Ecology is the study of the relationships between organisms and their environments. As such, it is subsumed into the larger subject of ecosystems, which encompasses both living and nonliving components of the environment. As a study of the biological aspect of ecosystems, ecology is properly a part of the biological sciences rather than the earth sciences; however, in practice it is difficult to draw a line between the disciplines. This is especially the case inasmuch as the study of the environment involves such aspects as soil science, where earth sciences and ecology meet. This fact, combined with increasing concerns over ecological stresses, such as the increase of greenhouse gases in the atmosphere, warrants the consideration of ecology in an earth sciences framework.


Ecosystems, biological Communities, Ecology

An ecosystem is the complete community of living organisms and the nonliving materials of their surroundings. It therefore includes components that represent the atmosphere, the hydrosphere (all of Earth’s waters, except for moisture in the atmosphere), the geosphere (the soil and extreme upper portion of the continental crust), and the biosphere. The biosphere includes all living things: plants (from algae and lichen to shrubs and trees); mammals, birds, reptiles, amphibians, aquatic life, and insects as well as all manner of microscopic forms, including bacteria and viruses. In addition, the biosphere draws together all formerly living things that have not yet decomposed.
The components of the biosphere are united not only by the fact that all of them are either living or recently living but also by the food web. The food web, discussed in much detail within the context of Ecosystems, is a complex network of feeding relationships and energy transfers between organisms. At various levels and stages of the food web are plants; herbivores, or plant-eating organisms; carnivores (meat-eating organisms); omnivores (organisms that eat both meat and plants); and, finally, decomposers and detritivores, which obtain their energy from the chemical breakdown of dead organisms.


When discussing the living components of an ecosystem—that is, those components drawn from the biosphere—the term biological community is used. This also may be called biota, which refers to all flora and fauna, or plant and animal life, respectively, in a particular region. The relationship between these living things and their larger environment, as we have noted, is called ecology. Pioneered by the German zoologist Ernst Haeckel (1834-1919), ecology was long held in disdain by the world scientific community, in part because it seemed to defy classification as a discipline. Though its roots clearly lie in biology, its broadly based, multidisciplinary approach seems more attuned to the earth sciences.
In any case, ecology long since has gained the respect it initially failed to receive, and much of that change has to do with a growing acceptance of two key concepts. On the one hand, there is the idea that all of life is interconnected and that the living world is tied to the nonliving, or inorganic, world. This is certainly a prevailing belief in the modern-day earth sciences, with its systems approach (see Earth Systems). On the other hand, there is the gathering awareness that certain aspects of industrial civilization may have a negative impact on the environment.
Clearly, the ecosystem as a whole is held together by tight bonds of interaction, but where the biological community is concerned, those bonds are even tighter. For the biological community to survive and thrive, a balance must be maintained between consumption and production of resources. Nature provides for that balance in numerous ways, but beginning in the late twentieth century, environmentalists in the industrialized world became increasingly concerned over the possibly negative effects their own societies exert on Earth’s ecosystems and ecological communities.

Climax and Succession

One of the concerns raised by environmentalists is the issue of endangered species, or varieties of animal whose existence is threatened by human activities. In fact, nature itself sometimes replaces biological communities in a process called succession. Succession involves the progressive replacement of earlier biological communities with others over time. Coupled with succession is the idea of climax, a theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession.
Succession typically begins with a disturbance exerted on the preexisting ecosystem, and this disturbance usually is followed by recovery. This recovery may constitute the full extent of the succession process, at which point the community is said to have reached its climax point. Whether or not this happens depends on such particulars as climate, the composition of the soil, and the local biota.
There are two varieties of succession, primary and secondary. Primary succession occurs in communities that have never experienced significant modification of biological processes. In other words, the community affected by primary succession is “virgin,” and primary succession typically involves enormous stresses. On the other hand, secondary succession happens after disturbances of relatively low intensity, such that the regenerative capacity of the local biota has not been altered significantly. Secondary succession takes place in situations where the biological community has experienced alteration.


Whereas climax and succession apply to broad biological communities, the term niche refers to the role a particular organism or species plays within the larger community. Though the concept of niche is abstract, it is unquestionable that each organism plays a vital role and that the totality of the ecosystem would suffer stress if a large enough group of organisms were removed from it. Furthermore, given the apparent interrelatedness of all components in a biological community, every species must have a niche—even human beings.
An interesting idea related to the niche is the concept of an indicator species: a plant or animal that by its presence, abundance, or chemical composition demonstrates a particular aspect of the character or quality of the environment. Indicator species can, for instance, be plants that accumulate large concentrations of metals in their tissues, thus indicating a preponderance of metals in the soil. This metal could indicate valuable deposits nearby, or it could serve as a sign that the soil is being contaminated.
In the rest of this essay, we explore a few examples of ecological stress—situations in which the relationship between organisms and environment has been placed under duress. We do not attempt to explore the ideas of succession, climax, niche, or indicator species with any consistency or depth; rather, our purpose in briefly discussing these terms is to illustrate a few of the natural mechanisms observed or hypothesized by ecologists in studying natural systems. The vocabulary of ecology, in fact, is as complex and varied as that of any natural science, and much of it is devoted to the ways in which nature responds to ecological stress.



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 disturbance. Actually, the term deforestation can describe any interruption in the ordinary progression of the forest’s life, including clear-cut harvesting, even if the forest fully recovers.
Rain forest destruction by fire in Madagascar. Such deforestation affects the carbon balance in the atmosphere and the diversity of species on Earth.
Rain forest destruction by fire in Madagascar. Such deforestation affects the carbon balance in the atmosphere and the diversity of species on Earth.
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.

Developed and developing nations

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 plants and animals of the rain forests, it would be too poor to grow anything. Therefore, when nations cut down their own rain forest lands, in effect, they are killing the golden goose to get the egg. Once the rain forest is gone, the land itself is worthless.
Old-growth forests are home to the northern spotted owl, recognized as an endangered species because of the destruction of its habitat.
Old-growth forests are home to the northern spotted owl, recognized as an endangered species because of the destruction of its habitat.

Consequences of deforestation

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

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.

The spotted owl

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.

The Greenhouse Effect

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.

The result of cutting mature forests

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.

Should we be worried?

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.)


Bioaccumulation: The buildup of toxic chemical pollutants in the tissues of individual organisms.
Biological community: The living components of an ecosystem.
Biomagnification: The increase in bioaccumulated contamination at higher levels of the food web. Biomagnification results from the fact that larger organisms consume larger quantities of food—and, hence, in the case of polluted materials, more toxins.
Biosphere: A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.
Biota: A combination of all flora and fauna (plant and animal life, respectively) in a region.
Canopy: The upper portion of the trees in a forest. In a closed-canopy forest, the canopy (which may be several hundred feet, or well over 50 meters, high) protects the soil and lower areas from sun and torrential rainfall.
Carnivore: A meat-eating organism.
Climax: A theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession.
Decomposers: Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.
Decomposition reaction: A chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. On Earth, this often is achieved through the help of detritivores and decomposers.
Detritivores: Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers; however, unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots.
Ecology: The study of the relationships between organisms and their environments.
Ecosystem: A community of interdependent organisms along with the inorganic components of their environment.
Energy transfer: The flow of energy between organisms in a food web.
Food web: A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these organisms consumes nutrients and passes them along to other organisms. Earth scientists typically prefer this name to food chain, an everyday term for a similar phenomenon. A food chain is a series ofsingu-lar organisms in which each plant or animal depends on the organism that precedes or follows it. Food chains rarely exist in nature.
Geosphere: The upper part of Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.
Greenhouse effect: Warming of the lower atmosphere and surface of Earth. This occurs because of the absorption of long-wavelength radiation from the planet’s surface by certain radiatively active gases, such as carbon dioxide and water vapor, in the atmosphere. These gases are heated and ultimately re-radiate energy to space at an even longer wavelength.
Herbivore: A plant-eating organism.
Hydrosphere: The entirety of Earth’s water, excluding water vapor in the atmosphere, but including all oceans, lakes, streams, groundwater, snow, and ice.
Niche: A term referring to the role that a particular organism plays within its biological community.
Omnivore: An organism that eats both plants and other animals.
Organic: At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.
Succession: The progressive replacement of earlier biological communities with others over time.
System: Any set of interactions that can be set apart from the rest of the universe for the purposes of study, observation, and measurement.
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.

Bioaccumulation and Biomagnification

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.

Addressing ecological concerns

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.

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