlesnake heat-sensing pits suggest infrared sensors and Zimbabwe’s Eastgate Centre (Figure below) was inspired by the air-conditioning efficiency of a termite mound (Figure below).
Figure 25.36
Aspirin originates in the bark of the white willow, pictured here.
Figure 25.37
Design of this Eastgate Centre, in Zimbabwe, which requires just 10% of the energy needed for a conventional building of the same size was inspired by a biological design (See Figure 17).
Figure 25.38
The air-conditioning efficiency of this termite mound was the inspiration for the Eastgate Centre (Figure 16).
At an ecological level, biodiversity provides ecosystem stability and productivity; the maintenance and renewal of soils, water supplies, and the atmosphere; nitrogen fixation and nutrient recycling; pollination, pest, and disease control; and waste disposal. Other benefits include the cultural, aesthetic, and spiritual values of biodiversity and its importance to many types of recreation.
Biodiversity is critically important for us and for the earth, and it is declining at a fast rate. What can you do to help to protect habitats, which are at the crux of biodiversity?
Protecting Habitats
There are lots of things we can do to protect biodiversity, some of which we’ve touched upon in prior sections of this lesson, including the need to reduce, reuse, and recycle of all resources; not contributing to introduction of invasive species; practicing sustainable management on your own land; adopting and spreading sustainable perspectives and philosophy; learning more about biodiversity; and taking action as a citizen to make sure biodiversity is protected.
We are going to focus now on what can be done, or has already been done, to protect habitats, the actual physical spaces, themselves, which, as we have seen, contributes to maintaining and increasing biodiversity. What do you think helps protect habitats and what can you do to help protect them?
Perhaps if you’ve taken a trip, or even in your own community, you’ve enjoyed some time exploring and enjoying the outdoors. Think of the areas you might have visited that seemed, even somewhat, undisturbed, in other words, areas where there was little disturbance from human influence. Maybe you were able to enjoy scenic landscape, enjoy some quiet where you could hear the sounds of nature, or maybe see very few people. Sometimes we need to get away from all the noise and pollution and be in a quiet place, not only to enjoy and appreciate the nature around us, but even to experience some quiet within ourselves.
If you think back on some of these places, what characteristics of the actual physical location did you observe? Does it require a huge amount of space to protect a habitat, or will even a small space do? From what we know about habitats and species, how much space is enough to ensure species will not become extinct or threatened?
There may not be a clear answer to this. It really depends on the species involved and what its requirements are. A large mammal, like a species of big cat, who has a large range, may need more land than a much smaller species, like a snail. Often, if we protect the habitat of a keystone species (See the From Populations to the Biosphere chapter, Lesson on Communities), which usually has a larger habitat than all the other species in that community, then all the other habitats of other species within that community will be protected as well.
The kinds of protected areas, we are talking about, that help protect species are usually in the form of national parks, nature reserves, state parks, and even community and town parks. Sometimes it is important to also protect interconnecting corridors between parks or reserves to protect those species that travel from one area to another for purposes of breeding or feeding, for example.
Even though many of these protected areas are already in existence, there is much you can do as a citizen to make sure these areas stay p
rotected and to help create other areas that need to be protected. Some of the things you can do are to get involved with your community or town’s efforts to protect local areas. Even if you don’t understand everything that goes on at a town meeting, you might want to attend one sometime to learn about some of the important local environmental issues that are being discussed.
Join local groups which monitor ecosystem health, such as Frog Watch, River Watch, or bird counts. Some national organizations have programs, such as National Audubon’s Great Backyard Bird Count and Operation Feeder Watch, and similar programs run by the National Wildlife Foundation, where you can keep track of what you see in your backyard and thereby contribute to a greater understanding of biodiversity.
Become aware of some of the habitat issues on a state and national level. Maybe you can write or e-mail your state representatives, for example, to urge them to help protect areas large enough to accommodate migration, flooding, buffer zones, pollution from nearby development, and even people and their activities. It is a challenge to balance the needs of an increasing population with natural resource needs, but we have to remember that people, as well as wildlife, depend on natural resources to flourish and survive!
Volunteer with local organizations that protect habitat. Help out at cleanup days in your community, where people gather together to pick up trash and make a habitat more hospitable for its inhabitants. Some of these cleanup days are even advertised through your school. Start an ecology club at your school, if there isn’t one already, and encourage your friends and classmates to join.
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