ecosystem

View more Science videos

For more information on ecosystem, visit Britannica.com.

Concept

An ecosystem is a complete community of living organisms and the nonliving materials of their surroundings. Thus, its components include plants, animals, and microorganisms; soil, rocks, and minerals; as well as surrounding water sources and the local atmosphere. The size of ecosystems varies tremendously. An ecosystem could be an entire rain forest, covering a geographical area larger than many nations, or it could be a puddle or a backyard garden. Even the body of an animal could be considered an ecosystem, since it is home to numerous microorganisms. On a much larger scale, the history of various human societies provides an instructive illustration as to the ways that ecosystems have influenced civilizations.

How It Works

The Biosphere

Earth itself could be considered a massive ecosystem, in which the living and nonliving worlds interact through four major subsystems: the atmosphere, hydrosphere (all the planet's waters, except for moisture in the atmosphere), geosphere (the soil and the extreme upper portion of the continental crust), and biosphere. The biosphere includes all living things: plants (from algae and lichen to shrubs and trees), mammals, birds, reptiles, amphibians, aquatic life, insects, and all manner of microscopic forms, including bacteria and viruses. In addition, the biosphere draws together all formerly living things that have not yet decomposed.

Several characteristics unite the biosphere. One is the obvious fact that everything in it is either living or recently living. Then there are the food webs that connect organisms on the basis of energy flow from one species to another. A food web is similar to the more familiar concept food chain, but in scientific terms a food chain—a series of singular organisms in which each plant or animal depends on the organism that precedes or follows it—does not exist. 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.

Food Webs

Food webs are built around the flow of energy between organisms, known as energy transfer, which begins with plant life. 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 through a process known as photosynthesis. In addition, plants take in nutrients from the soil, which contain energy in the forms of various chemical compounds. These compounds may be organic, which typically means that they came from living things, though, in fact, the term organic refers strictly to characteristic carbon-based chemical structures. Plants also receive inorganic compounds from minerals in the soil. (See Minerals. For more about the role of carbon in inorganic compounds, see Carbon Cycle.)

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 continually are circulated 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 Biogeochemical Cycles).

From Plants to Carnivores

As plants take up nutrients from the soil, they convert them into other forms, which provide usable energy to organisms who eat the plants. (An example of this conversion process is cellular respiration, discussed in Carbon Cycle.) When an herbivore, or plant-eating organism, eats the plant, it incorporates this energy.

Chances are strong that the herbivore will be eaten either by a carnivore, a meat-eating organism, or by an omnivore, an organism that consumes both herbs and herbivores—that is, both plants and animals. Few animals consume carnivores or omnivores, at least by hunting and killing them. (Detritivores and decomposers, which we discuss presently, consume the remains of all creatures, including carnivores and omnivores.) Humans are an example of omnivores, but they are far from the only omnivorous creatures. Many bird species, for instance, are omnivorous.

As nutrients pass from plant to herbivore to carnivore, the total amount of energy in them decreases. This is dictated by the second law of thermodynamics (see Energy and Earth), which shows that energy transfers cannot be perfectly efficient. Energy is not "lost"—the total amount of energy in the universe remains fixed, though it may vary with a particular system, such as an individual ecosystem—but it is dissipated, or directed into areas that do not aid in the transfer of energy between organisms. What this means for the food web is that each successive level contains less energy than the levels that precede it.

Detritivores and Decomposers

In the case of a food web, something interesting happens with regard to energy efficiency as soon as we pass beyond carnivores and omnivores to the next level. It might seem at first that there could be no level beyond carnivores or omnivores, since they appear to be "at the top of the food chain," but this only illustrates why the idea of a food web is much more useful. After carnivores and omnivores, which include some of the largest, most powerful, and most intelligent creatures, come the lowliest of all organisms: decomposers and detritivores, an integral part of the food web.

Decomposers, which include bacteria and fungi, obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. Detritivores perform a similar function: by feeding on waste matter, they break organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. The principal difference between detritivores and decomposers is that the former are relatively complex organisms, such as earthworms or maggots.

Both decomposers and detritivores 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 by organisms, and therefore such elements (which may appear individually or in compounds) need to be chemically processed through the body of a decomposer or detritivore. This processing involves chemical reactions in which the substance—whether an element or compound—is transformed into a more usable version.

By processing chemical compounds from the air, water, and geosphere, decomposers and detritivores deposit nutrients in the soil. These creatures feed on plant life, thus making possible the cycle we have described. Clearly this system, of which we have sketched only the most basic outlines, is an extraordinarily complex and well-organized one, in which every organism plays a specific role. In fact, earth scientists working in the realm of biosphere studies use the term niche to describe the role that a particular organism plays in its community. (For more about the interaction of species in a biological community, see Ecology and Ecological Stress.)

Real-Life Applications

The Fate of Human Civilizations

An interesting place to start in investigating examples of ecosystems is with a species near and dear to all of us: Homo sapiens. Much has been written about the negative effect industrial civilization has, or may have, on the natural environment—a topic discussed in Ecology and Ecological Stress—but here our concern is somewhat different. What do ecosystems, and specifically the availability of certain plants and animals, teach us about specific societies?

In his 1997 bestseller Guns, Germs, and Steel: The Fate of Human Societies, the ethnobotanist Jared M. Diamond (1937-) explained how he came to approach this question. While he was working with native peoples in New Guinea, a young man asked him why the societies of the West enjoyed an abundance of material wealth and comforts while those of New Guinea had so little. It was a simple question, but the answer was not obvious.

Diamond refused to give any of the usual pat responses offered in the past—for example, the Marxist or socialist claim that the West prospers at the expense of native peoples. Nor, of course, could he accept the standard answer that a white descendant of Europeans might have given a century earlier, that white Westerners are smarter than dark-skinned peoples. Instead, he approached it as a question of environment, and the result was his thought-provoking analysis contained in Guns, Germs, and Steel.

Advantages of Geography

As Diamond showed, those places where agriculture was first born were precisely those blessed with favorable climate, soil, and indigenous plant and animal life. Incidentally, none of these locales was European, nor were any of the peoples inhabiting them "white." Agriculture came into existence in four places during a period from about 8000 to 6000 B.C. In roughly chronological order, they were Mesopotamia, Egypt, India, and China. All were destined to emerge as civilizations, complete with written language, cities, and organized governments, between about 3000 and 2000 B.C.

Of course, it is no accident that civilization was born first in those societies that first developed agriculture: before a civilization can evolve, a society must become settled, and in order for that to happen, it must develop agriculture. Each of these societies, it should be noted, formed along a river, and that of Mesopotamia was born at the confluence of two rivers, the Tigris and Euphrates. No wonder, then, that the spot where these two rivers met was identified in the Bible as the site for the Garden of Eden or that historians today refer to ancient Mesopotamia as "the Fertile Crescent." (For a very brief analysis regarding possible reasons why modern Mesopotamia—that is, Iraq—does not fit this description, see the discussion of desertification in Soil Conservation.)

In the New World, by contrast, agriculture appeared much later and in a much more circumscribed way. The same was true of Africa and the Pacific Islands. In seeking the reasons for why this happened, Diamond noted a number of factors, including geography. The agricultural areas of the Old World were stretched across a wide area at similar latitudes. This meant that the climates were not significantly different and would support agricultural exchanges, such as the spread of wheat and other crops from one region or ecosystem to another. By contrast, the land masses of the New World or Africa have a much greater north-south distance than they do east to west.

Diversity of Species

Today such places as the American Midwest support abundant agriculture, and one might wonder why that was not the case in the centuries before Europeans arrived. The reason is simple but subtle, and it has nothing to do with Europeans' "superiority" over Native Americans. The fact is that the native North American ecosystems enjoyed far less biological diversity, or biodiversity, than their counterparts in the Old World. Peoples of the New World successfully domesticated corn and potatoes, because those were available to them. But they could not domesticate emmer wheat, the variety used for making bread, when they had no access to that species, which originated in Mesopotamia and spread throughout the Old World.

Similarly, the New World possessed few animals that could be domesticated either for food or labor. A number of Indian tribes domesticated some types of birds and other creatures for food, but the only animal ever adapted for labor was the llama. The llama, a cousin of the camel found in South America, is too small to carry heavy loads. Why did the Native Americans never harness the power of cows, oxen, or horses? For the simple reason that these species were not found in the Americas. After horses in the New World went extinct at some point during the last Ice Age (see Paleontology), they did not reappear in the Americas until Europeans brought them after A.D. 1500.

Diamond also noted the link between biodiversity and the practice, common among peoples in New Guinea and other remote parts of the world, of eating what Westerners would consider strange cuisine: caterpillars, insects—even, in some cases, human flesh. At one time, such practices served only to brand these native peoples further as "savages" in the eyes of Europeans and their descendants, but it turns out that there is a method to the apparent madness. In places such as the highlands of New Guinea, a scarcity of animal protein sources compels people to seek protein wherever they can find it.

By contrast, from ancient times the Fertile Crescent possessed an extraordinary diversity of animal life. Among the creatures present in that region (the term sometimes is used to include Egypt as well as Mesopotamia) were sheep, goats, cattle, pigs, and horses. With the help of these animals for both food and labor—people ate horses long before they discovered their greater value as a mode of transportation—the lands of the Old World were in a position to progress far beyond their counterparts in the New.

Greater Exposure to Micro-Organisms

Ultimately, these societies came to dominate their physical environments and excel in the development of technology; hence the "steel" and "guns" in Diamond's title. But what about "germs"? It is a fact that after Europeans began arriving in the New World, they killed vast populations without firing a shot, thanks to the microbes they carried with them. Of course, it would be centuries before scientists discovered the existence of microorganisms. But even in 1500, it was clear that the native peoples of the New World had no natural resistance to smallpox or a host of other diseases, including measles, chicken pox, influenza, typhoid fever, and bubonic plague.

Once again the Europeans' advantage over the Native Americans derived from the ecological complexity of their world compared with that of the Indians. In the Old World, close contact with farm animals exposed humans to germs and disease. So, too, did close contact with other people in crowded, filthy cities. This exposure, of course, killed off large numbers of people, but those who survived tended to be much hardier and possessed much stronger immune systems. Therefore, when Europeans came into contact with native Americans, they were like walking biological warfare weapons.

Evaluating Ecosystems

The ease with which Europeans subdued Native Americans fueled the belief that Europeans weresuperior, but, as Diamond showed, if anythingwas superior, it was the ecosystems of the Old World. This "superiority" relates in large part to the diversity of organisms an ecosystem possesses. Many millions of years ago, Earth's oceans and lands were populated with just a few varieties of single-cell organisms, but over time increasingdifferentiation of species led to the development of the much more complex ecosystems we know now.

Such differentiation is essential, given the many basic types of ecosystem that the world has to offer: forests and grasslands, deserts and aquatic environments, mountains and jungles. Among the many ways that these ecosystems can be evaluated, aside from such obvious parameters as relative climate, is in terms of abundance and complexity of species.

Abundance and Complexity

The biota (a combination of all flora and fauna, or plant and animal life, respectively) in a desert or the Arctic tundra is much less complex than that of a tropical rain forest or, indeed, almost any kind of forest, because far fewer species can live in a desert or tundra environment. For this reason, it is said that a desert or tundra ecosystem is less complex than a forest one. There may be relatively large numbers of particular species in a less complex ecosystem, however, in which case the ecosystem is said to be abundant though not complex in a relative sense.

Another way to evaluate ecosystems is in terms of productivity. This concept refers to the amount of biomass—potentially burnable energy—produced by green plants as they capture sunlight and use its energy to create new organic compounds that can be consumed by local animal life. Once again, a forest, and particularly a rain forest, has a very high level of productivity, whereas a desert or tundra ecosystem does not.

Forests

Now let us look more closely at a full-fledged ecosystem—that of a forest—in action. It might seem that all forests are the same, but this could not be less the case. A forest is simply any ecosystem dominated by tree-sized woody plants. Beyond that, the characteristics of weather, climate, elevation, latitude, topography, tree species, varieties of animal species, moisture levels, and numerous other parameters create the potential for an almost endless diversity of forest types.

In fact, the United Nations Educational, Scientific, and Cultural Organization (UNESCO) defines 24 different types of forest, which are divided into two main groups. On the one hand, there are those forests with a closed canopy at least 16.5 ft. (5 m) high. The canopy is the upper portion of the trees in the forest, and closed-canopy forests are so dense with vegetation that from the ground the sky is not visible. On the other hand, the UNESCO system encompasses open woodlands with a shorter, more sparse, and unclosed canopy. The first group tends to be tropical and subtropical (located at or near the equator), while the second typically is located in temperate and subpolar forests—that is, in a region between the two tropical latitudes and the Arctic and Antarctic circles, respectively. In the next paragraphs, we examine a few varieties of forest as classified by UNESCO.

Tropical and Subtropical Forests

Tropical rain forests are complex ecosystems with a wide array of species. The dominant tree type is an angiosperm (a type of plant that produces flowers during sexual reproduction), known colloquially as tropical hard-woods. The climate and weather are what one would expect to find in a place called a tropical rain forest, that is, rainy and warm. When the rain falls, it cools things down, but when the sun comes back out, it turns the world of the tropical rain forest into a humid, sauna-like environment.

Naturally, the creatures that have evolved in and adapted to a tropical rain forest environment are those capable of enduring high humidity, but they are tolerant of neither extremely cool conditions nor drought. Within those parameters, however, exists one of the most biodiverse ecosystems on Earth: the tropical rain forest is home to an astonishing array of animals, plants, insects, and microorganisms. Indeed, without the tropical rain forest, terrestrial (land-based) animal life on Earth would be noticeably reduced.

In the tropics, by definition the four seasons to which we are accustomed in temperate zones—winter, spring, summer, and fall—do not exist. In their place there is a rainy season and a dry season, but there is no set point in the year at which trees shed their leaves. In a tropical and subtropical evergreen forest conditions are much drier than in the rain forest, and individual trees or tree species may shed their leaves as a result of dry conditions. All trees and species do not do so at the same time, however, so the canopy remains rich in foliage year-round—hence the term evergreen. As with a rain forest, the evergreen forest possesses a vast diversity of species.

In contrast to the two tropical forest ecosystems just described, a mangrove forest is poor in species. In terms of topography and landform, these forests are found in low-lying, muddy regions near saltwater. Thus, the climate is likely to be humid, as in a rain forest, but only organisms that can tolerate flooding and high salt levels are able to survive there. Mangrove trees, a variety of angiosperm, are suited to this environment and to the soil, which is poor in oxygen.

Temperate and Subarctic Forests

Among the temperate and sub-arctic forest types are temperate deciduous forests, containing trees that shed their leaves seasonally, and temperate and subarctic evergreen conifer forests, in which the trees produce cones bearing seeds. These are forest types familiar to most people in the continental United States. The first variety is dominated by such varieties as oak, walnut, and hickory, while the second is populated by pine, spruce, or fir as well as other types, such as hemlock.

Less familiar to most Americans outside the West Coast are temperate winter-rain evergreen broadleaf forests. These forests are dominated by evergreen angiosperms and appear in regions that have both a pronounced wet season and a summer drought season. Such forests can be found in southern California, where an evergreen oak of the Quercus genus is predominant. Even less familiar to Americans is the temperate and subpolar evergreen rain forest, which is found in the Southern Hemisphere. Occurring in a wet, frost-free ocean environment, these forests are dominated by such evergreen angiosperms as the southern beech and southern pine.

Angiosperms Vs. 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 emphasizes these plants' sexual-type reproduction: an ovary.

Angiosperms are a beautiful example of how a particular group of organisms can adapt to specific ecosystems and do so in a way much more efficient than did their evolutionary forebear. Flowering plants evolved only about 130 million years ago, by which time Earth had long since been dominated by another variety of seed-producing plant, the gymnosperm, of which pines and firs are an example. Yet in a relatively short period of time, from the standpoint of the earth sciences, angiosperms have gone on to become the dominant plants in the world. Today, about 80% of all living plant species are flowering plants.

Angiosperm Vs. Gymnosperm Seeds

How did they do this? They did it by developing a means to coexist more favorably than gymnosperms with the insect and animal life in their ecosystems. Gymnosperms produce their seeds on the surface of leaflike structures, making the seeds vulnerable to physical damage and drying as the wind whips the branches back and forth. Furthermore, insects and other animals view gymnosperm seeds as a source of nutrition.

In an angiosperm, by contrast, the seeds are tucked away safely inside the ovary. Furthermore, the evolution of the flower not only has added a great deal of beauty to the world but also has provided a highly successful mechanism for sexual reproduction. This sexual reproduction makes it possible to develop new genetic variations, as genetic material from two individuals of differing ancestry come together to produce new offspring.

Gymnosperm Pollination

Gymnosperms reproduce sexually as well, but they do so by a less efficient method. In both cases, the trees have to overcome a significant challenge: the fact that sexual reproduction normally requires at least one of the individual plants to be mobile. Gymnosperms package the male reproductive component in tiny pollen grains, which are released into the wind. Eventually, the grains are blown toward the female component of another individual plant of the same species.

This method succeeds well enough to sustain large and varied populations of gymnosperms but at a terrific cost, as is evident to anyone who lives in a region with a high pollen count in the spring. A yellow dust forms on everything. So much pollen accumulates on window sills, cars, mailboxes, and roofs that only a good rain (or a car wash) can take it away, and one tends to wonder what good all this pollen is doing for the trees.

The truth is that pollination is wasteful and inefficient. Like all natural mechanisms, it benefit the overall ecosystem, in this case, by making nutrient-rich pollen grains available to the soil. Packed with energy, pollen grains contain large quantities of nitrogen, making them a major boost to the ecosystem if not to the human environment. 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 much more uncertain.

Pollen might make it to the right female component, and, in fact, it will, given the huge amounts of pollen produced. Yet the overall system is rather like trying to solve an economic problem by throwing a pile of dollar bills into the air and hoping that some of the money lands in the right place. For this reason, it is no surprise that angiosperms gradually are overtaking gymnosperms.

Angiosperm Pollination

The angiosperm overcomes its own lack of mobility by making use of mobile organisms. Whereas insects and animals pose a threat to gymnosperms, angiosperms actually put bees, butterflies, hummingbirds, and other flower-seeking creatures to work aiding their reproductive process. By evolving bright colors, scents, and nectar, the flowers of angiosperms attract animals, which travel from one flower to another, accidentally moving pollen as they do.

Because of this remarkably efficient system, animal-pollinated species of flowering plants do not need to produce as much pollen as gymnosperms. Instead, they can put their resources into other important functions, 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.

The Complexity of Ecosystems

The relationships between these two types of seed-producing plant and their environments illustrate, in a very basic way, the complex interactions between species in an ecosystem. Environmentalists often speak of a "delicate balance" in the natural world, and while there is some dispute as to how delicate that balance is—nature shows an amazing resilience in recovering from the worst kinds of damage—there is no question that a balance of some kind exists.

To put it another way, an ecosystem is an extraordinarily complex environment that brings together biological, geologic, hydrologic, and atmospheric components. Among these components are trees and other plants; animals, insects, and microorganisms; rocks, soil, minerals, and landforms; water in the ground and on the surface, flowing or in a reservoir; wind, sun, rain, moisture; and all the other specifics that make up weather and climate.

In the present context, we have not attempted to provide anything even approaching a comprehensive portrait of an ecosystem, drawing together all or most of the aspects described in the preceding paragraph. A full account of even the simplest ecosystem would fill an entire book. Given that level of complexity, it is safe to say that one should be very cautious before tampering with the particulars of an ecosystem. The essay on Ecology and Ecological Stress concerns what happens when such tampering occurs.

Where to Learn More

Beattie, Andrew J., and Paul Ehrlich. Wild Solutions: How Biodiversity Is Money in the Bank. New Haven, CT: Yale University Press, 2001.

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

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

Ecotopia (Web site). <http://www.ecotopia.com/>.

Jordan, Richard N. Trees and People: Forestland, Ecosystems, and Our Future. Lanham, MD: Regnery Publishing, 1994.

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

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.

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/>.

A functional system that includes an ecological community of organisms together with the physical environment, interacting as a unit. Ecosystems are characterized by flow of energy through food webs, production and degradation of organic matter, and transformation and cycling of nutrient elements. This production of organic molecules serves as the energy base for all biological activity within ecosystems. The consumption of plants by herbivores (organisms that consume living plants or algae) and detritivores (organisms that consume dead organic matter) serves to transfer energy stored in photosynthetically produced organic molecules to other organisms. Coupled to the production of organic matter and flow of energy is the cycling of elements. See also Ecological communities; Environment.

All biological activity within ecosystems is supported by the production of organic matter by autotrophs (organisms that can produce organic molecules such as glucose from inorganic carbon dioxide; see illustration). More than 99% of autotrophic production on Earth is through photosynthesis by plants, algae, and certain types of bacteria. Collectively these organisms are termed photoautotrophs (autotrophs that use energy from light to produce organic molecules). In addition to photosynthesis, some production is conducted by chemoautotrophic bacteria (autotrophs that use energy stored in the chemical bonds of inorganic molecules such as hydrogen sulfide to produce organic molecules). The organic molecules produced by autotrophs are used to support the organism's metabolism and reproduction, and to build new tissue. This new tissue is consumed by herbivores or detritivores, which in turn are ultimately consumed by predators or other detritivores.

General model of energy flow through ecosystems.

Terrestrial ecosystems, which cover 30% of the Earth's surface, contribute a little over one-half of the total global photosynthetic production of organic matter—approximately 60 × 10¹⁵ grams of carbon per year. Oceans, which cover 70% of the Earth's surface, produce approximately 51 × 10¹⁵ g C y⁻¹ of organic matter. See also Biomass.

Food webs

Organisms are classified based upon the number of energy transfers through a food web (see illustration). Photoautotrophic production of organic matter represents the first energy transfer in ecosystems and is classified as primary production. Consumption of a plant by a herbivore is the second energy transfer, and thus herbivores occupy the second trophic level, also known as secondary production. Consumer organisms that are one, two, or three transfers from photoautotrophs are classified as primary, secondary, and tertiary consumers. Moving through a food web, energy is lost during each transfer as heat, as described by the second law of thermodynamics. Consequently, the total number of energy transfers rarely exceeds four or five; with energy loss during each transfer, little energy is available to support organisms at the highest levels of a food web. See also Ecological energetics; Food web.

Biogeochemical cycles

In contrast to energy, which is lost from ecosystems as heat, chemical elements (or nutrients) that compose molecules within organisms are not altered and may repeatedly cycle between organisms and their environment. Approximately 40 elements compose the bodies of organisms, with carbon, oxygen, hydrogen, nitrogen, and phosphorus being the most abundant. If one of these elements is in short supply in the environment, the growth of organisms can be limited, even if sufficient energy is available. In particular, nitrogen and phosphorus are the elements most commonly limiting organism growth. This limitation is illustrated by the widespread use of fertilizers, which are applied to agricultural fields to alleviate nutrient limitation. See also Biogeochemistry; Nitrogen cycle.

Carbon cycles between the atmosphere and terrestrial and oceanic ecosystems. This cycling results, in part, from primary production and decomposition of organic matter. Rates of primary production and decomposition, in turn, are regulated by the supply of nitrogen, phosphorus, and iron. The combustion of fossil fuels is a recent change in the global cycle that releases carbon that has long been buried within the Earth's crust to the atmosphere. Carbon dioxide in the atmosphere traps heat on the Earth's surface and is a major factor regulating the climate. This alteration of the global carbon cycle along with the resulting impact on the climate is a major issue under investigation by ecosystem ecologists. See also Air pollution; Conservation of resources; Ecology, applied; Human ecology; Water pollution.

The health of humans, like all living organisms, is dependent on an ecosystem that sustains life. Healthy ecosystems are the sine qua non for healthy organisms. Yet there is abundant evidence that many life-support systems are far from healthy, placing an increased burden on human health. In some areas of the world, gains in life expectancy and quality of life made during the twentieth century are at risk of being reversed in the twenty-first century. The consequences of ecosystem degradation to human health are numerous, and include health risks from unsafe drinking water, polluted air, climate change, emerging new diseases, and the resurgence of old diseases owing to ecological imbalances. Reversing this damage is possible in some cases, but not in others. Prevention of ecological damage is by far the most efficient strategy.

Defining Ecosystems

An ecological system may be defined as a community of plants and animals interacting with each other and their abiotic, or natural, environment. Typically, ecosystems are differentiated on the basis of dominant vegetation, topography, climate, or some other criteria. Boreal forests, for example, are characterized by the predominance of coniferous trees; prairies are characterized by the predominance of grasses; the Arctic tundra is determined partly by the harsh climatic zone. In most areas of the world, the human community is an important and often dominant component of the ecosystem. Ecosystems include not only natural areas (e.g., forests, lakes, marine coastal systems) but also human-constructed systems (e.g., urban ecosystems, agroecosystems, impoundments). Human populations are increasingly concentrated in urban ecosystems, and it is estimated that, by the year 2010, 50 percent of the world's population will be living in urban areas.

A landscape comprises a mosaic of ecosystems, including towns, rivers, lakes, agricultural systems, and so on. Precise boundaries between ecosystems are often difficult to establish. Often regions slide into one another gradually, over a protracted "transition" zone, as for example between the boreal forest and the Taiga regions of Canada.

Ecosystem Health

It is important to recognize the inherent difficulties in defining "health," whether at the level of the individual, population, or ecosystem. The concept of health is somewhat of an enigma, being easier to define in its absence (sickness) than in its presence. Perhaps partially for that reason, ecologists have resisted applying the notion of "health" to ecosystems. Yet, ecosystems can become dysfunctional, particularly under chronic stress from human activity. For example, the discharge of nutrients from sewage, industrial waste, or agricultural runoff into lakes or rivers affects the normal functioning of the ecosystem, and can result in severe impairment. Excessive nutrient inputs from human activity was one of the major factors that severely compromised the health of the lower Laurentian Great Lakes (Lake Erie and Lake Ontario) and regions of the upper Great Lakes (Lake Michigan). Unfortunately, degraded ecosystems are becoming more the rule than the exception.

The study of the features of degraded systems, and comparisons with systems that have not been altered by human activity, makes it possible to identify the characteristics of healthy ecosystems. Healthy ecosystems may be characterized not only by the absence of signs of pathology, but also by signs of health, including measures of vigor (productivity), organization, and resilience.

Vigor can be assessed in terms of the metabolism (activity and productivity) of the system. Ecosystems differ greatly in their normal ranges of productivity. Estuaries are far more productive than open oceans, and marshes have higher productivity than deserts. Health is not evaluated by applying one standard to all systems. Organization can be assessed by the structure of the biotic community that forms an ecosystem and by the nature of the interactions between the species (both plants and animals). Invariably, healthy ecosystems have more diversity of biota than ecologically compromised systems. Resilience is the capacity of an ecosystem to maintain its structure and functions in the face of natural disturbances. Systems with a history of chronic stress are less likely to recover from normal perturbations such as drought than those systems that have been relatively less stressed.

Healthy ecosystems can also be characterized in economic, social, and human health terms. Healthy ecosystems support a certain level of economic activity. This is not to say that the ecosystem is necessarily self-sufficient, but rather that it supports economic productivity to enable the human community to meet reasonable needs. Inevitably, ecosystem degradation impinges on the long-term sustainability of the human economy that is associated with it, although in the short-term this may not be evident, as natural capital (e.g., soils, renewable resources) may be overexploited and temporarily enhance economic returns. Similarly, with respect to social well-being, healthy ecosystems provide a basis for and encourage community integration. Historically, for example, native Hawaiian groups managed their ecosystem through a well-developed social cohesiveness that provided a high degree of cooperation in fishing and farming activity.

Another reflection of ecosystem health lies directly in the public health domain. In spring 2000, a deadly strain of the bacterium E-coli (0157:H7) entered the public water supply in Walkerton, Ontario, Canada, causing seven deaths and making thousands sick. This small town, with a population of five thousand, is in a farming community. Inadequate manure management from cattle operations was the likely source of this tragedy.

How Healthy Ecosystems Become Pathological

Stress from human activity is a major factor in transforming healthy ecosystems to sick ecosystems. Chronic stress from human activity differs from natural disturbances. Natural disturbances (fires, floods, periodic insect infestations) are part of the dynamics of most ecosystems. These processes help to "reset" ecosystems by recycling nutrients and clearing space for recolonization by biota that may be better adapted to changing environments. Thus, natural perturbations help keep ecosystems healthy. In contrast, chronic and acute stress on ecosystems resulting from human activity (e.g., construction of large dams, release of nutrients and toxic substances into the air, water, and land) generally results in long-term ecological dysfunction.

Five major sources of human-induced (anthropogenic) stresses have been identified by D. J. Rapport and A. M. Friend (1979): physical restructuring, overharvesting, waste residuals, introduction of exotic species, and global change.

Physical Restructuring. Activities such as wetland drainage, removal of shoals in lakes, damming of rivers, and road construction fragment the landscape and alter and damage critical habitat. These activities also disrupt nutrient cycling, and cause the loss of biodiversity.

Overharvesting. Overexploitation is commonplace when it comes to harvesting of wildlife, fisheries, and forests. Over long periods of time, stocks of preferred species are reduced. For example, the giant redwoods that once thrived along the California coast now exist only in remnant patches because of overharvesting. When dominant species like the giant redwoods (arguably the world's tallest tree—one specimen was recorded at 110 meters tall with a circumference of 13.4 meters) are lost, the entire ecosystem becomes transformed. Overharvesting often results in reduced biodiversity of endemic species, while facilitating the invasion of opportunistic species.

Waste Residuals. Discharges from municipal, industrial, and agricultural sources into the air, water, and land have severely compromised many of the earth's ecosystems. The effects are particularly apparent in aquatic ecosystems. In some lakes that lack a natural buffering capacity, acid precipitation has eliminated most of the fish and other organisms. While the visual effect appears beneficial (water clarity goes up) the impact on ecosystem health is devastating. Systems that once contained a variety of organisms and were highly productive (biologically) become devoid of most lifeforms except for a few acid-tolerant bacteria and sediment-dwelling organisms.

Introduction of Exotic Species. The spread of exotics has become a problem in almost every ecosystem of the world. Transporting species from their native habitat to entirely new ecosystems can wreck havoc, as the new environments are often without natural checks and balances for the new species. In the Great Lakes Basin, the accidental introduction of two small pelagic fishes, the alewife and the rainbow smelt, combined with the simultaneous overharvesting of natural predators, such as the lake trout, led to a significant decline in native fish species. The introduction of the sea lamprey, an eel-like predacious fish that attacks larger fish, into Lake Erie and the upper Great Lakes further destabilized the native fish community. The sea lamprey contributed to the demise of the deepwater benthic fish community by preying on lake trout, whitefish, and burbot. This contributed to a shift in the fish community from one that had been dominated by large benthics to one dominated by small pelagics (fish found in the upper layers of the lake profile). This shift from bottom-dwelling fish (benthic) to surface-dwelling fish (pelagic) has now been partially reversed by yet another accidental introduction of an exotic: the zebra mussel. As the zebra mussel is a highly efficient filter of both phtyoplankton and zooplankton, its presence has reduced the available food in the surface waters for pelagic fish. However, while the benthic fish community has gained back its dominance, the preferred benthic fish species have not yet recovered owing to the degree of initial degradation. Overall, the increasing dominance by exotics not only altered the ecology, but also reduced significantly the commercial value of the fisheries.

Global Change. Rapid climate change (or climate warming) is an emerging potential global stress on all of the earth's ecosystems. In evolutionary time, there have of course been large fluctuations in climate. However, for the most part these fluctuations have occurred gradually over long periods of time. Rapid climate change is an entirely different matter. By altering both averages and extremes in precipitation, temperature, and storm events, and by destabilizing the El Niño Southern Oscillation (ENSO), which controls weather patterns over much of the southern Pacific region, many ecosystem processes can become significantly altered. Excessive periods of drought or unusually heavy rains and flooding will exceed the tolerance for many species, thus changing the biotic composition. Flooding and unusually high winds contribute to soil erosion, and at the same time add to nutrient load in rivers and coastal waters.

These anthropogenic stresses have compromised ecosystem function in most regions of the world, resulting in ecosystem distress syndrome (EDS). EDS is characterized by a group of signs, including abnormalities in nutrient cycling, productivity, species diversity and richness, biotic structure, disease prevalence, soil fertility, and so on. The consequences of these changes for human health are not inconsiderable. Impoverished biotic communities are natural harbors for pathogens that affect humans and other species.

Ecosystem Health and Human Health

An important aspect of ecosystem degradation is the associated increased risk to human health. Traditionally, the concern has been with contaminants, particularly industrial chemicals that can have adverse impacts on human development, neurological functions, reproductive functions, and that appear to be causative agents in a variety of carcinomas. In addition to these serious environmental concerns (where the remedies are often technological, including engineering solutions to reduce the release of contaminants), there are a large number of other risks to human health stemming from ecological imbalance.

Ecosystem distress syndrome results in the loss of valued ecosystem services, including flood control, water quality, air quality, fish and wildlife diversity, and recreation. One of the major signs of EDS is increased disease incidence, both in humans and other species. Human population health should thus be viewed within an ecological context as an expression of the integrity and health of the life-supporting capacity of the environment.

Ecological imbalances triggered by global climate change and other causes are responsible for increased human health risks.

Climate Change and Vector-Borne Diseases. The global infectious disease burden is on the order of several hundred million cases per year. Many vector-borne diseases are climate sensitive. Malaria, dengue fever, hantavirus pulmonary syndrome, and various forms of viral encephalitis are all in this category. All these diseases are the result of arthropod-borne viruses (arboviruses) which are transmitted to humans as a result of bites from blood-sucking arthropods.

Global climate change—particularly as it impacts both temperatures and precipitation—is highly correlated with the prevalence of vector-borne diseases. For example, viruses carried by mosquitoes, ticks, and other blood-sucking arthropods generally have increased transmission rates with rising temperatures. St. Louis encephalitis (SLE) serves as an example. The mosquito Culex tarsalis carries this virus. The percentage of bites that results in transmission of SLE is dependent on temperature, with greater transmission at higher temperatures.

The temperature dependence of vector-borne diseases is also well illustrated with malaria. Malaria is endemic throughout the tropics, with a high prevalence in Africa, the Indian subcontinent, Southeast Asia, and parts of South and Central America and Mexico. Approximately 2.4 billion people live in areas of risk, with some 350 million new infections occurring annually, resulting in approximately 2 million deaths, predominantly in young children. Untreated malaria can become a life-long affliction—general symptoms include fever, headache, and malaise.

The climate sensitivity of malaria arises owing to the nature of the interactions of parasites, vectors, and hosts, all of which impact the ultimate transmission rates to humans. The gestation time required for the parasite to become fully developed within the mosquito host (a process termed sporogony) is from eight to thirty-five days. When temperatures are in the range of 20°C to 27°C, the gestation time is reduced. Rainfall and humidity also have an influence. Both drought and heavy rains tend to reduce the population of mosquitoes that serve as vectors for malaria. In drier regions of the tropics, low rainfall and humidity restricts the survival of mosquitoes. Severe flooding can result in scouring of rivers and destruction of the breeding habitats for the mosquito vector, while intermediate rainfall enhances vector production.

Ecological Imbalances. Cholera is a serious and potentially fatal disease that is caused by the bacterium Vibrio cholerae. While not nearly so prevalent as malaria, cases are nonetheless numerous. In 1993, there were 296,206 new cases of cholera reported in South America; 9,280 cases were reported in Mexico; 62,964 cases in Africa; and 64,599 cases in Asia. Most outbreaks in Asia, Africa, and South America have originated in coastal areas. Symptoms of cholera include explosive watery diarrhea, vomiting, and abdominal pain. The most recent pandemic of cholera involved more regions than at any previous time in the twentieth century. The disease remains endemic in India, Bangladesh, and Africa. Vibrio cholerae has also been found in the United States—in the Gulf Coast region of Texas, Louisiana, and Florida; the Chesapeake Bay area; and the California coast.

The increase in prevalence of V. cholerae has been strongly linked to degraded coastal marine environments. Nutrient-enriched warmer coastal waters, resulting from a combination of climate change and the use of fertilizers, provides an ideal environment for reproduction and dissemination of V. cholerae. Recent outbreaks of cholera in Bangladesh, for example, are closely correlated with higher sea surface temperatures. V. cholerae attach to the surface of both freshwater and marine copepods (crustaceans), as well as to roots and exposed surfaces of macrophytes (aquatic plants) such as the water hyacinth, the most abundant aquatic plant in Bangladesh. Nutrient enrichment and warmer temperatures give rise to algae blooms and an abundance of macrophytes. The algae blooms provide abundant food for copepods, and the increasing copepod and macrophyte populations provide V. cholerae with habitat. Subsequent dispersal of V. cholerae into estuaries and fresh water bodies allows contact with humans who use these waters for drinking and bathing. Global distribution of marine pathogens such as V. cholerae is further facilitated by ballast water discharged from vessels. Ballast water contains a virtual cocktail of pathogens, including V. cholerae.

Two other examples of how ecological imbalances lead to human health burdens concern the increased prevalence of Lyme disease and hantavirus pulmonary disease. Lyme disease, sonamed because it was first positively identified in Lyme, Connecticut, is a crippling arthritic-type disease that is transmitted by spirochete-infected Ixodes ticks (deer ticks). Ticks acquire the infection from rodents, and spend part of their life cycle on deer. Three factors have combined to increase the risk to humans of contracting Lyme disease, particularly in North America: (1) the elimination of natural deer predators, particularly wolves; (2) reforestation of abandoned farmland has created more favorable habitat for deer; and (3) the creation of suburban estates, which the deer find ideal habitat for browsing. The net result is a rising deer population, which increases the chances of humans coming into more contact with ticks.

By 1995, in the southwestern United States, hantavirus infection was confirmed in ninety-four persons in twenty states, with 48 percent mortality. Variants of the strain that causes hantavirus pulmonary syndrome have also been found in other areas of the country, as well as in Asia and Europe. The virus is apparently asymptomatic in rodents, and it is transmitted in their saliva and excreta. In humans it has a flu-like presentation, which is followed by acute respiratory distress syndrome. The primary reservoir in the Four Corners area of the southwestern United States is the deer mouse. Climatic disturbances, which in recent years are thought to be exacerbated by human activity (e.g., global warming), appear to set up conditions that trigger outbreaks. In the early 1990s, ENSO events initially caused drought conditions to develop in the southwestern United States. This led to a decline in plant and animal populations, including natural predators of the deer mouse. Heavy rains followed the drought in 1993, resulting in a bumper crop of piñon nuts, a major food supply for the deer mouse. Subsequently the deer mouse population greatly increased, bringing about increased contact with humans and triggering the outbreak of hantavirus.

Antibiotic Resistance and Agricultural Practice Antibiotic resistance is a growing threat to public health. Antibiotic resistant strains of Streptococcus pneumoniae, a common bacterial pathogen in humans and a leading cause of many infections, including chronic bronchitis, pneumonia, and meningitis, have greatly increased in prevalence since the mid-1970s. In some regions of the world, up to 70 percent of bacterial isolates taken from patients proved resistant to penicillin and other b-lactam antibiotics. The use of large quantities of antibiotics in agriculture and aquaculture appears to have been a key factor in the development of antibiotic resistance by pathogens in farm animals that subsequently may also infect humans. One of the most serious risks to human health from such practices is vancomycin-resistant enterococci. The use of avoparcin, an animal growth promoter, appears to have compromised the utility of vancomycin, the last antibiotic effective against multi-drug-resistant bacteria. In areas where avoparcin has been used, such as on farms in Denmark and Germany, vancomycin-resistant bacteria have been detected in meat sold in supermarkets. Avoparcin was subsequently banned by the European Union. Another example is the use of ofloxacin to protect chickens from infection and thereby enhance their growth. This drug is closely related to ciprofloxacin, one of the most widely used antibiotics in the year 2000. There have been cases of resistance to ciprofloxacin directly related to its veterinary use. In the United Kingdom, ciprofloxacin resistance developed in strains of campylobacter, a common cause of diarrhea. Multi-drug-resistant strains of salmonella have been traced to European egg production.

Food and Water Security. Agricultural practices are also responsible for a growing number of threats to public health. Some of these are related to inadequate waste management, which has resulted in parasites and bacteria entering water supplies. Others are of entirely different origins and involve apparent transfer across species of pathogens that affect both animals and humans. The most recent and spectacular example is mad cow disease, known as variant Creutzfeldt-Jakob disease in humans, a neuro-degenerative condition that, in humans, is ultimately fatal. The first case of Bovine Spongiform Encephalopathy (BSE), the animal form of the disease, was identified in Southern England in November 1981. By the fall of 2000, an outbreak had also occurred in France, and isolated cases appeared in Germany, Switzerland, and Spain. More than one hundred deaths in Europe were attributed to what has come to be commonly called mad cow disease.

Improper manure management was the likely source of the outbreak of E. coli 0157:H7 in Walkerton, Ontario, Canada. Other health risks associated with malfunctioning agroecosystems include periodic outbreaks of cryptosporidiosis, a parasitic disease that is spread by surface runoff contaminated by feces of infected cattle. This parasite causes fever and diarrhea in immunocompetent individuals and severe diarrhea and even death in immunocompromised individuals.

Ecosystem Restoration

Ecosystem pathology in some cases can be reversed simply by removing the source of stress. In cases, for example, where ecosystem degradation is the result of point-source additions of nutrients or toxic chemicals, removal of these stresses may result in considerable recovery of ecosystem health. A classic case is Lake Washington (near Seattle, Washington). This lake had become highly anoxic (oxygen-depleted) owing to a sewage outfall entering the lake. Redirecting the sewage outfall away from the lake reversed many of the signs of pathology.

In cases where it is not feasible to remove the source of stress, more innovative engineering solutions have been tried. For example, in the Kyrönjoki and Lestijoki Rivers in western Finland, spring and fall runoff leads to sharp pulses of acidity. Spring runoff from snowmelt, which releases acid from tilled or dug soils, has been particularly damaging to fish, during the critical time of year for spawning. Fish reproduction is severely curtailed, if not all together eliminated in highly acidic water. Further there have been massive fish kills resulting from the highly acidic waters. One possible remedy is to replace the original drains which take runoff from the land to the rivers with new limed drains that can neutralize the acidity. This solution has been implemented on an experimental basis and appears to substantially reduce acidic runoff.

More radical treatments for damaged ecosystems involve "ecosystem surgery." In some cases, invading exotic vegetation (such as mangroves in Hawaii) have been removed from regions, and native vegetation has been replanted. In areas of North America where wetlands have been severely depleted owing to farming, urbanization, and industrial activity, efforts have been made to establish new wetlands.

More often than not, however, reversing ecosystem pathology is not possible. Efforts to restore the indigenous grasslands in the Jornada Experimental Range in the southwestern United States provide an example. Overgrazing by cattle has severely degraded the landscape and has lead to replacement of the native grasses by largely inedible shrubs, dominated by mesquite. Erosion by wind and episodic heavy rains have left areas between shrubs largely bare, and subsequently underlying sands have developed in dune-like fashion over a large part of the area. The resulting mesquite dunes have proven highly resistant to efforts to restore the native grasslands, although almost every intervention has been tried, including highly toxic defoliants (Agent Orange), fire, and bulldozing.

Even where it has been possible to restore some of the ecological functions of degraded ecosystems, and thus improve ecosystem health, the restoration seldom results in reestablishment of the pristine biotic community. The best that can be achieved in most cases is reestablishment of the key ecological functions that provide the required ecosystem services, such as the regulation of water, primary and secondary productivity, nutrient cycling, and pollination. In all such efforts, key indicators of ecosystem health (vigor, productivity, and resilience) are essential to monitor progress. Standard ecological indicators can be used for this purpose (e.g., measures of productivity, species composition, nutrient flows, soil fertility) along with socioeconomic and human health indicators.

Experience in efforts to restore highly damaged ecosystems suggests that ecosystem-health prevention is far more effective than restoration. For marine ecosystems, setting aside protective zones that afford a sanctuary for fish and wildlife has considerable promise. Many countries are adopting policies to establish such areas with the prospect that these healthy regions can serve as a reservoir for biota that have become depleted in the unprotected areas. Yet this remedy is not without its limits. Restoring ecosystem health is not simply a matter of replenishing lost or damaged biota. It is also a matter of reestablishing the complex interactions among ecosystem lifeforms. Having a ready source of healthy biota that could potentially recolonize damaged ecosystems is important, but it is only part of the solution.

Prevention of Ecosystem Disruptions

Given the difficulties in reversing ecosystem degradation, and the many associated human health risks that arise with the loss of ecosystem health, the most effective approach is simply the prevention of ecosystem disruption. However, like many common-sense approaches, this is easier said than done. In both developed and developing countries there is a strong inclination to continue economic growth, even at the cost of severe environmental damage. Apart from selfish motivations, the argument is made that economic growth has many obvious health benefits, such as providing more efficient means of distributing food supplies, providing more plentiful food, and providing better health services and funding for research to improve standards of living. These are indeed benefits of economic development, and have led to substantial increases in health status worldwide.

However, at the dawn of the twenty-first century, the past is not necessarily the best guide to the future. The human population is at an alltime high, and associated pressures of human activity have led to increasing degradation of the earth's ecosystems. As ultimately healthy ecosystems are essential for life of all biota, including humans, current global and regional trends are ominous. Under these circumstances, a tradeoff between immediate material gains and long-term sustainability of humans on the planet may be the only option. If so, the solution to sustaining human health and ecosystem health becomes one of devising a new politic that places sustaining lifesupport systems as a precondition for betterment of the human condition.

(SEE ALSO: Acid Rain; Ambient Air Quality [Air Pollution]; Ambient Water Quality; Biodiversity; Cholera; Ecological Footprint; Emerging Infectious Diseases; Global Burden of Disease; PCBs; Pesticides; Pollution; Vector-Borne Diseases)

Bibliography

Aldhous, P. (2000). "Inquiry Blames Missed Warnings for Scale of Britain's BSE Crisis." Nature 408:3–5.

Baquero, R., and Blazquez, J. (1997). "Evolution of Antibiotic Resistance." Trends in Ecology and Evolution 12:482–487.

Bright, C. (1998). Life Out of Bounds: Bioinvasion in a Borderless World. New York: W. W. Norton.

Colwell, R. R. (1996). "Global Climate and Infectious Disease: The Cholera Paradigm." Science 274:2025–2031.

Colwell, R. R., and Patz, J. A. (1998). Climate, Infectious Disease and Health: An Interdisciplinary Perspective. Washington, DC: American Academy of Microbiology.

Epstein, P. R. (1995). "Emerging Diseases and Ecosystem Instability: New Threats to Public Health." American Journal of Public Health 85(2):168–172.

Huq, A., and Colwell, R. R. (1996). "Vibrios in the Marine and Estuarine Environment: Tracking Vibrio Cholerae." Ecosystem Health 2:198–214.

Mageau, M. T.; Costanza, R.; and Ulanowicz, R. E. (1995). "The Development and Initial Testing of a Quantitative Assessment of Ecosystem Health." Ecosystem Health 1:201–213.

Rapport, D. J. (1989). "What Constitutes Ecosystem Health?" Perspectives in Biology and Medicine 33:120–132.

Rapport, D. J., and Friend, A. M. (1979). Towards a Comprehensive Framework for Environmental Statistics: A Stress-Response Approach. Ottawa: Statistics Canada.

Rapport, D. J., and Regier, H. A. (1980). "An Ecological Approach to Environmental Information." Ambio 9:22–27.

—— (1995). "Disturbance and Stress Effects on Ecological Systems." In Complex Ecology: The Part-WholeRelation in Ecosystems, ed. B. C. Patten and S. E. Jorgensen. Englewood Cliffs, NJ: Prentice Hall.

Rapport, D. J.; Costanza, R.; and McMichael, A. J. (1998). "Assessing Ecosystem Health: Challenges at the Interface of Social, Natural, and Health Sciences." Trends in Ecology and Evolution 13(10):397–401.

Rapport, D. J.; Christensen, N.; Karr, J. R.; and Patil, G. P. (1998). "The Centrality of Ecosystem Health in Achieving Sustainability in the Twenty-First Century: Concepts and Approaches to Environmental Management." In Human Survivability in the Twenty-First Century, ed. D. M. Hayne. Toronto: University of Toronto Press.

Rapport, D. J.; Costanza, R.; Epstein, P. R.; Gaudet, R.; and Levins, R., eds. (1998). Ecosystem Health. Malden, MA: Blackwell Science.

Rapport, D. J., and Whitford, W. (1999). "How Ecosystems Respond to Stress: Common Properties of Arid and Aquatic Systems." Bio Science 49(3):193–203.

Rapport, D. J.; Regier, H. A.; and Hutchinson, T. C. (1985). "Ecosystem Behavior under Stress." American Naturalist 125:617–640.

Reeves, W. C.; Hardy, J. L.; Reisen, W. K.; and Milby, M. M. (1994). "The Potential Effect of Global Warming on Mosquito-Borne Arboviruses." Journal of Medical Entomology 31(3):323–332.

Ruiz, G. M.; Rawlings, T. K.; Dobbs, F. C.; Drake, L. A.; Mullady, T.; Huq, A.; and Colwell, R. R.. (2000). "Global Spread of Microorganisms by Ships." Nature 408:49–50.

Watson R. T.; Zinyowera, M. C.; and Moss, R. H., eds. (1996). Second Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK: Cambridge University Press.

— DAVID J. RAPPORT

A community of plants and animals within a particular physical environment which is linked by a flow of materials through the non-living (abiotic) as well as the living (biotic) sections of the system. Thus, ecosystems can range in size from the whole earth to a drop of water, although in practice, the term ecosystem is generally used for units below the size of biomes, such as sand dunes, or an oak woodland.

A community of plants, animals, and their environment, functioning as a unit.

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

A collection of living things and the environment in which they live. For example, a prairie ecosystem includes coyotes, the rabbits on which they feed, and the grasses that feed the rabbits.

The fundamental unit in ecology, comprising the living organisms and the nonliving elements interacting in a certain defined area. In more sophisticated terms, a biotic community living in its biotope.

The sum total of all living and nonliving things that support the chain of life events within a particular area.

• Environment, Ecology, and Animal Behavior - ecosystem: all interactions of a community of organisms with its physical environment

Coral reefs are an example of a marine ecosystem

An ecosystem is a biological system consisting of all the living organisms or biotic components in a particular area and the nonliving or abiotic components with which the organisms interact, such as air, mineral soil, water, and sunlight.^[1] Key processes in ecosystems include the capture of light energy and carbon through photosynthesis, the transfer of carbon and energy through food webs, and the release of nutrients and carbon through decomposition. Biodiversity affects ecosystem functioning, as do the processes of disturbance and succession. Ecosystems provide a variety of goods and services upon which people depend; the principles of ecosystem management suggest that rather than managing individual species, natural resources should be managed at the level of the ecosystem itself.

Contents 1 Overview 2 History and development 3 Ecosystem processes 3.1 Primary production 3.2 Energy flow 3.3 Decomposition 3.4 Nutrient cycling 3.5 Function and biodiversity 3.6 Ecosystem goods and services 3.7 Ecosystem management 4 Ecosystem dynamics 4.1 Ecosystem ecology 5 Classification 5.1 Examples of ecosystems 6 See also 7 Notes 8 References 9 Literature cited 10 Further reading 11 External links

Overview

Rainforests often have a great deal of biodiversity with many plant and animal species. This is the Gambia River in Senegal's Niokolo-Koba National Park.

An ecosystem consists of a biological community together with its abiotic environment, interacting as a system.^[2] While the size of an ecosystem is not specifically defined it usually encompasses a specific, limited area^[3] (although it is sometimes said that it can encompass the entire planet^[4]). Ecosystems are defined by the network on interactions among organisms, and between organisms and their environment.^[5] They are linked together through nutrient cycle and energy flow.^[6]

Energy, water, nitrogen and soil minerals are other essential abiotic components of an ecosystem. The energy that flows through ecosystems is obtained primarily from the sun. It generally enters the system through photosynthesis, a process that also captures carbon from the atmosphere. By feeding on plants and on one-another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.^[7]

Ecosystems are controlled both by internal and external factors. External factors, also called state factors, control the overall structure an ecosystem and the way things work within it, but are not themselves influenced by the ecosystem. The most important of these is climate.^[8] Climate determines the biome in which the ecosystem is embedded. Rainfall patterns and temperature seasonality determine the amount of water available to the ecosystem and the supply of energy available (by influencing photosynthesis).^[8] Parent material, the underlying geological material that gives rise to soils, determines the nature of the soils present, and influences the supply of mineral nutrients. Topography also controls ecosystem processes by affecting things like microclimate, soil development and the movement of water through a system. This may be the difference between the ecosystem present in wetland situated in a small depression on the landscape, and one present on an adjacent steep hillside.^[8]

Other external factors that play an important role in ecosystem functioning include time and potential biota. Time plays a role in the development of soil from bare rock and the recovery of a community from disturbance.^[8] Similarly, the set of organisms that can potentially be present in an area can also have a major impact on ecosystems. Ecosystems in similar environments that are located in different parts of the world can end up doing things very differently simply because they have different pools of species present.^[8] The introduction of non-native species can cause substantial shifts in ecosystem function.

Unlike external factors, internal factors in ecosystems not only control ecosystem processes, but are also controlled by them. Consequently, they are often subject to feedback loops.^[8] While the resource inputs are generally controlled by external processes like climate and parent material, the availability of these resources within the ecosystem is controlled by internal factors like decomposition, root competition or shading.^[8] Other factors like disturbance, succession or the types of species present are also internal factors. Human activities are important in almost all ecosystems. Although humans exist and operate within ecosystems, their cumulative effects are large enough to influence external factors like climate.^[8]

History and development

Arthur Tansley, a British ecologist, was the first person to use the term "ecosystem" in a published work.^{[fn 1][9]} Tansley devised the concept to draw attention to the importance of transfers of materials between organisms and their environment.^[10] He later refined the term, describing it as "The whole system, … including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment".^[11] Tansley regarded ecosystems not simply as natural units, but as mental isolates.^[11] Tansley later^[12] defined the spatial extent of ecosystems using the term ecotope.

G. Evelyn Hutchinson, a pioneering limnologist who was a contemporary of Tansley's, combined Charles Elton's ideas about trophic ecology with those of Russian geochemist Vladimir Vernadsky to suggest that mineral nutrient availability in a lake limited algal production which would, in turn, limit the abundance of animals that feed on algae. Raymond Lindeman took these ideas one step further to suggest that the flow of energy through a lake was the primary driver of the ecosystem. Hutchinson's students, brothers Howard T. Odum and Eugene P. Odum, further developed a "systems approach" to the study of ecosystems, allowing them to study the flow of energy and material through ecological systems.^[10]

Ecosystem processes

Energy and carbon enter ecosystems through photosynthesis, are incorporated into living tissue, transferred to other organisms that feed on the living and dead plant matter, and eventually released through respiration.^[13] Most mineral nutrients, on the other hand, are recycled within ecosystems.^[14]

Primary production

Global oceanic and terrestrial phototroph abundance, from September 1997 to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it. Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE.

Main article: Primary production

Primary production is the production of organic matter from inorganic carbon sources. Overwhelmingly, this occurs through photosynthesis. The energy incorporated through this process supports life on earth, while the carbon makes up much of the organic matter in living and dead biomass, soil carbon and fossil fuels. It also drives the carbon cycle, which influences global climate via the greenhouse effect.

Through the process of photosynthesis, plants capture energy from light and use it to combine carbon dioxide and water to produce carbohydrates and oxygen. The photosynthesis carried out by all the plants in an ecosystem is called the gross primary production (GPP).^[15] About 48–60% of the GPP is consumed in plant respiration. The remainder, that portion of GPP that is not used up by respiration, is known as the net primary production (NPP).^[13] Total photosynthesis is limited by a range of environmental factors. These include the amount of light available, the amount of leaf area a plant has to capture light (shading by other plants is a major limitation of photosynthesis), rate at which carbon dioxide can be supplied to the chloroplasts to support photosynthesis, the availability of water, and the availability of suitable temperatures for carrying out photosynthesis.^[15]

Energy flow

Left: Energy flow diagram of a frog. The frog represents a node in an extended food web. The energy ingested is utilized for metabolic processes and transformed into biomass. The energy flow continues on its path if the frog is ingested by predators, parasites, or as a decaying carcass in soil. This energy flow diagram illustrates how energy is lost as it fuels the metabolic process that transform the energy and nutrients into biomass.
Right: An expanded three link energy food chain (1. plants, 2. herbivores, 3. carnivores) illustrating the relationship between food flow diagrams and energy transformity. The transformity of energy becomes degraded, dispersed, and diminished from higher quality to lesser quantity as the energy within a food chain flows from one trophic species into another. Abbreviations: I=input, A=assimilation, R=respiration, NU=not utilized, P=production, B=biomass.^[16]

Main article: Energy flow (ecology)

See also: Food web and Trophic level

The carbon and energy incorporated into plant tissues (net primary production) is either consumed by animals while the plant is alive, or it remains uneaten when the plant tissue dies and becomes detritus. In terrestrial ecosystems, roughly 90% of the NPP ends up being broken down by decomposers. The remainder is either consumed by animals while still alive and enters the plant-based trophic system, or it is consumed after it has died, and enters the detritus-based trophic system. In aquatic systems, the proportion of plant biomass that gets consumed by herbivores is much higher.^[17] In trophic systems photosynthetic organisms are the primary producers. The organisms that consume their tissues are called primary consumers or secondary producers—herbivores. Organisms which feed on microbes (bacteria and fungi) are termed microbivores. Animals that feed on primary consumers—carnivores—are secondary consumers. Each of these constitutes a trophic level.^[17] The sequence of consumption—from plant to herbivore, to carnivore—forms a food chain. Real systems are much more complex than this—organisms will generally feed on more than one form of food, and may feed at more than one trophic level. Carnivores may capture some prey which are part of a plant-based trophic system and others that are part of a detritus-based trophic system (a bird that feeds both on herbivorous grasshoppers and earthworms, which consume detritus). Real systems, with all these complexities, form food webs rather than food chains.^[17]

Decomposition

See also: Decomposition

The carbon and nutrients in dead organic matter are broken down by a group of processes known as decomposition. This releases nutrients that can then be re-used for plant and microbial production, and returns carbon dioxide to the atmosphere (or water) where it can be used for photosynthesis. In the absence of decomposition, dead organic matter would accumulate in an ecosystem and nutrients and atmospheric carbon dioxide would be depleted.^[18] Approximately 90% of terrestrial NPP goes directly from plant to decomposer.^[17]

Decomposition processes can be separated into three categories—leaching, fragmentation and chemical alteration of dead material. As water moves through dead organic matter, it dissolves and carries with it the water-soluble components. These are then taken up by organisms in the soil, react with mineral soil, or are transported beyond the confines of the ecosystem (and are considered "lost" to it).^[18] Newly shed leaves and newly dead animals have high concentrations of water-soluble components, and include sugars, amino acids and mineral nutrients. Leaching is more important in wet environments, and much less important in dry ones.^[18]

Fragmentation processes break organic material into smaller pieces, exposing new surfaces for colonization by microbes. Freshly shed leaf litter may be inaccessible due to an outer layer of cuticle or bark, and cell contents are protected by a cell wall. Newly dead animals may be covered by an exoskeleton. Fragmentation processes, which break through these protective layers, accelerate the rate of microbial decomposition.^[18] Animals fragment detritus as they hunt for food, as does passage through the gut. Freeze-thaw cycles and cycles of wetting and drying also fragment dead material.^[18]

The chemical alteration of dead organic matter is primarily achieved through bacterial and fungal action. Fungal hyphae produce enzymes which can break through the tough outer structures surrounding dead plant material. They also produce enzymes which break down lignin, which allows to them access to both cell contents and to the nitrogen in the lignin. Fungi can transfer carbon and nitrogen through their hyphal networks and thus, unlike bacteria, are not dependent solely on locally available resources.^[18]

Decomposition rates vary among ecosystems. The rate of decomposition is governed by three sets of factors—the physical environment (temperature, moisture and soil properties), the quantity and quality of the dead material available to decomposers, and the nature of the microbial community itself.^[19] Temperature controls the rate of microbial respiration; the higher the temperature, the faster microbial decomposition occurs. It also affects soil moisture, which slows microbial growth and reduces leaching. Freeze-thaw cycles also affect decomposition—freezing temperatures kill soil microorganisms, which allows leaching to play a more important role in moving nutrients around. This can be especially important as the soil thaws in the Spring, creating a pulse of nutrients which become available.^[19]

Decomposition rates are low under very wet or very dry conditions. Decomposition rates are highest in wet, moist conditions with adequate levels of oxygen. Wet soils tend to become deficient in oxygen (this is especially true in wetlands), which slows microbial growth. In dry soils, decomposition slows as well, but bacteria continue to grow (albeit at a slower rate) even after soils become too dry to support plant growth. When the rains return and soils become wet, the osmotic gradient between the bacterial cells and the soil water causes the cells to gain water quickly. Under these conditions, many bacterial cells burst, releasing a pulse of nutrients.^[19] Decomposition rates also tend to be slower in acidic soils.^[19] Soils which are rich in clay minerals tend to have lower decomposition rates, and thus, higher levels of organic matter.^[19] The smaller particles of clay result in a larger surface area that can hold water. The higher the water content of a soil, the lower the oxygen content^[20] and consequently, the lower the rate of decomposition. Clay minerals also bind particles of organic material to their surface, making them less accessibly to microbes.^[19] Soil disturbance like tilling increase decomposition by increasing the amount of oxygen in the soil and by exposing new organic matter to soil microbes.^[19]

The quality and quantity of the material available to decomposers is another major factor that influences the rate of decomposition. Substances like sugars and amino acids decompose readily and are considered "labile". Cellulose and hemicellulose, which are broken down more slowly, are "moderately labile". Compounds which are more resistant to decay, like lignin or cutin, are considered "recalcitrant".^[19] Litter with a higher proportion of labile compounds decomposes much more rapidly than does litter with a higher proportion of recalcitrant material. Consequently, dead animals decompose more rapidly than dead leaves, which themselves decompose more rapidly than fallen branches.^[19] As organic material in the soil ages, its quality decreases. The more labile compounds decompose quickly, leaving and increasing proportion of recalcitrant material. Microbial cell walls also contain a recalcitrant materials like chitin, and these also accumulate as the microbes die, further reducing the quality of older soil organic matter.^[19]

Nutrient cycling

Biological nitrogen cycling.

See also: Nutrient cycle and Biogeochemical cycle

Ecosystems continually exchange energy and carbon with the wider environment; mineral nutrients, on the other hand, are mostly cycled back and forth between plants, animals, microbes and the soil. Most nitrogen enters ecosystems through biological nitrogen fixation, is deposited through precipitation, dust, gases or is applied as fertilizer.^[14] Since most terrestrial ecosystems are nitrogen-limited, nitrogen cycling is an important control on ecosystem production.^[14]

Until modern times, nitrogen fixation was the major source of nitrogen for ecosystems. Nitrogen fixing bacteria either live symbiotically with plants, or live freely in the soil. The energetic cost is high for plants which support nitrogen-fixing symbionts—as much as 25% of GPP when measured in controlled conditions. Many members of the legume plant family support nitrogen-fixing symbionts. Some cyanobacteria are also capable of nitrogen fixation. These are phototrophs, which carry out photosynthesis. Like other nitrogen-fixing bacteria, they can either be free-living or have symbiotic relationships with plants.^[14] Other sources of nitrogen include acid deposition produced through the combustion of fossil fuels, ammonia gas which evaporates from agricultural fields which have had fertilizers applied to them, and dust.^[14] Anthropogenic nitrogen inputs account for about 80% of all nitrogen fluxes in ecosystems.^[14]

When plant tissues are shed or are eaten, the nitrogen in those tissues becomes available to animals and microbes. Microbial decomposition releases nitrogen compounds from dead organic matter in the soil, where plants, fungi and bacteria compete for it. Some soil bacteria use organic nitrogen-containing compounds as a source of carbon, and release ammonium ions into the soil. This process is known as nitrogen mineralization. Others convert ammonium to nitrite and nitrate ions, a process known as nitrification. Nitric oxide and nitrous oxide are also produced during nitrification.^[14] Under nitrogen-rich and oxygen-poor conditions, nitrates and nitrites are converted to nitrogen gas, a process known as denitrification.^[14]

Other important nutrients include phosphorus, sulfur, calcium, potassium, magnesium and manganese.^[21] Phosphorus enters ecosystems through weathering. As ecosystems age this supply diminishes, making phosphorus-limitation more common in older landscapes (especially in the tropics).^[21] Calcium and sulfur are also produced by weathering, but acid deposition is an important source of sulfur in many ecosystems. Although magnesium and manganese are produced by weathering, exchanges between soil organic matter and living cells account for a significant portion of ecosystem fluxes. Potassium is primarily cycled between living cells and soil organic matter.^[21]

Function and biodiversity

Savanna at Ngorongoro Conservation Area, Tanzania.

See also: Biodiversity and Ecosystem engineer

Ecosystem processes are broad generalizations that actually take place through the actions of individual organisms. The nature of the organisms—the species, functional groups and trophic levels to which they belong—dictates the sorts of actions these individuals are capable of carrying out, and the relative efficiency with which they do so. Thus, ecosystem processes are driven by the number of species in an ecosystem, the exact nature of each individual species, and the relative abundance organisms within these species.^[22] Biodiversity plays an important role in ecosystem functioning.^[23]

Ecological theory suggests that in order to coexist, species must have some level of limiting similarity—they must be different from one another in some fundamental way, otherwise one species would competitively exclude the other.^[24] Despite this, the cumulative effect of additional species in an ecosystem is not linear—additional species may enhance nitrogen retention, for example, but beyond some level of species richness, additional species may have little additive effect.^[22] The addition (or loss) of species which are ecologically similar to those already present in an ecosystem tends to only have a small effect on ecosystem function. Ecologically distinct species, on the other hand, have a much larger effect. Similarly, dominant species have a large impact on ecosystem function, while rare species tend to have a small effect. Keystone species tend to have a effect on ecosystem function that is disproportionate to their abundance in an ecosystem.^[22]

Ecosystem goods and services

Main articles: Ecosystem services and Ecological goods and services

See also: Ecosystem valuation and Ecological yield

Ecosystems provide a variety of goods and services upon which people depend.^[25] Ecosystem goods include the "tangible, material products"^[26] of ecosystem processes—food, construction material, medicinal plants—in addition to less tangible items like tourism and recreation, and genes from wild plants and animals that can be used to improve domestic species.^[19] Ecosystem services, on the other hand, are generally "improvements in the condition or location of things of value".^[26] These include things like the maintenance of hydrological cycles, cleaning air and water, the maintenance of oxygen in the atmosphere, crop pollination and even things like beauty, inspiration and opportunities for research.^[19] While ecosystem goods have traditionally been recognized as being the basis for things of economic value, ecosystem services tend to be taken for granted.^[26] While Gretchen Daily's original definition distinguished between ecosystem goods and ecosystem services, Robert Costanza and colleagues' later work and that of the Millennium Ecosystem Assessment lumped all of these together as "ecosystem services".^[26]

Ecosystem management

Main article: Ecosystem management

See also: Sustainability and Sustainable development

When natural resource management is applied to whole ecosystems, rather than single species, it is termed ecosystem management.^[27] A variety of definitions exist: F. Stuart Chapin and coauthors define it as "the application of ecological science to resource management to promote long-term sustainability of ecosystems and the delivery of essential ecosystem goods and services",^[28] while Norman Christensen and coauthors defined it as "management driven by explicit goals, executed by policies, protocols, and practices, and made adaptable by monitoring and research based on our best understanding of the ecological interactions and processes necessary to sustain ecosystem structure and function"^[25] and Peter Brussard and colleagues defined it as "managing areas at various scales in such a way that ecosystem services and biological resources are preserved while appropriate human use and options for livelihood are sustained".^[29]

Although definitions of ecosystem management abound, there is a common set of principles which underlie these definitions.^[28] A fundamental principle is the long-term sustainability of the production of goods and services by the ecosystem;^[28] "intergenerational sustainability [is] a precondition for management, not an afterthought".^[25] It also requires clear goals with respect to future trajectories and behaviors of the system being managed. Other important requirements include a sound ecological understanding of the system, including connectedness, ecological dynamics and the context in which the system is embedded. Other important principles include an understanding of the role of humans as components of the ecosystems and the use of adaptive management.^[25] While ecosystem management can be used as part of a plan for wilderness conservation, it can also be used in intensively managed ecosystems^[25] (see, for example, agroecosystems and close to nature forestry).

Ecosystem dynamics

Forest on San Juan Island

Loch Lomond in Scotland forms a relatively isolated ecosystem. The fish community of this lake has remained unchanged over a very long period of time.^[30]

Spiny forest at Ifaty, Madagascar, featuring various Adansonia (baobab) species, Alluaudia procera (Madagascar ocotillo) and other vegetation.

Arctic tundra on Wrangel Island, Russia.

Ecosystems are dynamic entities—invariably, they are subject to periodic disturbances and are in the process of recovering from some past disturbance.^[31] When an ecosystem is subject to some sort of perturbation, it responds by moving away from its initial state. The tendency of a system to remain close to its equilibrium state, despite that disturbance, is termed its resistance. On the other hand, the speed with which it returns to its initial state after disturbance is called its resilience.^[31]

From one year to another, ecosystems experience variation in their biotic and abiotic environments. A drought, an especially cold winter and a pest outbreak all constitute short-term variability in environmental conditions. Animal populations vary from year to year, building up during resource-rich periods and crashing as they overshoot their food supply. These changes play out in changes in NPP, decomposition rates, and other ecosystem processes.^[19] Longer-term changes also shape ecosystem processes—the forests of eastern North America still show legacies of cultivation which ceased 200 years ago, while methane production in eastern Siberian lakes is controlled by organic matter which accumulated during the Pleistocene.^[19]

Disturbance also plays an important role in ecological processes. F. Stuart Chapin and coauthors define disturbance as "a relatively discrete event in time and space that alters the structure of populations, communities and ecosystems and causes changes in resources availability or the physical environment".^[32] This can range from tree falls and insect outbreaks to hurricanes and wildfires to volcanic eruptions and can cause large changes in plant, animal and microbe populations, as well soil organic matter content.^[31] Disturbance is followed by succession, a "directional change in ecosystem structure and functioning resulting from biotically driven changes in resources supply."^[32]

The frequency and severity of disturbance determines the way it impacts ecosystem function. Major disturbance like a volcanic eruption or glacial advance and retreat leave behind soils that lack plants, animals or organic matter. Ecosystems that experience disturbances that sever undergo primary succession. Less severe disturbance like forest fires, hurricanes or cultivation result in secondary succession.^[19] More severe disturbance and more frequent disturbance result in longer recovery times. Ecosystems recover more quickly from less severe disturbance events.^[31]

The early stages of primary succession are dominated by species with small propagules (seed and spores) which can be dispersed long distances. The early colonizers—often algae, cyanobacteria and lichens—stabilize the substrate. Nitrogen supplies are limited in new soils, and nitrogen-fixing species tend to play an important role early in primary succession. Unlike in primary succession, the species that dominate secondary succession, are usually present from the start of the process, often in the soil seed bank. In some systems the successional pathways are fairly consistent, and thus, are easy to predict. In others, there are many possible pathways—for example, the introduced nitrogen-fixing legume, Myrica faya, alter successional trajectories in Hawai'ian forests.^[31]

The theoretical ecologist Robert Ulanowicz has used information theory tools to describe the structure of ecosystems, emphasizing mutual information (correlations) in studied systems. Drawing on this methodology and prior observations of complex ecosystems, Ulanowicz depicts approaches to determining the stress levels on ecosystems and predicting system reactions to defined types of alteration in their settings (such as increased or reduced energy flow, and eutrophication.^[33]

Ecosystem ecology

Main article: Ecosystem ecology

See also: Ecosystem model

Ecosystem ecology studies "the flow of energy and materials through organisms and the physical environment". It seeks to understand the processes which govern the stocks of material and energy in ecosystems, and the flow of matter and energy through them. The study of ecosystems can cover 10 orders of magnitude, from the surface layers of rocks to the surface of the planet.^[34]

There is no single definition of what constitutes an ecosystem.^[35] German ecologist Ernst-Detlef Schulze and coauthors defined an ecosystem as an area which is "uniform regarding the biological turnover, and contains all the fluxes above and below the ground area under consideration." They explicitly reject Gene Likens' use of entire river catchments as "too wide a demarcation" to be a single ecosystem, given the level of heterogeneity within such an area.^[36] Other authors have suggested that an ecosystem can encompass a much larger area, even the whole planet.^[4] Schulze and coauthors also rejected the idea that a single rotting log could be studied as an ecosystem because the size of the flows between the log and its surroundings are too large, relative to the proportion cycles within the log.^[36] Philosopher of science Mark Sagoff considers the failure to define "the kind of object it studies" to be an obstacle to the development of theory in ecosystem ecology.^[35]

Ecosystems can be studied through a variety of approaches—theoretical studies, studies monitoring specific ecosystems over long periods of time, those that look at differences between ecosystems to elucidate how they work and direct manipulative experimentation.^[37] Studies can be carried out at a variety of scales, from microcosms and mesocosms which serve as simplified representations of ecosystems, through whole-ecosystem studies.^[38] American ecologist Stephen R. Carpenter has argued that microcosm experiments can be "irrelevant and diversionary" if they are not carried out in conjunction with field studies carried out at the ecosystem scale, because microcosm experiments often fail to accurately predict ecosystem-level dynamics.^[39]

The Hubbard Brook Ecosystem Study, established in the White Mountains, New Hampshire in 1963, was the first successful attempt to study an entire watershed as an ecosystem. The study used stream chemistry as a means of monitoring ecosystem properties, and developed a detailed biogeochemical model of the ecosystem.^[40] Long-term research at the site led to the discovery of acid rain in North America in 1972, and was able to document the consequent depletion of soil cations (especially calcium) over the next several decades.^[41]

Classification

See also: Ecosystem diversity, Ecoregion, and Ecological land classification

The High Peaks Wilderness Area in the 6,000,000-acre (2,400,000 ha) Adirondack Park is an example of a diverse ecosystem.

Flora of Baja California Desert, Cataviña region, Mexico.

Classifying ecosystems into ecologically homogeneous units is an important step towards effective ecosystem management.^[42] A variety of systems exist, based on vegetation cover, remote sensing, and bioclimatic classification systems.^[42] American geographer Robert Bailey defines a hierarchy of ecosystem units ranging from microecosystems (individual homogeneous sites, on the order of 10 square kilometres (4 sq mi) in area), through mesoecosystems (landscape mosaics, on the order of 1,000 square kilometres (400 sq mi)) to macroecosystems (ecoregions, on the order of 100,000 square kilometres (40,000 sq mi)).^[43]

Bailey outlined five different methods for identifying ecosystems: gestalt ("a whole that is not derived through considerable of its parts"), in which regions are recognized and boundaries drawn intuitively; a map overlay system where different layers like geology, landforms and soil types are overlain to identify ecosystems; mulitvariate clustering of site attributes; digital image processing of remotely sensed data grouping areas based on their appearance or other spectral properties; or by a "controlling factors method" where a subset of factors (like soils, climate, vegetation physiognomy or the distribution of plant or animal species) are selected from a large array of possible ones are used to delineate ecosystems.^[44] In contrast with Bailey's methodology, Puerto Rican ecologist Ariel Lugo and coauthors identified ten characteristics of an effective classification system: that it be based on georeferenced, quantitative data; that it should minimize subjectivity and explicitly identify criteria and assumptions; that it should be structured around the factors that drive ecosystem processes; that it should reflect the hierarchical nature of ecosystems; that it should be flexible enough to conform to the various scales at which ecosystem management operates; that it should be tied to reliable measures of climate so that it can "anticipat[e] global climate change; that it be applicable worldwide; that it should be validated against independent data; that it take into account the sometimes complex relationship between climate, vegetation and ecosystem functioning; and that it should be able to adapt and improve as new data become available.^[42]

Examples of ecosystems

• Agroecosystem
• Aquatic ecosystem
• Chaparral
• Coral reef
• Desert
• Forest
• Greater Yellowstone Ecosystem
• Human ecosystem
• Large marine ecosystem
• Littoral zone
• Lotic
• Marine ecosystem
• Prairie
• Rainforest
• Riparian zone
• Savanna
• Steppe
• Subsurface Lithoautotrophic Microbial Ecosystem
• Taiga
• Tundra
• Urban ecosystem

A freshwater ecosystem in Gran Canaria, an island of the Canary Islands.

See also

	Earth_sciences portal
	Ecology portal
	Environment portal
	Weather portal
	Sustainable Development portal

• Biodiversity Action Plan
• Biosphere
• Biosphere 2
• Complex system
• Earth science
• Ecological economics
• Ecotope
• Human ecology
• Invasive species
• Landscape ecology
• Leopold Report
• Natural landscape
• Natural resource
• Novel ecosystem
• Spaceship Earth

Notes

1. ^ The term ecosystem was actually coined by Arthur Roy Clapham, who came up with the word at Tansley's request.(Willis 1997)

References

1. ^ Chapin et al. (2002), p. 380
2. ^ Tansley (1934); Molles (1999), p. 482; Chapin et al. (2002), p. 380; Schulze et al. (2005); p. 400; Gurevitch et al. (2006), p. 522; Smith & Smith 2012, p. G-5
3. ^ Chapin et al. (2002), p. 380; Schulze et al. (2005); p. 400
4. ^ ^{a b} Willis (1997), p.269; Chapin et al. (2002), p. 5; Krebs (2009). p. 572
5. ^ Schulze et al. (2005), p.400
6. ^ Odum, EP (1971) Fundamentals of ecology, third edition, Saunders New York
7. ^ Chapin et al. (2002), p. 10
8. ^ ^{a b c d e f g h} Chapin et al. (2002), pp. 11–13
9. ^ Willis (1997)
10. ^ ^{a b} Chapin et al. (2002), pp. 7–11)
11. ^ ^{a b} Tansley (1935)
12. ^ Tansley, AG (1939) The British islands and their vegetation. Volume 1 of 2. Cambridge University Press, United. Kingdom. 484 pg.
13. ^ ^{a b} Chapin et al. (2002), pp. 123–150
14. ^ ^{a b c d e f g h} Chapin et al. (2002), pp. 197–215
15. ^ ^{a b} Chapin et al. (2002), pp. 97–104
16. ^ Odum, H. T. (1988). "Self-organization, transformity, and information". Science 242 (4882): 1132–1139. doi:10.1126/science.242.4882.1132. JSTOR 1702630. PMID 17799729. 
17. ^ ^{a b c d} Chapin et al. (2002) pp. 244–264
18. ^ ^{a b c d e f} Chapin et al. (2002), pp. 151–157
19. ^ ^{a b c d e f g h i j k l m n o} Chapin et al. (2002), pp. 159–174
20. ^ Chapin et al. (2002), pp. 61–67
21. ^ ^{a b c} Chapin et al. (2002), pp. 215–222
22. ^ ^{a b c} Chapin et al. (2002), pp. 265–277
23. ^ Schulze et al. (2005), pp. 449–453
24. ^ Schoener, Thomas W. (2009). "Ecological Niche". In Simon A. Levin. The Princeton Guide to Ecology. Princeton: Princeton University Press. pp. 2–13. ISBN 978-0-691-12839-9. 
25. ^ ^{a b c d e} Christensen, Norman L.; Ann M. Bartuska; James H. Brown; Stephen Carpenter; Carla D'Antonio; Robert Francis; Jerry F. Franklin; James A. MacMahon; Reed F. Noss; David J. Parsons; Charles H. Peterson; Monica G. Turner; Robert G. Woodmansee (1996). "The Report of the Ecological Society of America Committee on the Scientific Basis for Ecosystem Management". Ecological Applications 6 (3): 665–691. 
26. ^ ^{a b c d} Brown, Thomas C.; John C. Bergstrom; John B. Loomis (2007). "Defining, valuing and providing ecosystem goods and services". Natural Resources Journal 47 (2): 329–376. http://lawlibrary.unm.edu/nrj/47/2/04_brown_goods.pdf. 
27. ^ Grumbine, R. Edward (1994). "What is ecosystem management?". Conservation Biology 8 (1): 27–38. http://www.pelagicos.net/MARS6920_spring2010/readings/Grumbine_1994.pdf. 
28. ^ ^{a b c} Chapin et al. (2002), pp. 362–365
29. ^ Brussard, Peter F.; J. Michael Reed; C. Richard Tracy (1998). "Ecosystem management: what is it really?". Landscape and Urban Planning 40 (1): 9–20. http://carmelacanzonieri.com/library/6108-LandscapeEcoPlanning/Brussard-EnvMgmntWhatIsIt.pdf. 
30. ^ Adams, C.E. (1994). "The fish community of Loch Lomond, Scotland : its history and rapidly changing status". Hydrobiologia 290 (1–3): 91–102. doi:10.1007/BF00008956. http://cat.inist.fr/?aModele=afficheN&cpsidt=3302548. 
31. ^ ^{a b c d e} Chapin et al. (2002), pp. 281–304
32. ^ ^{a b} Chapin et al. (2002), p. 285
33. ^ Robert Ulanowicz (1997). Ecology, the Ascendant Perspective. Columbia Univ. Press. ISBN 0-231-10828-1.
34. ^ Chapin et al. (2002), pp. 3–7
35. ^ ^{a b} Sagoff, Mark (2003). "The plaza and the pendulum: Two concepts of ecological science". Biology and Philosophy 18 (4): 529–552. doi:10.1023/A:1025566804906. 
36. ^ ^{a b} Schulze et al. 300–402
37. ^ Carpenter, Stephen R.; Jonathan J. Cole; Timothy E. Essington; James R. Hodgson; Jeffrey N. Houser; James F. Kitchell; Michael L. Pace (1998). "Evaluating Alternative Explanations in Ecosystem Experiments". Ecosystems 1 (4): 335–344. 
38. ^ Schindler, David W. (1998). "Replication versus Realism: The Need for Ecosystem-Scale Experiments". Ecosystems 1 (4): 323–334. 
39. ^ Carpenter, Stephen R. (1996). "Microcosm Experiments have Limited Relevance for Community and Ecosystem Ecology". Ecology 77 (3): 677–680. 
40. ^ Lindenmayer, David B.; Gene E. Likens (2010). "The Problematic, the Effective and the Ugly – Some Case Studies". Effective Ecological Monitoring. Collingwood, Australia: CSIRO Publishing. pp. 87–145. ISBN 978-1-84971-145-6. 
41. ^ Likens, Gene E. (2004). "Some perspectives on long-term biogeochemical research from the Hubbard Brook Ecosystem Study". Ecology 85 (9): 2355–2362. http://www.ci.uri.edu/CIIP/SummerPracticum/Docs2007/Likens_LongTermResearch_Ecology2004.pdf. 
42. ^ ^{a b c} Lugo, A. E.; S.L. Brown; R. Dodson; T.S. Smith; H.H. Shugart (1999). "The Holdridge life zones of the conterminous United States in relation to ecosystem mapping". Journal of Biogeography 26: 1025–1038. http://www.fs.fed.us/global/iitf/pubs/ja_iitf_1999_lugo002.pdf. 
43. ^ Bailey (2009), Chapter 2, pp. 25–28
44. ^ Bailey (2009), Chapter 3, pp. 29–40

Literature cited

• Bailey, Robert G. (2009). Ecosystem Geography (Second ed.). New York: Springer. 
• Chapin, F. Stuart; Pamela A. Matson; Harold A. Mooney (2002). Principles of Terrestrial Ecosystem Ecology. New York: Springer. ISBN 0-387-95443-0. 
• Gurevitch, Jessica; Samuel M. Scheiner; Gordon A. Fox (2006). The Ecology of Plants (Second ed.). Sunderland, Massachusetts: Sinauer Associates. ISBN 978-0-87893-294-8. 
• Krebs, Charles J. (2009). Ecology: The Experimental Analysis of Distribution and Abundance (Sixth ed.). San Francisco: Benjamin Cummings. ISBN 978-0-321-50743-3. 
• Lindenmayer, David B.; Gene E. Likens (2010). Effective Ecological Monitoring. Collingwood, Australia: CSIRO Publishing. ISBN 978-1-84971-145-6. 
• Molles, Manuel C. (1999). Ecology: Concepts and Applications. Boston: WCB/McGraw-HIll. ISBN 0-07-042716-X. 
• Schulze, Ernst-Detlef; Erwin Beck; Klaus Müller-Hohenstein (2005). Plant Ecology. Berlin: Springer. ISBN 3-540-20833-X. 
• Tansley, AG (1935). "The use and abuse of vegetational terms and concepts". Ecology 16 (3): 284–307. doi:10.2307/1930070. JSTOR 1930070. 
• Smith, Thomas M.; Robert Leo Smith (2012). Elements of Ecology (Eighth ed.). Boston: Benjamin Cummings. ISBN 978-0-321-73607-9. 
• Willis, A.J. (1997). "The Ecosystem: An Evolving Concept Viewed Historically". Functional Ecology 11 (2): 268–271. doi:10.1111/j.1365-2435.1997.00081.x. 

Further reading

• Ecological Society of America, Ecosystem Services, 25 May 2007
• Lawton, John H., What Do Species Do in Ecosystems?, Oikos, December, 1994. vol.71, no.3.
• Lindeman, R.L. 1942. The trophic-dynamic aspect of ecology. Ecology '23': 399–418.
• Modlin, Richard F. 2011. "Ecosystems." Encyclopedia of Environmental Issues. Rev. ed. Ed. Craig W. Allin. Pasadena: Salem Press. pp. 415–417. ISBN 978-1-58765-737-5
• Ranganathan, J and Irwin, F. (2007, May 7). Restoring Nature's Capital: An Action Agenda to Sustain Ecosystem Services

External links

Wikimedia Commons has media related to: Ecosystems

Look up ecosystem in Wiktionary, the free dictionary.

• Millennium Ecosystem Assessment (2005)
• The State of the Nation's Ecosystems (U.S.)
• Bering Sea Climate and Ecosystem (Current status)
• Arctic Climate and Ecosystem (Current status)
• Teaching about Ecosystems

v t e Modelling ecosystems – trophic components General Abiotic component Abiotic stress Behaviour Biogeochemical cycle Biomass Biotic component Biotic stress Carrying capacity Competition Ecosystem Ecosystem ecology Ecosystem model Keystone species List of feeding behaviours Metabolic theory of ecology Productivity Producers Autotrophs Chemosynthesis Chemotrophs Foundation species Mixotrophs Myco-heterotrophy Mycotroph Organotrophs Photoheterotrophs Photosynthesis Photosynthetic efficiency Phototrophs Primary nutritional groups Primary production Consumers Apex predator Bacterivore Carnivores Chemoorganotroph Foraging Generalist and specialist species Intraguild predation Herbivores Heterotroph Heterotrophic nutrition Insectivore Mesopredator release hypothesis Omnivores Optimal foraging theory Predation Prey switching Decomposers Chemoorganoheterotrophy Decomposition Detritivores Detritus Microorganisms Archaea Bacteriophage Environmental microbiology Lithoautotroph Lithotrophy Microbial cooperation Microbial ecology Microbial food web Microbial intelligence Microbial loop Microbial mat Microbial metabolism Phage ecology Food webs Biomagnification Ecological efficiency Ecological pyramid Energy flow Food chain Trophic level Example webs Cold seeps Hydrothermal vents Intertidal Kelp forests Lakes North Pacific Subtropical Gyre Rivers San Francisco Estuary Soil Tidal pool Processes Ascendency Bioaccumulation Cascade effect Climax community Competitive exclusion principle Consumer-resource systems Copiotrophs Dominance Ecological network Ecological succession Energy quality Energy Systems Language f-ratio Feed conversion ratio Feeding frenzy Mesotrophic soil Nutrient cycle Oligotroph Paradox of the plankton Trophic cascade Trophic mutualism Trophic state index Defense/counter Animal coloration Antipredator adaptations Camouflage Herbivore adaptations to plant defense Mimicry Plant defense against herbivory Predator avoidance in schooling fish

v t e Modelling ecosystems – other components Population ecology Abundance Allee effect Depensation Ecological yield Effective population size Intraspecific competition Logistic function Malthusian growth model Maximum sustainable yield Overpopulation in wild animals Overexploitation Population cycle Population dynamics Population modeling Population size Predator–prey equations Recruitment Resilience Small population size Stability Species Biodiversity Density-dependent inhibition Ecological effects of biodiversity Ecological extinction Endemic species Flagship species Gradient analysis Indicator species Introduced species Invasive species Latitudinal gradients in species diversity Minimum viable population Neutral theory Occupancy-abundance relationship Population viability analysis Priority effect Rapoport's rule Relative abundance distribution Relative species abundance Species diversity Species homogeneity Species richness Species distribution Species-area curve Umbrella species Species interaction Antibiosis Biological interaction Commensalism Community ecology Ecological facilitation Interspecific competition Mutualism Storage effect Spatial ecology Biogeography Cross-boundary subsidy Ecocline Ecotone Ecotype Disturbance Edge effect Foster's rule Habitat fragmentation Ideal free distribution Intermediate Disturbance Hypothesis Island biogeography Landscape ecology Landscape epidemiology Landscape limnology Metapopulation Patch dynamics r/K selection theory Source–sink dynamics Niche Ecological niche Ecological trap Ecosystem engineer Environmental niche modelling Guild Habitat Limiting similarity Niche apportionment models Niche construction Niche differentiation Other networks Assembly rules Bateman's principle Bioluminescence Ecological collapse Ecological debt Ecological deficit Ecological energetics Ecological indicator Ecological threshold Ecosystem diversity Emergence Extinction debt Kleiber's law Liebig's law of the minimum Marginal value theorem Thorson's rule Xerosere Other Allometry Alternative stable state Balance of nature Biological data visualization Constructal theory Ecocline Ecological economics Ecological footprint Ecological forecasting Ecological humanities Ecological stoichiometry Ecopath Ecosystem based fisheries Endolith Evolutionary ecology Functional ecology Industrial ecology Macroecology Microecosystem Natural environment Systems ecology Theoretical ecology List of ecology topics

v t e Hierarchy of life Biosphere >  Ecosystem > Community (Biocoenosis) > Population >  Organism > Organ system > Organ > Tissue > Cell > Organelle > Molecule (Macromolecule · Biomolecule) > Atom

v t e The Earth Continents Africa Antarctica Asia Australia Europe North America South America Oceans Arctic Ocean Atlantic Ocean Indian Ocean Pacific Ocean Southern Ocean Earth Earth science Future of the Earth Geological history of Earth Geology History of the Earth Plate tectonics Structure of the Earth Natural environment Biome Ecology Ecosystem Nature Wilderness Related articles Earth Day Geology of solar terrestrial planets Solar System World Category Portal

v t e Elements of nature Universe Space Time Matter Energy Earth Earth science Future of the Earth Geological history of Earth Geology History of the Earth Plate tectonics Structure of the Earth Weather Atmosphere of Earth Climate Meteorology Environment Ecology Ecosystem Wilderness Life Biology Eukaryota Plants/Flora Animals/Fauna Fungi Protista Evolutionary history of life Hierarchy of life Life on Earth Origin of life Prokaryotes Archaea Bacteria Viruses Category Portal

v t e Systems and systems science Systems categories Systems theory Systems science Systems scientists Conceptual Physical Social Systems Biological Complex Complex adaptive Conceptual Database management Dynamical Economical Ecosystem Formal Global Positioning System Human anatomy Information systems Legal systems of the world Systems of measurement Metric system Multi-agent system Nervous system Nonlinearity Operating system Physical system Political system Sensory system Social system Solar System Systems art Theoretical fields Chaos theory Complex systems Control theory Cybernetics Living systems Sociotechnical systems theory Systems biology System dynamics Systems ecology Systems engineering Systems neuroscience Systems psychology Systems science Systems theory World-systems theory Systems scientists Russell L. Ackoff William Ross Ashby Béla H. Bánáthy Gregory Bateson Richard E. Bellman Stafford Beer Ludwig von Bertalanffy Murray Bowen Kenneth E. Boulding C. West Churchman George Dantzig Heinz von Foerster Jay Wright Forrester Charles A S Hall George Klir Edward Lorenz Niklas Luhmann Humberto Maturana Margaret Mead Donella Meadows Mihajlo D. Mesarovic James Grier Miller Howard T. Odum Talcott Parsons Ilya Prigogine Anatol Rapoport Claude Shannon Qian Xuesen Francisco Varela Kevin Warwick Norbert Wiener Anthony Wilden

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - økosystem

Nederlands (Dutch)
ecosysteem

Français (French)
n. - écosystème

Deutsch (German)
n. - Ökosystem

Ελληνική (Greek)
n. - (βιολ.) οικοσύστημα

Italiano (Italian)
ecosistema

Português (Portuguese)
n. - ecossistema (m)

Русский (Russian)
экологическая система

Español (Spanish)
n. - ecosistema

Svenska (Swedish)
n. - ekologiskt system

中文（简体）(Chinese (Simplified))
生态系统

中文（繁體）(Chinese (Traditional))
n. - 生態系統

한국어 (Korean)
n. - 생태계

日本語 (Japanese)
n. - 生態系

العربيه (Arabic)
‏(الاسم) نظام بيئي‏

עברית (Hebrew)
n. - ‮קהילה של יצורים המקיימים יחסים הדדיים וסביבתה הטבעית, מערכת אקולוגית‬

If you are unable to view some languages clearly, click here.