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Food Chain/Web

Food chain/web


Food chains and food webs are methods of describing an ecosystem by describing how energy flows from one species to another.

First proposed by the English zoologist Charles Elton in 1927, food chains and food webs describe the successive transfer of energy from plants to the animals that eat them, and to the animals that eat those animals, and so on. A food chain is a model for this process which assumes that the transfer of energy within the community is relatively simple. A food chain in a grassland ecosystem, for example, might be: Insects eat grass, and mice eat insects, and fox eat mice. But such an outline is not exactly accurate, and many more species of plants and animals are actually involved in the transfer of energy. Rodents often feed on both plants and insects, and some animals, such as predatory birds, feed on several kinds of rodents. This more complex description of the way energy flows through an ecosystem is called a food web. Food webs can be thought of as interconnected or intersecting food chains.

The components of food chains and food webs are producers, consumers, and decomposers . Plants and chemosynthetic bacteria are producers. They are also called primary producers or autotrophs ("self-nourishing") because they produce organic compounds from inorganic chemicals and outside sources of energy. The groups that eat these plants are called primary consumers or herbivores. They have adaptations that allow them to live on a purely vegetative diet which is high in cellulose. They usually have teeth modified for chewing and grinding; ruminants such as deer and cattle have well-developed stomachs, and lagomorphs such as rabbits have caeca which aid their digestion. Animals that eat herbivores are called secondary consumers or primary carnivores, and predators that eat these animals are called tertiary consumers. Decomposers are the final link in the energy flow . They feed on dead organic matter, releasing nutrients back into the ecosystem. Animals that eat dead plant and animal matter are called scavengers, and plants that do the same are known as saprophytes.

The components of food chains and food webs exist at different stages in the transfer of energy through an ecosystem. The position of every group of organisms obtaining their food in the same manner is known as a trophic level . The term comes from a Greek word meaning "nursing," and the implication is that each stage nourishes the next. The first tropic level consists of autotrophs, the second herbivores, the third primary carnivores. At the final trophic level exists what is often called the "top predator." Organisms in the same trophic level are not necessarily connected taxonomically; they are connected ecologically, by the fact they obtain their energy in the same way. Their trophic level is determined by how many steps it is above the primary producer level. Most organisms occupy only one trophic level; however some may occupy two. Insectivorous plants like the venus flytrap are both primary producers and carnivores. Horseflies are another example: the females bite and draw blood, while the males are strictly herbivores.

In 1942, Raymond Lindeman published a paper entitled "The Tropic-Dynamic Aspect of Ecology." Although a young man and only recently graduated from Yale University, he revolutionized ecological thinking by describing ecosystems in the terminology of energy transformation. He used data from his studies of Cedar Bog Lake in Minnesota to construct the first energy budget for an entire ecosystem. He measured harvestable net production at three trophic levels, primary producer, herbivore, and carnivore. He did this by measuring gross production minus growth, reproduction, respiration , and excretion. He was able to calculate the assimilation efficiency at each tropic level, and the efficiency of energy transfers between each level. Lindeman's calculations are still widely regarded today, and his conclusions are usually generalized by saying that the ecological efficiency of energy transfers between trophic levels averages about 10%.

Lindeman's calculations and some basic laws about physics reveal important truths about food chains, food webs, and ecosystems in general. The First Law of Thermodynamics states that energy cannot be created or destroyed; energy input must equal energy output. The Second Law of Thermodynamics states that all physical processes proceed in such a way that the availability of the energy involved decreases. In other words, no transfer of energy is completely efficient. Using the generalized 10% figure from Lindeman's study, a hypothetical ecosystem with 1,000 kcal of energy available (net production) at the primary-producer level would mean that only 100 kcal would be available to the herbivores at the second trophic level, 10 kcal to the primary carnivores at the third level, and 1 kcal to the secondary carnivores at the fourth level. Thus, no matter how much energy is assimilated by the autotrophs at the first level of an ecosystem, the eventual number of trophic levels is limited by the laws which govern the transfer of energy. The number of links in most natural food chains is four.

The relationships between trophic levels has sometimes been compared to a pyramid, with a broad base which narrows to an apex. Trophic levels represent successively narrowing sections of the pyramid. These pyramids can be described in terms of the number of organisms at each trophic level. This was first proposed by Charles Elton, who observed that the number of plants usually exceeded the number of herbivores, which in turn exceeded the number of primary carnivores, and so on. Pyramids of number can be inverted, particularly at the base; an example of this would be the thousands of insects which mfeed on a single tree. The pyramid-like relationship between trophic levels can also be expressed in terms of the accumulated weight of all living matter, known as biomass . Although upper-level consumers tend to be large, the population of organisms at lower trophic levels are usually much higher, resulting in a larger combined biomass. Pyramids of biomass are not normally inverted, though they can be under certain conditions. In aquatic ecosystems, the biomass of the primary producers may be less than that of the primary consumers because of the rate at which they are being consumed; phytoplankton can be eaten so rapidly that the biomass of zooplankton and other herbivores are greater at any particular time. The relationship between trophic levels can also described in terms of energy, but pyramids of energy cannot be inverted. There will always be more energy at the bottom than there is at the top.

Humans are the top consumer in many ecosystems, and they exert strong and sometimes damaging pressures on food chains. For example, overfishing or overhunting can cause a large drop in the number of animals, resulting in changes in the food-web interrelationships. On the other hand, overprotection of some animals like deer or moose can be just as damaging. Another harmful influence is that of biomagnification . Toxic chemicals such as mercury and DDT released into the environment tend to become more concentrated as they travel up the food chain. Some ecologists have proposed that the stability of ecosystems is associated with the complexity of the internal structure of the food web and that ecosystems with a greater number of interconnections are more stable. Although more studies must be done to test this hypothesis, we do know that food chains in constant environments tend to have a greater number of species and more trophic links, whereas food chains in unstable environments have fewer species and trophic links.

[John Korstad and Douglas Smith ]

RESOURCES

BOOKS

Krebs, C. J. Ecology: The Experimental Analysis of Distribution and Abundance. 3rd ed. New York: Harper & Row, 1985.

PERIODICALS

Hairston, N. G., F. E. Smith, and L. B. Slobodkin. "Community Structure, Population Control, and Competition." American Naturalist 94 (1960): 421-25.

Lindeman, R. L. "The Trophic-Dynamic Aspect of Ecology." Ecology 23 (1942): 399-418.

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