Ecosystems and Ecology

views updated

ECOSYSTEMS AND ECOLOGY

CONCEPT

Composed of living organisms and the remains of living things as well as the nonliving materials in their surroundings, an ecosystem is a complete community. Its components include plants, animals, and microorganisms, both living and dead; soil, rocks, and minerals; water sources above and below ground; and the local atmosphere. An ecosystem can consist of an entire rain forest, geographically larger than many nations, or it can be as small as the body of an animal, which is likely to comprise far more microorganisms than there are people on Earth. Among the most significant areas of interest in the realm of ecosystems is ecology, or the study of the relationships between organisms and their environments. Forests, a broad category of ecosystem, provide a living laboratory in which to investigate the ways in which organisms interact with their environments. They also aptly illustrate the stresses placed on ecosystems by human activities.

HOW IT WORKS

The Biosphere and Food Webs

In the sciences, a system is any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement. Modern earth scientists regard the planet as a massive ecosystem, the stage on which four extraordinarily complex earth systems interact. These systems are the atmosphere, the hydrosphere (all the planet's waters, except for moisture in the atmosphere), the geosphere (the soil and the extreme upper portion of the continental crust), and the biosphere.

The biosphere consists of all living things—microorganisms, plants, insects, birds, marine life, and all other forms of animals—as well as formerly living things that have not yet decomposed. (Decomposed remains of organic materials become part of the geosphere, specifically the soil.) Organisms in the biosphere, whether living or formerly living, are united by the interrelation of energy transfer that takes place through the food web.

A food web is similar to the more well known expression food chain. In scientific terms, however, a food chain is defined as a series of singular organisms in which each plant or animal depends on the organism that precedes or follows it. This rarely exists in naturas instead, the feeding relationships between organisms in the real world are much more complex and are best described as a web rather than a chain. (For more on the biosphere and earth systems, see The Biosphere.)

Energy Flow and Nutrients

Energy is the ability of objects or systems to accomplish work. (The latter is defined as the exertion of force over a given distance: for example, a plant growing from the ground, an insect or bird flying, or a human or pack animal moving an object.) Food webs are built around energy transfer, or the flow of energy between organisms, which begins with plant life. Hence the importance of plants to ecosystems, as we illustrate later in discussing various types of forest, which are defined by their dominant varieties of tree.

Plants absorb energy in two ways. From the Sun they receive electromagnetic energy in the form of visible light and invisible infrared waves, which they convert to chemical energy by means of photosynthesis. In addition, plants take in nutrients from the soil, which contains energy in the forms of various chemical compounds. These compounds may be organic, which typically means that they came from living things, though, strictly speaking, organic refers to characteristic carbon-based chemical structures. Plants also receive inorganic compounds from minerals in the soil. (See Paleontology for an explanation of the scientific distinction between organic and inorganic.)

Contained in these minerals are six chemical elements essential to the sustenance of life on planet Earth: hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. These are the elements involved in biogeochemical cycles, through which they are circulated continually between the living and nonliving worlds—that is, between organisms on the one hand and the inorganic realms of rocks, minerals, water, and air on the other. (See The Biosphere for more about biogeochemical cycles.)

FROM PLANTS TO MEAT EATERS.

As plants take up nutrients from the soil, they convert them into other forms. Eventually, the plants themselves become food either for herbivores (plant-eating organisms) or for omnivores (organisms that eat both plants and other animals), thus passing along these usable, energy-containing chemical compounds to other participants in the food web. It is likely that an herbivore will be eaten in turn either by an omnivore (for example, humans as well as a number of bird species and many others) or by a carnivore, an organism that eats only meat.

Carnivores and omnivores are not usually prey for other carnivores or omnivores, but this does happen in the case of what are known as tertiary consumers (see Food Webs). There is also the matter of cannibalism, discussed in Biological Communities. For the most part, however, the only creatures that eat carnivores and omnivores do so after these organisms have died or been killed.

DETRITIVORES AND DECOMPOSERS.

An animal that obtains its energy in this way, from consuming the carcasses of carnivores and omnivores (as well as herbivores and perhaps even plants), is known as a detritivore. Large and notable examples include vultures and hyenas, though most detritivores—earthworms or maggots, for instance—are much smaller.

Nonetheless, detritivores are relatively large, complex organisms compared with another variety of species that occupies a level or position in the food web that comes "after" carnivores and omnivores: decomposers, including bacteria and fungi.

This illustrates the reason why ecological sciences treat the expression food chain with disfavor. There is no such thing as "the top of the food chain," rather, there are simply stages, like a circular assembly line, with detritivores occupying a position between meat-eating animals and plants. Earthworms, in particular, help convert animal bodies into soil nutrients useful to the growth of plant life, yet even these and other detritivores must themselves eventually be converted to soil as well.

This "final" stage of conversion—that is, the last stop after the animal portion of the food web and before the cycle comes back around to ordinary plants—is occupied by decomposers, such as bacteria and fungi. These decomposers obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. Like detritivores, they aid in decomposition, a chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements.

Often an element such as nitrogen appears in forms that are not readily usable to organisms, and therefore such elements (which may appear individually or in compounds) need to be processed chemically through the body of a decomposer or detritivore. This process brings about a chemical reaction in which the substance (whether an element or compound) is transformed into a more usable version. By processing these chemical compounds, decomposers and detritivores provide nutrients necessary to plant growth.

REAL-LIFE APPLICATIONS

Forests and Ecology

One easily understandable example of ecosystems and ecology in action is the forest. Virtually everyone has visited a forest at one time or another, and those who are enthusiasts for the great outdoors may spend a great deal of time in one. In the past, of course, people interacted with forests not so much out of choice, and certainly not with recreation as the foremost aim in mind, but simply because they depended on the forest for survival. Not only did the forest provide hunters and food-gatherers with an abundance of wildlife and fruit, but trees provided material for building dwellings. It is no wonder, then, that many early human settlements tended to be in, or at the edges of, forests.

A forest is simply an ecosystem dominated by trees. There are many varieties of forest, however, because so many factors go into determining the character of a forest ecosystem. The fact that the forest is an ecosystem means that its qualities are defined by far more than just the varieties of trees, which are simply the most visible among many biological forms in the forest. Numerous abiotic, or nonbiological, factors also affect the characteristics of a forest as well. For instance, there is weather, defined as the condition of the atmosphere at a given time and place, and climate, the overall patterns of weather for extended periods.

These play a clear role, for instance, in defining the tropical rain forest, a place where constant rainfall ensures that there are always plenty of plants in flower. Because the trees and other species of vegetation do not all shed at the same time, the rain forest canopy—the upper layer of trees in the forest—remains rich in foliage year-round. Hence, the tropical rain forest is an example of an evergreen forest. Climate can determine the type of life forms capable of surviving in the forest ecosystem. This can be illustrated by referring to a forest almost perfectly opposite in character to a rain forest: the taiga, or boreal forest, that spans much of northern Eurasia.

The taiga is a deciduous forest, meaning that its trees shed their leaves seasonally; indeed, because of the very cold climate in taiga regions, where the temperature during winter is usually well below the freezing point, trees spend a great portion of the year bare. Rainfall is much, much lower than in a rain forest, of course: only about 10-20 in. (250-500 mm) per year, as compared with more than 70 in. (1,800 mm) for a typical rain forest. The dry, inhospitable climate of the taiga makes it a forbidding place for reptiles and amphibians, though the taiga is home to many endothermic (warm-blooded) creatures such as mammals or birds.

LATITUDE, ALTITUDE, AND FORESTS.

Elevation or relief—that is, height above sea level—also determines the character of a forest, as do latitude (distance north or south of the equator) and topography, or the overall physical configuration of Earth's surface in a given area. Rain forests can exist anywhere, but by definition a tropical rain forest, such as those along the Amazon River in South America or the Congo River in Africa, must lie between the Tropic of Cancer in the north and the Tropic of Capricorn in the south.

Naturally, in the tropical rain forest, temperatures are high—typically, about 86°F (30°C) during the day, cooling down to about 68°F (20°C) at night. By contrast, there are much cooler rain forests in the temperate zones. An example is the Cherokee National Forest on the border between North Carolina and Tennessee, which, though located in the southeastern United States, is chilly even in the summer months.

Just as latitude affects a forest, so does altitude. Rain forests at relatively high elevations, such as the highlands of New Guinea, are known as montane forests. These forests, though they may be located at the same latitude as tropical rain forests—most montane forests are in eastern Brazil, southeastern Africa, northern Australia, and parts of southeast Asia—are much cooler. Lush by comparison to most non-rain forests, their vegetation is nonetheless much less dense than in a typical tropical rain forest.

In addition to its role in defining the overall character of the forest, differences in relative altitude or elevation resulting from the great height of trees in the rain forest also influence the formation of differing biological communities. For example, monkeys, flying squirrels, and other animals capable of swinging, gliding, or otherwise moving from tree to tree inhabit the canopy, which is rich in well-watered leaves and other food sources. These top-dwellers seldom even need to come down to the ground for anything. The rain forest floor, by contrast, is mostly bare, since the trees above shade it. On this level live creatures such as chimpanzees and gorillas, who feed off of low-lying plant forms. Other biological communities exist above or below the forest floor. (For more on these subjects, including the various types of forests, see Biomes. See The Biosphere for a discussion of soil quality in the rain forest.)

HUMANS AND FORESTS.

Earlier we noted the fact that humans' early history kept them, like other primates, close to the forest. In modern times, a growing awareness of ecology, and of the distance that technology has placed between modern society and the forests, led to the movement for the establishment of national parks in general, and of national forests in particular.

The first of these preserves—places where commercial development is forbidden and commercial activity is limited—was Yellowstone National Park in Wyoming, established by the administration of President Ulysses S. Grant in 1872. Though Yellowstone contains enormous areas of forest land, the first national forest reserve (as national forests were called at the time) was Sequoia National Park, established in 1891. Home to some of the largest, most awe-inspiring trees in the world, Sequoia is part of a group of national forests and parks to the northeast of Bakersfield, California.

The United States Forest Service was actually founded earlier (1905) than the National Park Service (1916), a fact that illustrates the importance of pristine forests to maintaining a proper balance between humans and their environment. Since the establishment of the forest service under the aegis of the U.S. Department of Agriculture, the lands controlled by the forest service have grown to encompass about 191 million acres (77.3 million hectares), an area larger than Texas. During the same period, the U.S. example of national parks and forests has inspired nations around the world to create their own preserves.

Coupled with the rise of national parks and forests at the turn of the nineteenth century was a growing interest in conservation and management of environmental resources. This interest manifested across a broad spectrum, from environmentalists who urged that the forests be left in their original state to industrial foresters who view the forest as a resource that can be utilized. Both sides have their merits, and both have their complaints about the other. The close historical ties between conservationism and the science of forestry (the management of forest ecosystems for purposes such as harvesting timber), both of which had their origins during the nineteenth century, suggest that there is no inherent reason that the two sides should be in conflict. If anything, responsible forestry goes hand-in-hand with an attitude of conserving as many resources as is feasible.

Comparing Angiosperms and Gymnosperms

Several times we have referred to angiosperms, a name that encompasses not just certain types of tree but also all plants that produce flowers during sexual reproduction. The name, which comes from Latin roots meaning "vessel seed," is a reference to the fact that the plant keeps its seeds in a vessel whose name, the ovary, emphasizes the sexual quality of the reproductive process it undergoes.

Angiosperms are a beautiful example of how a particular group of organisms can adapt to specific ecosystems and do so in a highly efficient manner, such that the evolutionary future heralds only greater dominance for these species. This is all the more interesting in light of the contrast between the success of the angiosperm and the rather less impressive results achieved by another broad category of sexually reproducing plant, one that formerly dominated Earth's forest: the gymnosperm.

Flowering plants evolved only about 130 million years ago, by which time gymnosperms (of which modern pines are an example) had long since evolved and proliferated. Yet in a relatively short period of time, angiosperms have become the dominant plants in the world today. About 80% of all living plant species are flowering plants, and based on the record of angiosperms and gymnosperms heretofore, it is likely that the world of 100 million years from now will be one in which the forests are typified by angiosperms. Gymnosperms, meanwhile, may well become a dying, if not a dead, breed.

POLLINATION BY GYMNOSPERMS.

Gymnosperms reproduce sexually as well, but they do so by a much less efficient method than that of the angiosperm. Whereas the angiosperm keeps its seeds safely tucked away inside the ovary and coexists with its ecosystem most favorably by putting the insect and animal life to work, gymnosperm reproduction is an altogether less effective—and, indeed, less pleasant—process.

For starters, gymnosperms produce their seeds on the surface of leaflike structures, and this makes the seeds vulnerable to physical damage and drying as the wind whips the trees' branches back and forth. Furthermore, insects and other animals view gymnosperm seeds as a source of nutrition, as indeed they are. And in contrast to the angiosperm, which attracts bees and other creatures to it, gymnosperms package the male reproductive component in tiny pollen grains, which it releases into the wind.

Eventually, the grains make their way toward the female component of another individual within the same species, but the fact that they do is an example of the wonder inherent in life itself and not of the efficiency of gymnosperm reproduction. Gymnosperms shower their ecosystems with pollen, a fact familiar to anyone who lives in a place with a high gymnosperm population—and hence a high pollen count in the spring. In gymnosperm-heavy environments, yellowish dust forms on everything, and where humans interact with the natural world, this can create a great deal of discomfort in the form of hay fever and allergies. Meanwhile, cars, windowsills, mailboxes, and virtually every other available surface takes on a yellow film that usually is not relieved until a good rain falls or, more likely, pollination ends for the year.

Though pollen is unpleasant to humans, it should be noted that like all natural mechanisms, it benefits the overall ecosystem. Packed with energy, pollen grains contain large quantities of nitrogen, making them a major boost to the nutrient content in the soil. But it costs the gymnosperm a great deal, in terms of chemical and biological energy and material, to produce pollen grains, and the benefits are uncertain.

POLLINATION BY ANGIOSPERMS.

If the gymnosperm and angiosperm varieties of pollination were compared to marketing campaigns, gymnosperm reproduction would involve the client (i.e., the gymnosperms themselves) investing maximum capital for minimal returns. In a very real sense, gymnosperm pollination is like the marketing of a company that bombards a neighborhood with leaflets, such that advertising rapidly becomes another form of trash simply to be swept up and thrown away.

By contrast, the "marketing" of angiosperms is like that of a company that uses carefully targeted, researched advertising, utilizing as many free means as possible for getting out word about itself. Just as a smart marketer sets in place the conditions to get consumers talking about a product—thus using advertising that is both free and extremely effective—the angiosperm enlists the aid of mobile organisms in its environment.

In addition, the angiosperm puts a great deal of its energy into producing reproductive structures, an effort that pays off bountifully. By evolving bright colors, scents, and nectar, the flowers of angiosperms attract animals, which travel from one flower to another, unintentionally moving pollen as they go. Thus, whereas insects and animals pose a threat to gymnosperms, angiosperms actually put bees, butterflies, hummingbirds, and other flower-seeking creatures to work assisting their reproductive process.

Because of this remarkably efficient system, animal-pollinated species of flowering plants do not need to produce as much pollen as gymnosperms. They can put their resources into other important functions instead, such as growth and greater seed production. In this way, the angiosperm solves its own problem of reproduction—and, as a side benefit, adds enormously to the world's beauty.

Deforestation

Returning to the subject of forests in general, if a forest experiences significant disturbance, it may undergo deforestation. Despite the finality in the sound of the word, deforestation does not necessarily imply complete destruction of the forest. In fact, deforestation can describe any interruption in the ordinary progression of a forest's life, including clear-cut harvesting—even if the forest fully recovers.

Deforestation can occur 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 consequence of human activities. Usually deforestation is driven by the need to clear land to harvest trees for fuel or, in some cases, to obtain building materials in the form of lumber. Though deforestation has been a problem the world over, since the 1970s it has become an issue primarily in developing countries.

In developed nations such as the United States, environmental activism has raised public awareness concerning deforestation and led to curtailment of large-scale cutting in forests that are deemed important environmental habitats. By contrast, developing nations, such as Brazil, are cutting down their forests at an alarming rate. Generally, economics is the dominant factor, with the need for new agricultural land or the desire to obtain wood and other materials typically driving the deforestation process.

CONSEQUENCES OF DEFORESTATION.

The deforestation of valuable reserves such as the Amazon rain forest is an environmental disaster in the making. As discussed in the essay The Biosphere, the soil in rain forests as a rule is "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, they are in effect killing the golden goose to get at the egg: once the rain forest is gone, the land itself is worthless.

Deforestation has several other 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 (see Biological Communities), 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 environ mental asset that contributes to the planet's bio diversity—and to its oxygen supply.

Even from a human standpoint, deforesta tion 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, while still other political and social horrors may lurk in the shadows. For example, Brazil's forests are home to charcoal factories that amount to virtual slave-labor camps. Aboriginal peoples (i.e., "Indians") are lured from cities with promises of high income and benefits, only to arrive and find that the sit uation is quite different from what was adver tised. 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.

The Greenhouse Effect

The most potentially serious aspect of cutting down forests may well be the greenhouse effect, which some scientists and activists believe is causing an overall warming of the planet. Today, thanks to the popularity of environmental causes among entertainment figures and on college campuses, terms such as "the greenhouse effect" and "global warming" are commonplace. However, these phrases are used so frequently, and sometimes so confusingly or misleadingly, that it is worthwhile to address their meaning briefly; then, we can conclude our discussion by looking at what impact the steady reduction in forest lands has had on the increasing release of greenhouse gases.

The greenhouse effect itself is not a consequence of any action on the part of human beings; rather, it is a part of life on Earth. In fact, without it, there could be no life on Earth. Though the planet receives an incredible amount of energy from the Sun, much of it is lost by being absorbed or reflected in the atmosphere or on the surface. So-called greenhouses gases such as carbon dioxide, however, help to trap this energy, keeping much more of the Sun's warmth within Earth's atmosphere, much as a greenhouse helps trap heat. Without the greenhouse effect, Earth would be so cold that the oceans would freeze.

Obviously, then, the greenhouse effect is a good thing—but only if greenhouse gases are kept at certain levels. Earth, after all, is not the only planet in the solar system that experiences a greenhouse effect; there is also Venus, a hellish place where surface temperatures are as high as 932°F (500°C). To many environmentalists, there is a grave danger that Earth could be slowly going the way of Venus, building up greenhouse gases such that the temperature is slowly increasing. This is the phenomenon of global warming, which threatens to melt the polar ice caps and submerge much of Earth's land surface. At least, that is the opinion of environmentalists and others who subscribe to the idea that Earth is steadily warming as a result of human pollution and industrial activity.

There is a considerable body of scientific knowledge that challenges the environmentalist position on global warming and the greenhouse effect, but it is not our purpose here to judge the various positions. Rather, our concern is the link between forests and the increase of greenhouses gases in the atmosphere. Old-growth or mature forests of the type discussed in Succession and Climax contain vast amounts of carbon—the basis for all living things—and when these forests are cut down, that carbon has to go somewhere. Specifically, carbon, in the form of carbon dioxide, will be released into the atmosphere, increasing the amount of greenhouse gases there.

This release may occur quickly, as when wood is burned, or more slowly, if the timber from the forest is used over long periods of time—for instance, in the building of houses or other structures. Statistics suggest an alarming change in the amount of carbon in the forests as compared with that in the atmosphere: since about 1850, the amount of carbon stored in forests had dropped by about one-third, while the amount of carbon dioxide in the atmosphere has increased by a comparable factor. Thus, the effort to keep greenhouse gases at viable levels is inextricably tied to the movement to preserve forest ecosystems.

WHERE TO LEARN MORE

Ashworth, William, and Charles E. Little. Encyclopedia of Environmental Studies. New York: Facts on File, 2001.

Diamond, Jared M. Guns, Germs, and Steel: The Fates of Human Societies. New York: W. W. Norton, 1997.

The Ecological Society of America: Issues in Ecology (Web site). <http://esa.sdsc.edu/issues.htm>.

The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts (Web site). <http://ecosystems.mbl.edu/>.

Living Things: Habitats and Ecosystems (Web site). <http://www.fi.edu/tfi/units/life/habitat/habitat.html>.

Markley, O. W., and Walter R. McCuan. 21st Century Earth: Opposing Viewpoints. San Diego, CA: Green-haven Press, 1996.

Martin, Patricia A. Woods and Forests. Illus. Bob Italiano and Stephen Savage. New York: Franklin Watts, 2000.

Nebel, Bernard J., and Richard T. Wright. Environmental Science: The Way the World Works. Upper Saddle River, NJ: Prentice Hall, 2000.

Philander, S. George. Is The Temperature Rising?: The Uncertain Science of Global Warming. Princeton, NJ: Princeton University Press, 1998.

Rybolt, Thomas R., and Robert C. Mebane. Environmental Experiments About Life. Hillside, NJ: Enslow Publishers, 1993.

The State of the Nation's Ecosystems (Web site). <http://www.us-ecosystems.org/>.

Sustainable Ecosystems Institute (Web site). <http://www.sei.org/>.

KEY TERMS

ANGIOSPERM:

A type of plant that produces flowers during sexual reproduc tion.

ATMOSPHERE:

Earth's atmosphere is a blanket of gases that includes nitrogen (78%), oxygen (21%), argon (0.93%), and a combination of water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%). Most of these gases are con tained in the troposphere, the lowest layer, which extends to about 10 mi. (16 km) above the planet's surface.

BIODIVERSITY:

The degree of variety among the species represented in a partic ular ecosystem.

BIOLOGICAL COMMUNITY:

The living components of an ecosystem.

CANOPY:

The upper portion or layer of the trees in a forest. A forest with a closed canopy is one so dense with vegeta tion that the sky is not visible from the ground.

CARNIVORE:

A meat-eating organism, or an organism that eats only meat (as distinguished from an omnivore).

COMPOUND:

A substance made up of atoms, chemically bonded to one another, of more than one chemical element.

CONIFER:

A type of tree that produces cones bearing seeds.

DECIDUOUS:

A term for a tree or other form of vegetation that sheds its leaves seasonally.

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. In the earth system, this often is achieved through the help of detritivores and decomposers.

DEFORESTATION:

A term for any interruption in the ordinary progression of a forest's life.

DETRITIVORES:

Organisms that feed on waste matter, breaking down organic material 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 relation ships between organisms and their environments.

ECOSYSTEM:

A community of inter dependent organisms along with the in organic components of their environment.

ELEMENT:

A substance made up of only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances.

ENERGY:

The ability of an object (or in some cases a non object, such as a mag netic force field) to accomplish work.

ENERGY BUDGET:

The total amount of energy available to a system or, more specifically, the difference between the energy flowing into the system and the energy lost by it.

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 them consumes nutrients and passes it 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 of singular organisms in which each plant or animal depends on the organism that precedes it. Food chains rarely exist in nature.

FOREST:

In general terms, a forest is simply any ecosystem dominated by tree-size woody plants. A number of other characteristics and parameters (for example, weather, altitude, and dominant species) further define types of forests, such as tropical rain forests.

GREENHOUSE EFFECT:

Warming of the lower atmosphere and surface of Earth. This occurs because of the absorp tion of long-wave length 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 at an even longer wavelength to space. (Wave-length and energy levels are related inverse ly; hence, the longer the wavelength, the less the energy.)

GYMNOSPERM:

A type of plant that reproduces sexually through the use of seeds that are exposed, not hidden in an ovary as with an angiosperm.

HERBIVORE:

A plant-eating organism.

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.