The idea of a food chain is common in everyday life, so much so that it has become a metaphor applied in many situations. A high achiever in business or other endeavors is said to be "at the top of the food chain," and images of big fish eating little fish abound in cartoons. Yet in the study of the biological sciences, the concept of food chains is part of the much larger idea of a food web. Whereas a food chain is a linear series of organisms dependent on each other for food, a food web is an interconnected set of food chains in the same ecosystem. Food webs make possible the transfer of energy from plants through herbivores to carnivores and omnivores, and ultimately to the detritivores and decomposers that enrich the soil with organic waste. Just as a food web can transfer materials essential to the life of organisms, it is also a devastatingly efficient conduit for the transfer of poisons.
An ecosystem is a community of independent organisms along with the inorganic components (chiefly soil, water, air, and rocks) that make up their environment. A biome is a large ecosystem, characterized by its dominant life-forms—for example, the Amazonian rain forest.
That portion of an ecosystem composed only of living things, as opposed to the formerly living or never living components, is known as a biological community. This community includes creatures from all five kingdoms of living organisms (including bacteria, algae, and fungi), whereas the term biota typically refers only to plant and animal life within a biological community or ecosystem. (For more on these subjects, see Ecosystems and Ecology and Biological Communities.)
The organisms in a biological community are linked in their need to obtain energy from food, which derives from the Sun through plant life. (There are, however, some communities, in areas such as deep-ocean rifts, that are not dependent on sunlight at all.) The Sun's energy is electromagnetic and travels in the form of radiation, which Earth receives as light and heat. Plants, known as primary producers, convert this electromagnetic energy into chemical energy through the process of photosynthesis.
The plants are eaten by herbivores (plant-eating animals), known also as primary consumers, examples of which include squirrels, rabbits, mice, deer, cows, horses, sheep, and seed-eating birds. These creatures, in turn, are eaten by secondary consumers, which are either carnivores, which are creatures that eat only meat, or omnivores—creatures, such as humans, that eat meat and plants.
There may even be tertiary, or third-level, consumers. These are animals that eat secondary consumers; examples are mountain lions and hawks, both of which eat such second-order consumers as snakes and owls. Human societies that eat dogs or cats, as well as those that engage in cannibalism, also behave as tertiary consumers. (See Biological Communities for a biological explanation of what otherwise is considered an abhorrent and immoral practice—not to mention a dangerous one, due to the risk of such diseases as kuru, a type of spongiform encephalopathy.) In any case, the further along in the chain of trophic levels or stages of the food web, the fewer consumers there are.
It is fairly obvious that when a creature is "higher on the food chain" (to use the common expression), it has fewer natural predators. The reason for this is that at each successive trophic level, there are simply fewer organisms; this, in turn, is due to the fact that the energy available to each level is progressively smaller, and the organisms themselves progressively larger. This, in turn, stems from one of the most intriguing, maddening concepts in the entire universe: the second law of thermodynamics, which we discuss shortly.
Because of the diminishing number of organisms at each trophic level, the food web often is depicted as a pyramid, a concept we explore further later in this essay. The number of organisms begins to increase again at the next trophic level beyond secondary or tertiary consumers, that of decomposers. Large omnivores and carnivores may not be prey for other creatures in life, but everything dies eventually, and anything that has ever lived is food for detritivores, or organisms that feed on waste matter.
Detritivores, which range in size and complexity from maggots to vultures, may not be the most appealing creatures on Earth, but without them life itself would suffer. By consuming the remains of formerly living things, they break organic material down into inorganic substances. In other words, their internal systems chemically process compounds containing the element carbon in characteristic structures. They then release that carbon into the atmosphere and soil in such a way that what remains is inorganic material that enriches the soil for the growth of new plant life.
But detritivores are not the last stop on the food web. The final trophic level, before the cycle comes back around to plants, contains the largest number of organisms in the entire food web—perhaps billions and billions, even in a space smaller than a coffee cup. These are decomposers, or organisms that, as with detritivores, obtain their energy from the chemical breakdown of dead organisms. The decomposers, however, break down the nutrients in decayed organic matter to a far greater extent than do detritivores.
Typically, decomposers are microorganisms, including bacteria and fungi, and they process materials in such a way that complex compounds undergo the chemical reaction of decomposition. Through decomposition, compounds are broken down into simpler forms, or even into their constituent elements, which provide the environment with nutrients necessary to the growth of more plant life.
The organisms in the food web can be viewed in three groups: producers (plants), consumers (primary-and secondary-consuming animals, whether herbivores, carnivores, or omnivores), and decomposers (that is, both detritivores and true decomposers). Producers and consumers are part of a larger structure known as the grazing food web, in which food is "on its way up the food chain," as it were. Decomposers and detritivores make up the decomposer food web, which brings food back "down" to the soil.
Producers also are called autotrophs, from Greek roots meaning "self-feeders," because they are not dependent on other organisms as a source of energy. Beyond the level of the primary producers, all consumers are known as heterotrophs, or "other-feeders." These creatures feed on other organisms to obtain their energy and are classified according to the types of food they eat—herbivores, carnivores, omnivores, as we already have discussed. Detritivores and decomposers also are considered heterotrophs.
Rather than depending on other organisms for energy, autotrophs obtain energy from the Sun and carbon dioxide from the atmosphere. From these components, they build the large organic molecules that they need to survive. Green plants do this through the process of photosynthesis, a chemical reaction that can be represented as follows:
solar energy + carbon dioxide (CO2) + water (H2O) → glucose (sugar: C6H12O6) + oxygen (O2)
Actually, in order to produce what chemists call a balanced equation, it would be necessary to show this equation as a reaction between the energy and six molecules each of carbon dioxide and water, which would produce a glucose molecule and six oxygen molecules. In any case, what we have described here is an amazing thing and one of the great wonders of nature. Sunlight aids plants in converting carbon dioxide, which they receive from the respiration of animals, along with water (which also may come from animal respiration, though this is not necessarily the case), into a sugar molecule for the plant's sustenance. Furthermore, oxygen, essential to the life of virtually all animals, also is produced—yet from the standpoint of the plant, it is simply a waste by-product!
The productivity of plants, which is measured in terms of biomass (the combined mass of all organisms at a particular trophic level in a food web), determines the amount of "fixed," or usable, energy available to other trophic levels on the food web. The amount of energy available always will be less for each successive trophic level, through the point where consumers end and decomposers begin—that is, through the level of the secondary or perhaps tertiary consumer.
If there is any scientific equivalent of the curse in the Garden of Eden (the punishment for the sins of Adam and Eve, according to Judeo-Christian belief), it is the second law of thermodynamics. Just as the expulsion from Paradise in the biblical story ensured that life would be much more difficult for humans than it would have been in Eden, so the second law thwarts all ambitions toward transcending the limits of physical reality.
The first law of thermodynamics states that it is impossible to obtain more energy from a system than is put into it. Thus, for instance, a car will go only as far as is allowed by the amount of energy that is pumped into its tank. The first law, discovered in the mid-nineteenth century, effectively ruled out any hopes of a perpetual-motion machine, but the second law, derived a few decades later, delivered even worse news.
Though it can be stated in a number of ways, the second law essentially means that it is impossible to extract as much energy from a system as one puts into it. Thus, in the case of an automobile, most of the energy contained in the gas does not go toward moving the car; rather, it is dissipated in the form of heat and sound, as a natural by-product of operating the engine. Even without running an air conditioner or other energy-consuming device, only about 30% of the energy from the gas goes to turning the wheels.
What this means for the food web is that there is bound to be a loss of energy in the transfer from one trophic level to another. Organisms never manage to retrieve 100% of the energy from the materials they eat; in fact, the figure is more like 10%. A rabbit that eats a carrot gets only about 10% of the energy in it, and an owl that eats the rabbit gets only about 10% of the energy from the rabbit, or 1% of the energy in the carrot. Because of these diminishing returns, there are always fewer organisms at each successive trophic level on the grazing food web. This fact is expressed in a model known as the ecological pyramid, or energy pyramid, which shows that as the amount of total energy decreases with each trophic level, so does the biomass. As a result, it may take 1,000 carrots to support 100 rabbits, 10 owls, and one hawk.
The picture changes as the shift is made from the grazing web to the decomposer web. Detritivores and decomposers are extraordinarily efficient feeders, reworking detritus over and over and extracting more fixed energy as they do. Eventually, they break the waste down into simple inorganic chemicals, which, as we have noted, then may be reused by the primary producers. The number of organisms in the decomposing food web dwarfs that of all others combined, though decomposers themselves are very small, and their combined population takes up very little physical space.
The keystone in an archway is a wedge-shaped stone at the top of the arch. It's position is extraordinarily important: if the keystone is removed, the arch will collapse. Thus, the key-stone has become an often-used metaphor in other circumstances, as, for instance, in the nickname of Pennsylvania, the "Keystone State." In the realm of ecology, the term keystone species refers to those organisms that, like a keystone in
Within a food web, for instance, a keystonespecies can have a powerful influence, one that is far out of proportion to its relative biomass orproductivity. Typically, a keystone species is a toppredator (that is, a large secondary or even tertiary consumer), though occasionally an herbivore can occupy the keystone position. Often, the role of the keystone species becomes apparent only once it is removed, either experimentally or by natural forces, from an ecosystem.
In temperate ecosystems on the west coast of North America, for instance, removal of a certain species of starfish (Pisaster ochraceous) was found to cause a rapid growth in the numbers and biomass of the mussel Mytilus californianus. The latter then forced out other species and proceeded to dominate the biological community. As it turned out, the starfish acted as a keystone predator by consuming these mussels.
Specifically, the starfish prevented the mussel from gaining dominance that it otherwise would have gained, owing to its competitive superiority in relation to other species within this particular coastal ecosystem. Yet the starfish could not eliminate the mussel, because it was incapable of feeding on larger individuals of that species. The result was that the community enjoyed a much greater degree of diversity and complexity than it would have if the mussel had been allowed to dominate.
Another keystone species in a geographic area close to that of the starfish we have just described is the sea otter, native to western North America. Its principal food source is the sea urchin, an herbivore that, in turn, survives by consuming kelp, a large form of algae. By controlling the numbers and densities of sea urchins, sea otters allowed kelp to retain a relatively large biomass within the community, thus facilitating the growth of "kelp forests."
When humans began hunting sea otters for their fur during the late eighteenth and into the nineteenth centuries, however, the ecological effect soon was felt in the form of declining kelp forests. Fortunately, hunting did not render the species extinct, and since the 1930s, sea otters have been colonizing many of their former habitats. This colonization has resulted in a corresponding increase in the density of surrounding kelp forests.
A concept similar to that of the keystone species is the idea of an indicator species: plants or animals that, by their presence, abundance, or chemical composition, demonstrate a distinctive aspect of the character or quality of the environment. For instance, in an ecosystem affected by pollution, examination of indicator plant species may reveal the pollution patterns. By their presence, indicator species also may serve to show the quality or integrity of an ecosystem. Such is the case, for instance, with the spotted owl, or Strix occidentalis, and other species that depend on old-growth forests. (See Succession and Climax for more on this subject.) Because the needs of these species are so particular, their presence or absence can illustrate the health or lack thereof of the biome in question.
Other indicator plants also can be used to determine the presence of valuable mineral deposits in the soil, because those minerals make their way into the tissues of the plants themselves. Nickel concentrations as great as 10% have been found in the tissues of Russian plants from the mustard family, and a mintlike species called Becium homblei has proved useful for locating copper deposits in parts of Africa. Since the plant can tolerate more than 7% copper in soil (a great amount and many times the percentage of copper in the human body, for instance), it can and does live near enormous copper deposits.
Some plants can serve as indicators of serpentine minerals, varieties of compounds that can be toxic in large concentrations. In California, for instance, where serpentine soils are not uncommon, there plant species unique to specific ecosystems high in serpentine mineral content. Elsewhere, there are types of lichens that are sensitive to toxic gases, such as sulfur dioxide, and thus these lichens can be monitored as a way of keeping tabs on air pollution.
In semiarid regions where soils contain large quantities of the element selenium, plants can accumulate such large concentrations of the element that they become poisonous to primary consumers (for example, rabbits) who eat them. The result may be temporary or even permanent blindness. In such situations, legumes of the genus Astragalus, which can accumulate as much as 15,000 ppm (parts per million)—a comparatively enormous concentration—serve as indicators. Their heavy selenium concentration gives them a noticeably unpleasant smell.
Aquatic invertebrates and fish often have been surveyed for what they can show as to the quality of water and the health of aquatic ecosystems.
The subject of indicator species leads us, naturally enough, to the topic of pollution in the food web, which can be seen on a small scale in the phenomenon of bioaccumulation and which manifests on a much broader scale as biomagnification. One of the key concepts in ecological studies is the idea that a disturbance in one area can lead to serious consequences elsewhere. The interconnectedness of components in the environment thus makes it impossible for any event or phenomenon to be truly isolated.
Nowhere is this principle better illustrated that in the processes of bioaccumulation and biomagnification. The first of these is the buildup of toxins, and particularly chemical pollutants, in the tissues of individual organisms. Part of the danger in these toxins is the fact that the organism cannot easily process them either by metabolizing them (i.e., incorporating them into the metabolic system, as one does food or water) or by excreting them through urine or other substances produced by the body.
The only way for the organism to release toxins, in fact, is by passing them on to other members of the food web. Because organisms at each successive trophic level must consume more biomass to meet their energy requirements, they experience an increase in contamination, a phenomenon known as biomagnification. The following example illustrates how toxins enter the food web and gradually make their way down the line, growing in proportion as they do.
Particles of pollutant may stick to algae, for instance, which are so small that the toxin does little damage at this level of the food web. But even a small herbivore, such as a zooplankton, takes in larger quantities of the pollutant when it consumes the algae, and so begins the cycle of biomagnification. By the time the toxin has passed from a zooplankton to a small fish, the amount of pollutant in a single organism might be 100 times what it was at the level of the algae. The reason, again, is that the fish can consume 10 zooplankton that each has consumed 10 algae.
These particular numbers, of course, are used simply for the sake of convenience, as were those cited earlier in relation to the ecological pyramid. Note, incidentally, the similarity of the relationships between this "pyramid of poison" and the ecological pyramid, whereby energy, which is beneficial, passes between trophic levels. The higher the trophic level, the smaller the amounts of energy that the organism extracts from its food—but the higher the amount of toxic content. By the time the toxins have passed on to a few more levels in the food web, they might be appearing in concentrations as great as 10,000 times their original amount.
Among the most prominent examples of chemical pollutants that are bioaccumulated are such pesticides as DDT (dichlorodiphenyltrichloroethane). DDT is an insecticide of the hydrocarbon family, a large group of chemical compounds of which the many varieties of petroleum are examples. Because it is based in hydrocarbons, DDT is hydrophobic ("water-fearing") and instead mixes with oils—including the fat of organisms.
In the twenty or so years leading up to 1972, Americans used vast amounts of DDT for the purpose of controlling mosquitoes and other pests. DDT appeared to be a remarkably successful killer, and in fact it turned out to be a little too successful. As it found its way into water sources, DDT entered the bodies of fish, and then those of predatory birds such as osprey, peregrine falcons, brown pelicans, and even the bald eagle.
The detrimental effect of DDT on America's national symbol, a bird protected by law since 1940, aptly illustrates the ravages exacted by this powerful insecticide. DDT levels in birds became so high that the birds' eggshells were abnormally thin, and adult birds sitting on the nest would accidentally break the shells of unhatched chicks. As a result, baby birds died, and populations of these species dwindled. Environmentalists in the late 1960s and early 1970s raised public awareness of this phenomenon, and this led to the banning of DDT spraying in 1972. The period since that time has seen dramatic increases in bird populations.
Because the species of bird affected were not ones that people normally consume for food, DDT biomagnification did not have a wide-ranging effect on human populations. However, tests showed that some DDT had made its way into the fat deposits of some members of the population. In any case, bioaccumulation and biomagnification have threatened humans, for instance in the late 1940s and early 1950s, when cows fed on grass that had been exposed to nuclear radiation, and this radioactive material found its way into milk.
Traces of the radioactive isotope strontium-90, a by-product of nuclear weapons testing in the atmosphere from the late 1940s onward, fell to earth in a fine powder that coated the grass. Later the cows ate the grass, and strontium-90 wound up in the milk they produced. Because of its similarities to calcium, the isotope became incorporated into the teeth and gums of children who drank the milk, posing health concerns that helped bring an end to atmospheric testing in the early 1960s.
Humans themselves can serve as repositories for contaminants, a particular concern for a mother nursing her baby. Assuming the mother's own system has been contaminated by toxins, it is likely that her milk contains traces of the harmful chemical, which will be passed on to her child. Obviously, this is a very serious matter. Nursing mothers, babies, and their loved ones are not the only people affected by bioaccumulation and the storing of toxins in fatty tissues. In fact, some physicians and nutritionists maintain that one of the reasons for the buildup of fat on a person's body (though certainly not the only reason) may be as a response to toxins, the idea being that the body produces fat cells as a means of keeping the toxins away from the bloodstream.
Another case of large-scale bioaccumulation occurred during the 1970s and 1980s, when fish such as tuna were found to contain bioaccumulated levels of mercury. In the face of such concerns, governments have intervened in several ways, including the banning of DDT spraying, as mentioned earlier. The U.S. federal government and the governments of some states have issued warnings against the consumption of certain types of fish, owing to bioaccumulated levels of toxic pollutants. Bioaccumulation is particularly serious in the case of species that live a long time, because a longer life allows for much longer periods of bioaccumulation. For this reason, some governments warn against consuming fish over a certain age or size: the older and larger the creature, the more contaminated it is likely to be.
A to Z of Food Chains and Webs (Web site). <http://www.education.leeds.ac.uk/~edu/technology/epb97/forest/azfoodcw.htm>.
Bioaccumulation and Biomagnification (Web site). <http://www.marietta.edu/~biol/102/2bioma95.html>.
Busch, Phyllis S. Dining on a Sunbeam: Food Chains and Food Webs. Photos Les Line. New York: Four Winds Press, 1973.
Extension Toxicology Network (EXTOXNET): Toxicology Information Briefs (Web site). <http://ace.orst.edu/info/extoxnet/tibs/bioaccum.htm>.
Food Chains and Food Webs: An Introduction (Web site). <http://www.si.edu/sites/educate/troprain/foodchai.htm>.
Food Webs: Build Your Own (Web site). <http://www.gould.edu.au/foodwebs/kids_web.htm>.
Fox, Nicols. Spoiled: The Dangerous Truth About a Food Chain Gone Haywire. New York: Basic Books, 1997.
Introduction to Biogeography and Ecology: Trophic Pyramids and Food Webs. Fundamentals of Physical Geography (Web site). <www.geog.ouc.bc.ca/physgeog/contents/9o.html>.
Pimm, Stuart L. Food Webs. New York: Chapman and Hall, 1982.
Wallace, Holly. Food Chains and Webs. Chicago: Heinemann Library, 2001.
Primary producers. Autotroph means "self-feeder," and these organisms are distinguished by the fact that they do not depend on other organisms as a source of energy. Instead, they obtain energy from the Sun and carbondioxide from the atmosphere, and from these constituents they build the large organic molecules that they need to survive.
The buildup of toxic chemical pollutants in the fatty tissues of organisms.
The changes that particular elements undergo as they pass back and forth through the various earth systems (e.g., the biosphere) and particularly between living and non-living matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.
The living components of an ecosystem.
The increase in bioaccumulated contamination at higher levels of the food web. Biomagnification results from the fact that larger organisms consume larger quantities of food—and, hence, in the case of polluted materials, more toxins.
The combined mass of all organisms at a particular trophic level in a food web.
A large ecosystem, characterized by its dominant life-forms.
A combination of all living things on Earth—plants, mammals, birds, amphibians, reptiles, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.
A combination of all flora and fauna (plant and animal life, respectively) in a region.
A meat-eating organism, or an organism that eats only meat (as distinguished from an omnivore).
That portion of the food web occupied by detritivores and decomposers. (Compare with grazing food web.)
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.
A chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the biosphere, this often is achieved through the help of detritivores and decomposers.
Organisms that feed on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers;however, unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots.
The study of the relationships between organisms and their environments.
A community of interdependent organisms along with the inorganic components of their environment.
The flow of energy between organisms in a food web.
A law of physics stating that the amount of energy in a system remains constant, and therefore it is impossible to perform work that results in an energy output greater than the energy input.
A series of singular organisms in which each plant or animal depends on the organism that precedes it. Food chains rarely exist in nature; therefore, scientists prefer the term food web.
A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these organisms consumes nutrients and passes them along to other organisms (or, in the case of the decomposer food web, to the soil and environment). The food web maybe thought of as a bundle or network of food chains, but since the latter rarely existseparately, scientists prefer the concept of a food web to that of a food chain.
That portion of the food web occupied by autotrophs, herbivores, carnivores, and omnivores.(Compare with decomposer food web.)
A plant-eating organism.
Secondary consumers, or "other-feeders." These creatures feed on other organisms to obtain their energy and are classified according to the types of food they eat. Thus, they are known as herbivores, carnivores, and so on.
A plant or animal that, by its presence, abundance, or chemical composition, demonstrates a particular aspect of the character or quality of the environment.
A species that plays a crucial role in the functioning of its ecosystem or that has a disproportionate influence on the structure of its ecosystem.
A term referring to the role that a particular organism plays within its biological community.
An organism that eats both plants and other animals.
At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.
The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants.
Animalsthat eat green plants. (Compare with sec on dary consumers.)
Green plants that depend on photosynthesis for their nourishment.
Animals that eat other animals.
A law of physics, which can be stated in several ways, all of which mean the same thing. According to one version of the second law, it is impossible to transfer energy with perfect efficiency, because some energy always will be lost in the transfer. This is the same as saying that it is impossible to extract from a system the same amount of energy that was put into it.
Various stages within a food web. For instance, plants are on one trophic level, herbivores on another, and so on.