ECOSYSTEMS

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.

David J. Rapport

(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 )

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ECOSYSTEMS

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 withnative 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/>.

KEY TERMS

ABUNDANCE:

A measure of the degree to which an ecosystem possesses large numbers of particular species. An abundant ecosystem may or may not have a wide array of different species. Compare with complexity.

ANGIOSPERM:

A type of plant that produces flowers during sexual reproduction.

BIOGEOCHEMICAL CYCLES:

The changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.

BIOTA:

A combination of all flora and fauna (plant and animal life, respectively) in a region.

CANOPY:

The upper portion of the trees in a forest. In a closed-canopy forest the canopy (which may be several hundredfeet, or well over 50 meters, high) protects the soil and lower areas from sun and torrential rainfall.

CARNIVORE:

A meat-eating organism.

COMPLEXITY:

A measure of the degree to which an ecosystem possesses a wide array of species. These species may or may not appear in large numbers. Compare with abundance.

COMPOUND:

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

DECOMPOSERS:

Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.

DECOMPOSITION REACTION:

A chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. On Earth, this often is achieved through the help of detritivores and decomposers.

DETRITIVORES:

Organisms that feed on waste matter, breaking down organic material into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers, but unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots.

ECOSYSTEM:

A community of interdependent organisms along with the inorganic components of their environment.

ELEMENT:

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

ENERGY TRANSFER:

The flow of energy between organisms in a food web.

FOOD WEB:

A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each consumes nutrients and passes it along to other organisms. Earth scientists typically prefer this name to food chain, an everyday term for a similar phenomenon. A food chain is a series of singular organisms in which each plant or animal depends on the organism that precedes or follows it. Food chains rarely exist in nature.

GEOSPHERE:

The upper part of Earth's continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.

GYMNOSPERM:

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

HERBIVORE:

A plant-eating organism.

HYDROSPHERE:

The entirety of Earth's water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.

NICHE:

A term referring to the role that a particular organism plays within its biological community.

OMNIVORE:

An organism that eats both plants and other animals.

ORGANIC:

At one time, chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals) and oxides, such as carbondioxide.

PHOTOSYNTHESIS:

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants.

SYSTEM:

Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.

Ecosystems

Introduction

Ecosystems are the biological community of organisms as well as the physical components of a specific area. A biological community is all of the populations of different species that live in a certain environment. An ecosystem can be as large as an entire forest or as small as a clump of moss that provides a habitat for plants, microscopic invertebrates, and bacteria.

Ecosystems can be thought of and studied in various contexts. Ecologists often study the flow of energy through ecosystems. Another focus of study of ecosystems involves understanding the interactions between the organisms that live within a community. Finally, ecological researchers study the interactions between organisms and the physical environment.

There are several important types of ecosystems on Earth. These ecosystems are often defined by the amount of precipitation they receive and the typical temperatures that they experience. Some of the more important ecosystems are the tropical rain forest, desert, temperate forest, tundra, and savanna or grasslands. Common aquatic ecosystems include lakes, rivers, estuaries, coral reefs, kelp forests, the open ocean, and the deep ocean.

Historical Background and Scientific Foundations

An ecosystem is the collection of organisms, or the biological community, that lives in a certain area as well as the physical components of the environment. The biological community includes all of the populations of animals, plants, fungi, and bacteria. A population is all the members of a certain species that live in a location. In terrestrial ecosystems, the physical environment includes the type and conditions of soil or rocky terrain, the general climatic conditions, the amount of precipitation, and the amount of sunlight available. In aquatic ecosystems, the physical environment includes the salinity, temperature, and acidity of the water, the amount of sunlight available, the amount of sedimentation in the water, the amount of nutrients dissolved in the water, and the type of substrate below the water.

The flow of energy through an ecosystem plays a key role in the stability and functioning of an ecosystem. With few exceptions, the energy for life originates with light energy from the sun. Plants convert this energy to chemical energy stored in carbohydrates through photosynthesis. Herbivores then consume the carbohydrates in plants and use the energy from the chemical bonds for growth and reproduction. Carnivores consume the herbivores and assimilate the energy stored in the chemical bonds of the herbivores. Finally, the carnivores die, and their organic remains are decomposed by bacteria and fungi. These decomposers use the stored chemical energy in organis’s chemical bonds and convert the organic material into inorganic material that plants require to perform photosynthesis. With each successive step in the food chain, or food web in complex ecosystems, energy is lost as heat to the environment. Approximately 10% of the energy harvested from the sun by plants gets passed on to the herbivores. Similarly, only 10% of the energy stored in the chemical bonds of the herbivores is passed to the carnivores. As a result, most ecosystems have significantly more plants, or primary producers, than predators.

Every organism in an ecosystem depends on many other organisms in order to grow and reproduce. In order to grow, organisms require food. Both the struggle to obtain food and the struggle to avoid becoming food are major goals of the biological interactions in ecosystems. Predation is the consumption of one species by another species. The predator is the species that consumes the prey. Predators have developed adaptations that allow them to hunt their prey more effectively. For example, tuna have special muscles that allow them to swim quickly after smaller fish. Lions have large jaws and sharp teeth to capture prey. Angler fish have special outgrowths from their foreheads that look like worms and lure smaller fish into striking distance. Halibut can disguise themselves so that they are nearly invisible to prey swimming near the seafloor. However, prey has developed adaptations to avoid predation. Roses and cacti have thorns that discourage grazing. Tobacco produces nicotine, which is toxic to many insects. Skunks spray acrid chemicals on potential predators.

Another type of ecological interaction is competition for resources. Resources are anything that an organism needs to grow and reproduce such as shelter, territory, food, sunlight, and water. Members of the same species may also compete for mates. Just as organisms have developed numerous adaptations in response to predator and prey relationships, so too have they developed numerous adaptations to improve their ability to compete in an ecosystem. Tubeworms have developed long tentacles to sieve more plankton out of the water than their competitors. Several species of desert plants release chemicals into the soil that prevent the growth of other plants nearby. This decreases the competition for soil nutrients and water.

The sum of the resource requirements for an organism is called its ecological niche. When organisms have ecological niches that overlap, competition is more intense. Because individuals of the same species have identical resource requirements, competition within a species is often quite intense. Any resource that is in such short supply in an ecosystem that it prevents the growth of a population is called a limiting factor. Limiting factors often control the available ecological niches within an ecosystem. The number of available ecological niches within an ecosystem is often related to the biodiversity, or species richness of an ecosystem. When an ecosystem has more ecological niches, the number of species in the ecosystem is greater.

Although the flow of energy moves through ecosystems in one direction, from plants to herbivores and then to carnivores, many of the physical components of ecosystems are recycled. For example, chemicals that are critical to growth and reproduction in animals, water, carbon, nitrogen, and phosphorus all flow through both the biotic, or living, and abiotic, or nonliving, components of ecosystems. The patterns of flow are called biogeochemical cycles. Each of these cycles varies depending on the chemical properties of the matter. For example, the generalized biogeochemical cycle for carbon begins with carbon dioxide from the atmosphere becoming assimilated into carbohydrates during plant photosynthesis. It is passed to animals as the plants are consumed. The animals release carbon dioxide to the atmosphere through respiration. If plants are not con-

WORDS TO KNOW

BIOME: A well-defined terrestrial environment (e.g., desert, tundra, or tropical forest) and the complex of living organisms found in that region.

BYCATCH: Non-target species killed in the process of fishing.

COMMUNITY: All of the populations of species living in a certain environment.

ECOLOGICAL NICHE: The sum of the environmental requirements necessary for an individual to survive and reproduce.

PRECIPITATION: Moisture that falls from clouds as a result of condensation in the atmosphere.

TRANSPIRATION: Loss of water taken in by roots from leaves through evaporation.

sumed, they die and become buried. In some cases, over long periods of time, these buried plants become fossil fuels. When fossil fuels are burned, carbon dioxide is returned to the atmosphere. Alternatively, the hydrologic cycle involves evaporation of water from lakes and oceans. Precipitation returns water to Earth where plants and animals use it to grow. A process known as transpiration returns water to the atmosphere from plants. Although water is in the atmosphere, the movement of air transports water from region to region, controlling the climate.

Ecosystems vary in size and structure depending on the physical properties of an ecosystem. In addition, ecosystems can be thought of as nested within ecosystems. For example, the ocean as a whole is an ecosystem. So too, a rocky shoreline is an ecosystem. Further, a tide pool within a rocky shoreline consists of animals, plants, and decomposers that make it an ecosystem unto itself. Within a tide pool, the back of a decorator crab might be an ecosystem, complete with moss, barnacles, limpets, shrimp, and even a parasite or two.

Throughout the world, several broad types of ecosystems are recognized. Because these forms of ecosystems are found in many different locations, they are also referred to as biomes. On land, the major biomes include the tropical rain forest, desert, temperate forest, tundra, and savanna or grasslands. Terrestrial ecosystems depend on the amount of precipitation they receive as well as the typical temperatures they experience.

The wettest and warmest type of ecosystem is the tropical rain forest. These ecosystems are the most diverse in the world, which means that they contain the most species as well as the most interactions among species. Rain forests are particularly diverse because they have

abundant ecological resources and therefore possess many different environmental niches. Rain forests receive more than 80 in (200 cm) of precipitation per year. In addition, temperatures are consistently warm, so organisms do not need adaptations to survive extremes. Rain forests are characterized by trees that form multiple layers of canopy. Within the trees live numerous species of insects, birds, reptiles, amphibians, sloths, and monkeys.

The driest ecosystems on Earth are desert ecosystems. Deserts are found in places where the climate is hot or temperate. Because there is very little water in

IN CONTEXT: ECOLOGY

In the 1700s, the science of biology exploded with new discoveries. Swedish botanist Carl Linnaeus (1707–1778) developed a vast classification system for living things, still in use today. Scientific knowledge about life on Earth became detailed enough to allow a sense of how the web of life interacts.

Charles Darwin (1809–1882) furthered this sense of life as an interconnected web in the mid-nineteenth century, especially with his On the Origin of Species (1859). He argued that interactions with other living things are among the most important factors shaping the course of natural selection. Western thinkers were quickly learning to see living communities as self-interacting, self-shaping, and self-sustaining—the essence of “ecological” thought. The word “ecology” itself was coined about 1870 by German biologist and Darwin supporter Ernst Haeckel (1834–1919).

desert ecosystems, plants have evolved adaptations to decrease water loss to evaporation. Instead of leaves, which have a large surface area over which water can evaporate, desert plants have tiny leaves or spines. Other plants only grow leaves during rainy periods and then shed them during dry periods. Many desert plants have a waxy coating on the outsides of their stems and leaves to prevent evaporation. Animals that live in deserts tend to be small. Many are nocturnal, hunting at night and hiding in shadows where it is cooler during the day. Typical animals include desert reptiles like tortoises, iguanas, and other lizards. Mammals include rodents like kangaroo rats and gerbils. Many species of birds and cats also hunt in deserts.

The coldest ecosystems are known as taiga or tundra. These places have long, cold winters. The snow only melts for a short time each year. There is not much precipitation, less than 10 in (25 cm) a year. Because the soil is frozen, trees have difficulty taking root and most of the plants are small and scrublike. Mosses, lichens, grasses, and sedges are the most common plants. Animals include those that are adapted to the harsh climate, such as lemmings, arctic foxes, snowshoe hares, and musk oxen. Many species of birds migrate to the tundra during the warm season. During this time, the tundra swarms with insects that provide food for the migrating birds.

Temperate forests are found in places where winter is cold and summer is warm, generally at mid-latitudes throughout North America, Europe, and Asia, and to a smaller extent in the southern portion of South America. Precipitation tends to be significant, between 30 and 50 in (75 to 125 cm) a year. Typically the trees in temperate forests lose their leaves each winter. The nutrients from these leaves are recycled into the soil by decomposers like bacteria and fungi. Many large mammals such as puma, bear, wolves, and bison are native to temperate forests. Numerous species of insects, reptiles, and amphibians are also found in temperate forests.

Grasslands are found in places where temperatures are moderate, but where the precipitation is somewhat less than that of temperate forests, approximately 10 to 30 in (25 to 75 cm) per year. They are common throughout central Asia, Australia, Africa, central North America, and South America. The major vegetation is grass, and few trees are found near water reservoirs. Grasslands are often home to large herds of herbivores like bison and elk. Fewer numbers of predators, like coyotes and wolves, hunt these grazers.

Common aquatic ecosystems include lakes, rivers, estuaries, coral reefs, kelp forests, the open ocean, and the deep ocean. The plants include phytoplankton, which are photosynthetic microorganisms that live in the surface layers of the water where sunlight is available. Other plants include kelp and sea grasses, many of which have adaptations that hold them firmly to the sea floor, especially in places where wave action is powerful. Many aquatic animals have adaptations that allow them to filter food out of the water. Clams extend a tube into the water, corals and anemones have tentacles, barnacles use modified leg appendages that look like feathers, and ctenophores spin mucous webs. Small fish often feed on these filter-feeders. Larger fish and marine mammals feed on the smaller fish. When organisms die in aquatic systems, they sink to the ocean or lake floor where they are decomposed by bacteria, returning inorganic nutrients to the water for phytoplankton to use in photosynthesis.

Impacts and Issues

Maintaining the health of ecosystems is important for numerous reasons. Natural ecosystems break down pollutants, recycle wastes, provide flood and erosion control, and create freshwater in aquifers. They also provide habitat for organisms, including pests, diverting these organisms from urban centers. The diversity of plants in the many ecosystems is the source for many medicines that humans use to treat disease. All of the oxygen on the planet results from photosynthesis from plants in forests and phytoplankton in the oceans.

Ecosystems throughout the world are threatened by human activity. Development, agriculture, mining, and grazing by ranchers have the most significant impact on land because they destroy native habitats. Pollution and climate change threaten the health of organisms in ecosystems, just as it threatens the health of humans. Because organisms within ecosystems depend on one another, impacts to one type of organism have repercussions throughout the entire ecosystem. Although every

IN CONTEXT: AUTHORITATIVE ASSESSMENTS OF CLIMATE CHANGE AND ECOSYSTEMS

According to the Intergovernmental Panel on Climate Change (IPCC): “The resilience of many ecosystems is likely to be exceeded this century by an unprecedented combination of climate change, associated disturbances (e.g., flooding, drought, wildfire, insects, ocean acidification), and other global change drivers (e.g., landuse change, pollution, overexploitation of resources).”

“Over the course of this century, net carbon uptake by terrestrial ecosystems is likely to peak before mid-century and then weaken or even reverse, thus amplifying climate change.”

“Approximately 20-30% of plant and animal species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5-2.5°C.”

“For increases in global average temperature exceeding 1.5-2.5°C and in concomitant atmospheric carbon dioxide concentrations, there are projected to be major changes in ecosystem structure and function, species’ ecological interactions, and species’ geographical ranges, with predominantly negative consequences for biodiversity, and ecosystem goods and services e.g., water and food supply.”

Source: Parry, M. L., et al. IPCC, 2007: Summary for Policymakers. Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

ecosystem is affected by these human activities, each type of biome has its own challenges due to its specific structure. Some of these more important impacts to specific ecosystems are discussed below.

Desert soils are usually thin and easily damaged. In addition, desert soils are slow to recover from damage. For example, tire tracks left by military vehicles training during World War II (1939–1945) are still visible in California deserts. Many grasslands are being converted to deserts through a process known as desertification. Desertification occurs when grasslands in semiarid regions are overgrazed. This overgrazing exposes the soil to wind, which removes the fertile top layers. Plants are unable to take root and the land becomes unable to support biological communities. Agriculture and grazing are no longer viable on desertified lands and people living nearby often migrate away. It has been estimated that up to 90% of all semiarid land in North America has been moderately to severely desertified. Conservation and management have attempted to slow the effects of desertification. These practices include controlling the distribution of grazing animals on rangeland and seeding in places where plant cover is sparse. Although these management practices have contributed to some range recovery, they are both slow and costly.

Forests, both tropical and temperate, are threatened by deforestation, which occurs when trees are removed for economic use, either for the wood itself or to clear the land for grazing or farming. Not only does deforestation remove trees that function as a basis for the ecosystems, tree removal exposes soils leading to erosion and it stops the influx of organic materials into soils making them less fertile. In wet areas, erosion leads to sedimentation of waterways, harming aquatic ecosystems. In dry areas, erosion can lead to desertification. Many species have become extinct as a result of deforestation, and deforestation affects the migratory patterns of birds and butterflies. Between 2000 and 2005, the United Nations Food and Agriculture Organization (FAO) estimates that 10.4 million hectares of tropical rain forest were destroyed, which is about double the rate of destruction during the prior decade.

More than half of the world’s population lives along coastlines. Aquatic ecosystems are threatened by urbanization, engineering (projects such as dams and levees), pollution, mining, and overfishing. Wetlands, in particular, are disappearing at alarming rates. In the contiguous 48 states, only about half the original wetlands remain undeveloped. In 1972 a section of the Clean Water Act was aimed at preventing additional loss of coastal wetlands. In 1986 the Emergency Wetlands Resource Act further authorized the U.S. Fish and Wildlife Service to acquire important wetlands. Though the loss of wetlands has slowed, questions of authority over wetlands complicate application of the legislation. In the open ocean, pollution is a major issue threatening ecosystems. In particular plastics break down into small spheres that both float and resemble fish eggs. Many organisms ingest these plastic spheres, which become incorporated into their tissues. In the center of the North Pacific Ocean, where winds and currents are slack, plastics have accumulated into a large flotilla. Net samples through this region collect six times more plastic than biological material. In addition, overfishing and bycatch of large marine mammals disrupt aquatic food webs.

The National Biological Service undertook a broad study of endangered ecosystems as a means of assessing human impact on the environment. The study found that 85% of the forests had been destroyed by the 1980s. More than 90% of the old growth forests were destroyed. About 98% of all streams were degraded so as to be unworthy of designation as wild or scenic waterways. Overall, the greatest losses to ecosystems in the United States were in the Northeast, the South, the Midwest, and in California. In response studies on threatened ecosystems, conservation agencies and governments have begun buying land and setting aside areas with particularly sensitive or endangered ecosystems. In some cases, government incentives encourage land owners to restore ecosystems to natural conditions.

See Also Aquatic Ecosystems; Benthic Ecosystems; Biodiversity; Coastal Ecosystems; Conservation; Estuaries; Extinction and Extirpation; Forests; Freshwater and Freshwater Ecosystems; Grasslands; Habitat Loss; Marine Ecosystems; Overfishing; Overgrazing; Reef Ecosystems; Wetlands

BIBLIOGRAPHY

Books

Odum, Eugene, and Gary W. Barrett. Fundamentals of Ecology, 5th ed. Detroit: Brooks Cole, 2004.

Raven, Peter H., Linda R. Berg, and George B. Johnson. Environment. Hoboken, NJ: Wiley, 2002.

Web Sites

American Museum of Natural History. “Center for Biodiversity and Conservation.” 2008. http://www.nhptv.org/natureworks/nwepecosystems.htm (accessed February 21, 2008).

Environmental Literacy Council. “Ecosystems.” May 14, 2007. http://www.enviroliteracy.org/category.php/3.html (accessed February 21, 2008).

Marietta College. “Environmental Biology—Ecosystems.” http://www.marietta.edu/~biol/102/ecosystem.html (accessed February 21, 2008).

New Hampshire Public Television. “NatureWorks—Ecosystems.” 2007. http://stort.unep-wcmc.org/imaps/gb2002/book/viewer.htm (accessed February 18, 2008).

University Corporation for Atmospheric Research. “Ecosystems.” January 15, 2008. http://www.windows.ucar.edu/tour/link=/earth/ecosystems.html&edu=elem (accessed February 21, 2008).

U.S. Environmental Protection Agency (EPA). “Ecosystems.” February 20, 2008. http://www.epa.gov/ebtpages/ecosystems.html (accessed February 21, 2008).

U.S. Geological Survey (USGS). “Endangered Ecosystems of the United States: A Preliminary Assessment of Loss and Degradation.” http://biology.usgs.gov/pubs/ecosys.htm (accessed February 27, 2008).

Juli Berwald

Ecosystem

An ecosystem is all the living organisms in an area along with the nonliving, or abiotic, parts of their environment. The abiotic parts of an ecosystem include physical substances such as soil, air, and water; forces such as gravity and wind; and conditions such as temperature, light intensity, humidity, or salinity.

Components and Boundaries

Physical substances can include organic materials that were once alive, such as bits of wood from trees, rotting plant material, and animal wastes and dead organisms. The physical substance of an ecosystem also includes inorganic materials such as minerals , nitrogen, and water, as well as the overall landscape of mountains, plains, lakes, and rivers.

The organisms and the physical environment of an ecosystem interact with one another. The atmosphere, water, and soil allow life to flourish and limit what kind of life can survive. For example, a freshwater lake provides a home for certain fish and aquatic plants. Yet, the same lake would kill plants and animals adapted to a saltwater estuary.

Just as the environment affects organisms, organisms affect their environment. Lichens break down rock. Trees block sunlight, change the acidity and moisture content of soil, and release oxygen into the atmosphere. Elephants may uproot whole trees in order to eat their leaves, beavers dam streams and create meadows, and rabbits nibble grasses right down to the ground.

Ecosystems are not closed; in fact, an ecosystem's boundaries are usually fuzzy. A pond, for example, blends little by little into marsh, and then into a mixture of open meadow and brush. A stream brings nutrients and organisms from a nearby forest and carries away materials to other ecosystems. Even large ecosystems interact with other ecosystems. Seeds blow from place to place, animals migrate, and flowing water and air carry organisms—and their products and remains—from ecosystem to ecosystem.

All ecosystems taken together make up the biosphere, all living organisms on the earth and their physical environment. The biosphere differs from other ecosystems in having fixed boundaries. The biosphere covers the whole surface of the earth. It begins underground and extends into the highest reaches of the atmosphere.

Feeding Relations

Ecologists divide the living, biotic part of an ecosystem into two groups of organisms: the autotrophs and the heterotrophs. Autotrophs, also called primary producers, are organisms that make their own food. The vast majority of autotrophs (literally self-nourishers) are either plants, algae, or bacteria that use sunlight to make sugars from carbon dioxide in the air through photosynthesis.

Heterotrophs (which means "nourished by others"), also called consumers, are organisms that consume other organisms. Heterotrophs include animals, protists, and bacteria, or fungi. Animals that eat plants, such as deer and caterpillars, are called herbivores. Animals that eat other animals, such as mountain lions and wasps, are called carnivores.

Decomposers are heterotrophs that feed from the carcasses of dead animals or dead plants. If they are animals, such as millipedes, lobsters, starfish, clams, and catfish, scientists sometimes call them scavengers. Many animals, including starfish, lions, hyenas, and humans, change from carnivore to scavenger and back, depending on what food source is available.

Some of the most important decomposers are nearly invisible. These are the detritivores: fungi, bacteria, and other organisms that feed on the remains of dead plants and other organisms. Each year, detritivores break down the remains of millions of tons of dead plant and animal material, recycling nutrients back into ecosystems around the world.

Because animals eat one another, they can be linked in food chains, where, for example, a hawk eats a snake, which has eaten a ground squirrel, which has eaten a seed. Every ecosystem has numerous food chains that interlink to form a food web . A food web can change over time. In one year, a population explosion of oak moths means that insect predators focus on oak moth caterpillars. In another year, oak moths are rare, and predators eat a diversity of other herbivores.

Ecologists assign the organisms in a food web to different trophic levels, depending on where they get their energy. Plants, which get their energy directly from the sun, are in the first trophic level; caterpillars, which get their energy from plants, are in the second; birds that eat caterpillars are in the third. Predators that eat the birds would be in a fourth trophic level. Predators may eat at more than one level. (Humans are an example.)

Productivity and Nutrient Cycling

Every ecosystem is unique, yet similar ecosystems share fundamental characteristics, including climate, productivity, total mass of living organisms, and numbers of species. For example, tropical rain forests have higher species diversity than temperate forests.

In the same way, marshes all have high productivity and deserts all have low productivity. Primary productivity is the amount of energy captured by primary producers during photosynthesis on a square meter of land each year. One factor that determines productivity is latitude and its effect on sunshine. A square meter of land near the North Pole, for example, receives about 700,000 kcals (kilocalories) of sunshine per year, while the same area at the equator receives nearly 2.5 times that much sunshine. So all things being equal, the tropical region has the potential for higher productivity. However, even in the same latitude, primary productivity varies enormously from ecosystem to ecosystem. A marsh, for example, is twice as productive as a temperate forest, four times as productive as a wheat field, and thirty-five times as productive as a desert.

Another important characteristic of ecosystems is total biomass, the dry weight of all the organisms living in it. Rain forests have more organisms per square meter and therefore more total biomass than other ecosystems, more even than the superproductive marshes.

On land, the biomass of plants is usually greater than the biomass of herbivores, which is greater than the biomass of carnivores. The reason for this is that every chemical process releases energy in the form of heat. So producers can use only part of the energy from the sun to build their bodies; the rest is lost as heat. In the same way, consumers can use only part of the energy in plants to build their own bodies; the rest is lost as heat. Each trophic level passes along only about 10 percent of the energy from the one below. This generalization is called the 10 percent law.

The 10 percent law explains why ecosystems have so few trophic levels and so few individuals at the highest trophic levels. If on a square meter of land, primary consumers store 15,000 kcal/year, herbivores will be able to consume only about 1,500 kcal/year from that meter, and herbivore-eating carnivores will only get 150 kcals, about as many calories as are in a cup of spaghetti. Carnivores must, therefore, roam over large areas to obtain enough to eat.

All sunlight energy eventually escapes from the biosphere in the form of heat. In contrast, the biosphere constantly recycles water, carbon, and other materials. As materials move from one trophic level to another, they may change form, but they rarely escape from the biosphere entirely. A single carbon atom in a fingernail may have been, at different times, part of an apple, part of a trilobite in the ocean, part of a mountain range, part of a dinosaur, or part of the oil in a Texas oil well. Carbon, oxygen, nitrogen, phosphorus, and other materials all pass through many forms—both biotic and abiotic—in a system called a biogeochemical cycle. The biogeochemical cycles of materials such as carbon and oxygen involve the whole biosphere.

see also Biogeochemical Cycles; Community; Desert; Estuaries; Forest, Boreal; Forest, Temperate; Forest, Tropical; Landscape Ecology; Plankton; Population Dynamics

Jennie Dusheck

Bibliography

Brewer, Richard. The Science of Ecology, 2nd ed. Philadelphia, PA: W. B. Saunders, Co., 1988.

Kareiva, Peter M., ed. Exploring Ecology and Its Applications: Readings from American Scientists. Sunderland, MA: Sinauer Associates, Inc., 1982–97.

Molles, Manuel C. Ecology: Concepts and Applications. Boston: WCB/McGraw-Hill, 1999.

Ecosystem

The term ecosystem was coined in 1935 by the Oxford ecologist Arthur Tansley to encompass the interactions among biotic and abiotic components of the environment at a given site. It was defined in its presently accepted form by Eugene Odum as follows: "Any unit that includes all of the organisms (i.e, the community) in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (i.e., exchange of materials between living and non-living parts) within the system." Tansley's concept had been expressed earlier in 1913 by the Oxford geographer A. J. Herbertson, who suggested the term "macroorganism" for such a combined biotic and abiotic entity. He was, however, too far in advance of his time and the idea was not taken up by ecologists. On the other hand Tansley's concept—elaborated in terms of the transfer of energy and matter across ecosystem boundaries–was utilized within the next few years by Evelyn Hutchinson, Raymond Lindeman, and the Odum brothers, Eugene and Howard.

The boundaries of an ecosystem can be somewhat arbitrary, reflecting the interest of a particular ecologist in studying a certain portion of the landscape. However, such a choice may often represent a recognizable landscape unit such as a woodlot, a wetland, a stream or lake, or—in the most logical case—a watershed within a sealed geological basin, whose exchanges with the atmosphere and outputs via stream flow can be measured quite precisely. Inputs and outputs imply an open system , which is true of all but the planetary or global ecosystem, open to energy flow but effectively closed in terms of materials except in the case of large-scale asteroid impact.

Ecosystems exhibit a great deal of structure, as may be seen in the vertical partitioning of a forest into tree, shrub, herb, and moss layers, underlain by a series of distinctive soil horizons. Horizontal structure is often visible as a mosaic of patches, as in forests with gaps where trees have died and herbs and shrubs now flourish, or in bogs with hummocks and hollows supporting different kinds of plants. Often the horizontal structure is distinctly zoned, for instance around the shallow margin of a lake; and sometimes it is beautifully patterned, as in the vast peatlands of North America that reflect a very complicated hydrology .

Ecosystems exhibit an interesting functional organization in their processing of energy and matter. Green plants, the primary producers of organic matter, are consumed by herbivores, which in turn are eaten by carnivores that may in turn be the prey of other carnivores. Moreover, all these animals may have parasites as another set of consumers. Such sequences of producers and successive consumers constitute a food chain, which is always part of a complicated, inter-linked food web along which energy and materials pass. At each step along the food chain some of the energy is egested or passed through the organisms as feces. Much more is used for metabolic processes and—in the case of animals—for seeking food or escaping predators; such energy is released as heat. As a consequence only a small fraction (often of the order of 10%) of the energy captured at a given step in the food chain is passed along to the next step.

There are two main types of food chains. One is made up of plant producers and animal consumers of living organisms, which constitute a grazing food chain. The other consists of organisms that break down and metabolize dead organic matter, such as earthworms, fungi , and bacteria. These constitute the detritus food chain. Humans rely chiefly on grazing food chains based on grasslands , whereas in a forest it is usual for more than 90% of the energy trapped by photosynthesis to pass along the detritus food chain.

Whereas energy flows one way through ecosystems and is dispersed finally to the atmosphere as heat, materials are partially and often largely recycled. For example, nitrogen in rain and snow may be taken up from the soil by roots, built into leaf protein that falls with the leaves in autumn, there to be broken down by soil microbes to ammonia and nitrate and taken up once again by roots. A given molecule of nitrogen may go through this nutrient cycle again and again before finally leaving the system in stream outflow. Other nutrients, and toxins such as lead and mercury , follow the same pathway, each with a different residence time in the forest ecosystem.

Mature ecosystems exhibit a substantial degree of stability , or dynamic equilibrium, as the endpoint of what is often a rather orderly succession of species determined by the nature of the habitat . Sometimes this successional process is a result of the differing life spans of the colonizing species, at other times it comes about because the colonizing species alter the habitat in ways that are more favorable to their competitors, as in an acid moss bog that succeeds a circumneutral sedge fen that has in its turn colonized a pond as a floating mat. Equilibrium may sometimes be represented on a large scale by a relatively stable mosaic of small-scale patches in various stages of succession, for instance in fire-dominated pine forests. On the millennial time scale, of course, ecosystems are not stable, changing very gradually owing to immigration and emigration of species and to evolutionary changes in the species themselves.

The structure, function, and development of ecosystems are controlled by a series of partially independent environmental factors: climate , soil parent material, topography , the plants and animals available to colonize a given site, and disturbances such as fire and windthrow. Each factor is, of course, divisible into a variety of components, as in the case of temperature and precipitation under the general heading of climate.

There are many ways to study ecosystems. Evelyn Hutchinson divided them into two main categories, holistic and meristic. The former treats an ecosystem as a "black box" and examines inputs, storages, and outputs, for example in the construction of a lake's heat budget or a watershed's chemical budget. This is the physicist's or engineer's approach to how ecosystems work. The meristic point of view emphasizes analysis of the different parts of the system and how they fit together in their structure and function, for example the various zones of a wetland or a soil profile , or the diverse components of food webs. This is the biologist's approach to how ecosystems work.

Ecosystem studies can also be viewed as a series of elements. The first is, necessarily, a description of the system, its location, boundaries, plant and animal communities, environmental characteristics, etc. Description may be followed by any or all of a series of additional elements, including: 1) a study of how a given ecosystem compares with others locally, regionally, or globally; 2) how it functions in terms of hydrology, productivity, and biogeochemical cycling of nutrients and toxins; 3) how it has changed over time; and 4) how various environmental factors have controlled its structure, function, and development. Such studies involve empirical observations about relationships within and among ecosystems, experiments to test the causality of such relationships, and model-building to assist in forecasting what may happen in the future.

The ultimate in ecosystem studies is a consideration of the structure, function, and development of the global or planetary ecosystem, with a view to understanding and mitigating the deleterious impacts upon it of current human activities.

See also Biotic community

[Eville Gorham Ph.D. ]

RESOURCES

BOOKS

Hagen, J. B. An Entangled Bank, the Origins of Ecosystem Ecology. New Brunswick, NJ: Rutgers University Press, 1992.

PERIODICALS

Herbertson, A. J. "The Higher Units: A Geological Essay." Scientia 14 (1913): 199-212.

Tansley, A. G. "The Use and Abuse of Vegetational Concepts and Terms." Ecology 16 (1935): 284-307.