In Ecosystems, we discuss a number of forest types, whose makeup is determined by climate and the dominant tree varieties. Here let us consider what happens to a forest—particularly an old-growth forest—that experiences significant
Deforestation can take place naturally, as a result of changes in the soil and climate, but the most significant cases of deforestation over the past few thousand years have been the result of human activities. Usually, deforestation is driven by the need to clear land or to harvest trees for fuel and, in some cases, building. Though deforestation has been a problem the world over, since the 1970s it has become more of an issue in developing countries.
In developed nations such as the United States, environmental activism has raised public awareness concerning deforestation and has led to curtailment of large-scale cutting in forests deemed important environmental habitats. By contrast, developing nations, such as Brazil, are cutting down their forests at an alarming rate. Generally, economics is the driving factor, with the need for new agricultural land or the desire to obtain wood and other materials driving the deforestation process.
Yet the deforestation of such valuable reserves as the Amazon rain forest is an environmental disaster in the making: as noted in Soil Conservation, the soil in rain forests is typically "old" and leached of nutrients. Without the constant reintroduction of organic material from the
Deforestation has several extremely serious consequences. From a biological standpoint, it greatly reduces biodiversity, or the range of species in the biota. In the case of tropical rain forests as well as old-growth forests, certain species cannot survive once the environmental structure has been ruptured. From an environmental perspective, it leads to dangerous changes in the carbon content of the atmosphere, discussed later in this essay. In the case of old-growth forests or rain forests, deforestation removes an irreplaceable environmental asset that contributes to the planet's biodiversity—and to its oxygen supply.
Even from a human standpoint, deforestation takes an enormous toll. Economically, it depletes valuable forest resources. Furthermore, deforestation in many developing countries often is accompanied by the displacement of indigenous peoples. Other political and social horrors sometimes lurk in the shadows: for example, Brazil's forests are home to charcoal plants that amount to virtual slave-labor camps. Indians are lured from cities with promises of high income and benefits, only to arrive and find that the situation is quite different from what was advertised. Having paid the potential employer for transportation to the work site, however, they are unable to afford a return ticket and must labor to repay the cost.
Old-growth forests represent a climax ecosystem—one that has come to the end of its stages of succession. They are dominated by trees of advanced age (hence the name old-growth ), and the physical structure of these ecosystems is extraordinarily complex. In some places the canopy, or "rooftop," of the forest is dense and layered, while in others it has gaps. Tree sizes vary enormously, and the forest is littered with the remains of dead trees.
An old-growth forest, by definition, takes a long time to develop. Not only must it have been free from human disturbance, but it also must have been spared various natural types of disturbance that bring about succession: catastrophic storms or wildfire, for instance. For this reason, most old-growth forests are rain forests in tropical and temperate environments. Among North American old-growth forests are those of the United States Pacific Northwest as well as those in adjoining regions of southwestern Canada.
These old-growth forests are home to a bird that, in the 1980s and 1990s, became well known both to environmentalists and to their critics: the northern spotted owl, or Strix occidentalis caurina. A nonmigratory bird, the spotted owl has a breeding pattern such that it requires large tracts of old-growth, moist-to-wet conifer forest—that is, a forest dominated by cone-producing trees—as its habitat. Given the potential economic value of old-growth forests in the region, the situation became one of heated controversy.
On the one hand, environmentalists insisted that the spotted owl's existence would be threatened by logging, and, on the other hand, representatives of the logging industry and the local community maintained that prevention of logging in the old-growth forests would cost jobs and livelihoods. The question was not an easy one, pitting the interests of the environment against those of ordinary human beings. By the early 1990s, the federal government had stepped in on the side of the environmentalists, having recognized the spotted owl as a threatened species under the terms of the U.S. Endangered Species Act of 1973. Nonetheless, controversy over the spotted owl—and over the proper role of environmental, economic, and political concerns in such situations—continues.
Deforestation and other activities pose potential dangers to our atmosphere. In particular, such activities have led to an increasing release of greenhouse gases, which may cause the warming of the planet. As discussed in Energy and Earth, the greenhouse effect, in fact, is a natural process. Though it is typically associated, in the popular vocabulary at least, with the destructive impact of industrial civilization on the environment, it is an extremely effective mechanism whereby Earth makes use of energy from the Sun.
Rather than simply re-radiating solar radiation, Earth traps some of this heat in the atmosphere with the help of greenhouse gases, such as carbon dioxide. As in the case of most natural processes, however, if a little bit of carbon dioxide in the atmosphere is good, this does not mean that a lot is better.
As noted in the essay Carbon Cycle, all living things contain carbon in certain characteristic structures; hence, the term organic refers to this type of carbon content. Though carbon dioxide is not an organic compound, it is emitted by animals: they breathe in oxygen, which undergoes a chemical reaction in their carbon-based bodies, and, as a result, carbon dioxide is released. Plants, on the other hand, receive this carbon dioxide and, through a chemical process in their own cellular structures, take in the carbon while releasing the oxygen.
Mature forests, such as those of the old-growth variety, contain vast amounts of carbon in the form of living and dead organic material: plants, animals, and material in the soil. Because this quantity is much greater than in a younger forest, when deforestation occurs in a mature forest ecosystem, the mature forest will be replaced by an ecosystem that contains much smaller amounts of carbon.
Ultimately, the carbon from the former ecosystem will be released to the atmosphere in the form of carbon dioxide. This will happen quickly, if the biomass of the forest is burned, or more slowly, if the timber from the forest is used for a long periods of time, for instance, in the building of houses or other structures.
Before humans began cutting down forests, Earth's combined vegetation stored some 990 billion tons (900 billion metric tons) of carbon, 90% of it appeared in forests. Today only about 616 billion tons (560 billion metric tons) of carbon are stored in Earth's vegetation, and the amount is growing smaller as time passes. At the same time, the amount of carbon dioxide in the atmosphere has increased from about 270 parts per million (ppm) in 1850 to about 360 ppm in 2000, and, again, the increase continues.
Given this rise in atmospheric carbon dioxide as a result of deforestation—not to mention the more well-known cause, burning of fossil fuels—it is no wonder that atmospheric scientists and environmentalists are alarmed. Some of these scientists hypothesize that larger concentrations of carbon dioxide in the atmosphere will lead to increased intensity of the greenhouse effect. If this is true, it is possible that global warming will ensue, an eventuality that could have enormous implications for human survival. As a worst-case scenario, the polar ice cap (see Glaciology) could melt, submerging the cities of Earth.
Before succumbing to the sort of doomsday thinking and scaremongering for which many environmentalists are criticized, however, it is important to recognize that several contingencies are involved: if carbon dioxide in the atmosphere causes an increase in the intensity of the greenhouse effect, it could cause global warming. The fact is that despite a few mild winters at the end of the twentieth century, it is far from clear that the planet is warming. The winter of 1993, for instance, produced one of the worst blizzards that the eastern United States has ever seen.
As recently as the mid-1970s some environmentalists claimed that Earth actually is cooling —a response to a spate of cold winters in that period. The fact of the matter is that climate cycles are difficult to determine and require the perspective of several centuries' worth of data (at least), rather than just a few years' worth. (See Glaciology for a discussion of the Little Ice Age, which took place just a few centuries ago.)
Nonetheless, it is important to be aware of the legitimate environmental concerns raised by the increased presence of carbon dioxide in the atmosphere due to human activities. Atmospheric scientists continue to monitor levels of greenhouse gases and to form hypotheses regarding the ultimate effect of such activities as deforestation and the burning of fossil fuels.
As we have seen in a number of ways, one of the key concepts of ecological studies is also a core principle in the modern approach to the earth sciences. In both cases, there is the idea that a disturbance in one area can lead to serious consequences elsewhere. The interconnectedness of components in the environment thus makes it impossible for any event or phenomenon to be truly isolated.
A good example of this is biomagnification. Biomagnification is the result of bioaccumulation, or the buildup of toxic chemical pollutants in the tissues of individual organisms. Part of what makes these toxins dangerous is the fact that the organism cannot process them easily either by metabolizing them (i.e., incorporating them into the metabolic system, as one does food or water) or by excreting them. Yet the organism ultimately does release some toxins—by passing them on to other members of the food web. This increase in contamination at higher levels of the food web is known as biomagnification.
Among the most prominent examples of chemical pollutants that are bioaccumulated are such pesticides as DDT (dichlorodiphenyl-trichloroethane). DDT is a chlorinated hydrocarbon (see Economic Geology) used as an insecticide. Because of its hydrocarbon base, DDT is highly soluble in oils—and in the fat of organisms. Once pesticides such as DDT have been sprayed, rain can wash them into creeks and, finally, lakes and other bodies of water, where they are absorbed by creatures that drink or swim in the water.
Atmospheric deposition, for instance, from industrial smokestacks or automobile emissions, is another source of toxins. Sludge from a sewage treatment plant can make its way into water sources, spreading all sorts of pollutants to the food web. Whatever the case, these toxins usually enter the food web by attaching to the smallest components. Particles of pollutant may stick to algae, which are so small that the toxin does little damage at this level of the food web. But even a small herbivore, such as a zooplankton, when it consumes the algae, takes in larger quantities of the pollutant, and thus begins the cycle of bio-magnification.
By the time the toxin has passed from a zoo-plankton to a small fish, the amount of pollutant in a single organism might be 100 times what it was at the level of the algae. The reason, again, is that the fish can consume 10 zooplankton that each has consumed 10 algae. (These particular numbers, of course, are used simply for the sake of convenience.) By the time the toxins have passed on to a few more levels in the food web, they might be appearing in concentrations as great as 10,000 times their original amount.
For a period of about two decades before 1972, DDT was used widely in the United States to help control the populations of mosquitoes and other insects. Eventually, however, it found its way into water sources and fish species through the process we have described. Predatory birds, such as osprey, peregrine falcons, and brown pelicans, consumed these fish. So, too, did the bald eagle, which has long been a protected species owing to its role as America's national symbol.
DDT levels became so high that the birds' eggshells became abnormally thin, and adult birds sitting on nests accidentially would break the shells of unhatched eggs. As a result, baby birds died, and populations of these species also died. Public awareness of this phenomenon, raised by environmentalists in the late 1960s and early 1970s, led to the banning of DDT spraying in 1972. Since that time, populations of many predatory birds have increased dramatically.
In the case of DDT biomagnification, humans were not directly involved, because the species of birds affected were not ones that people consume for food. Yet bioaccumulation and biomagnification have threatened humans. For example, in the 1950s, cows fed on grass that had been exposed to nuclear radiation and this radioactive material found its way into milk. Another example occurred during the 1970s and 1980s, when fish, such as tuna, were found to contain abnormally high levels of mercury.
This led the federal government and some states to issue warnings against the consumption of certain types of fish, owing to bioaccumulated levels of toxic pollutants. Obviously, such measures, however well intentioned, are just cosmetic fixes for larger problems. In the long run, what is needed is a systemic ecological approach that attempts to address problems such as biomagnification and the accumulation of greenhouse gases by approaching the root causes.
Ashworth, William, and Charles E. Little. Encyclopedia of Environmental Studies. New York: Facts on File, 2001.
The Ecological Society of America: Issues in Ecology (Web site). <http://esa.sdsc.edu/issues.htm> .
Ecology WWW Page (Web site). <http://pbil.univ-lyon1.fr/Ecology/Ecology-WWW.html> .
Endangered Species on EE-Link (Web site). <http://126.96.36.199/EndSpp/endangeredspecies-mainpage.html> .
Markley, O. W., and Walter R. McCuan. 21st Century Earth: Opposing Viewpoints. San Diego, CA: Green-haven Press, 1996.
The Marshall Cavendish Illustrated Encyclopedia of Plants and Earth Sciences. New York: Marshall Cavendish, 1988.
The Need to Know Library—Ecology and Environment Page (Web site). <http://www.peak.org/~mageet/tkm/ecolenv.htm> .
Philander, S. George. Is the Temperature Rising?: The Uncertain Science of Global Warming. Princeton, NJ: Princeton University Press, 1998.
Schultz, Warren. The Organic Suburbanite: An Environmentally Friendly Way to Live the American Dream. Emmaus, PA: Rodale, 2001.