The food chain concept allows ecologists to interconnect the organisms living in an ecosystem and to trace mathematically the flow of energy from plants through animals to decomposers.
The food chain concept provides the basic framework for production biology and has major implications for agriculture, wildlife biology, and calculating the maximum amount of life that can be supported on the earth. As early as 1789, naturalists such as Gilbert White described the many sequences of animals eating plants and themselves being eaten by other animals.
However, the use of the term “food chain” dates from 1927, when Charles Elton described the implications of the food chain and food web concept in a clear manner. His solid exposition advanced the study of two important biological concepts: the complex organization and interrelatedness of nature, and energy flow through ecosystems.
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Food Chains in Ecosystem Description
Stephen Alfred Forbes, founder of the Illinois Natural History Survey, contended in 1887 that a lake comprises a system in which no organism or process can be understood unless its relationship to all the parts is understood.
Forty years later, Elton’s food chains provided an accurate way to diagram these relationships. Because most organisms feed on several food items, food chains were cross-linked into complex webs with predictive power. For instance, algae in a lake might support an insect that in turn was food for bluegill.
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| Food chain illustration |
If unfavorable conditions eliminated this algae, the insect might also disappear. However, the bluegill, which fed on a wider range of insects, would survive because the loss of this algae merely increases the pressure on the other food sources.
This detailed linkage of food chains advanced agriculture and wildlife management and gave scientists a solid overview of living systems. When Arthur George Tansley penned the term ecosystem in the 1930’s, it was food-chain relationships that described much of the equilibrium of the ecosystem.
Today most people still think of food chains as the basis for the “balance of nature.” This phrase dates from the controversial 1960 work of Nelson G. Hairston, Frederick E. Smith, and Lawrence B. Slobodkin.
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They proposed that if only grazers and plants are present, grazing limits the plants. With predators present, however, grazers are limited by predation, and the plants are free to grow to the limits of the nutrients available. Such explanations of the “balance of nature” were commonly taught in biology books throughout the 1960’s and 1970’s.
Food Chains in Production Biology
Elton’s explanation of food chains came just one year after Nelson Transeau of Ohio State University presented his calculations on the efficiency with which corn plants converted sunlight into plant tissue. Ecologists traced this flow of stored chemical energy up the food chain to herbivores that ate plants and on to carnivores that ate herbivores.
Food chains therefore under girded the new “production biology” that placed all organisms at various trophic levels and calculated the extent towhich energy was lost or preserved as it passed up the food chain.
With data accumulating from many ecologists, Elton extended food chains into a pyramid of numbers. The food pyramid, in which much plant tissue supports some herbivores that are in turn eaten by fewer carnivores, is still referred to as an Eltonian pyramid.
In 1939 August Thienemann added decomposers to reduce unconsumed tissues and return the nutrients of all levels back to the plants. Early pyramids were based on the amount of living tissues, or biomass.
Calculations based on the amount of chemical energy at each level, as measured by the heat released when food is burned (calories), provided even more accurate budgets.
Because so much energy is lost at each stage in a food chain, it became obvious that this inefficiency was the reason food chains are rarely more than five or six links long and why large, fierce animals are uncommon.
It also became evident that because the earth intercepts a limited amount of sunlight energy per year, there is a limit on the amount of plant life—and ultimately upon the amount of animal life and decomposers—that can be fed. Food chains are also important in the accounting of carbon, nitrogen, and water cycling.
Value of Food Chains in Environmental Science
Unlike calories, which are dramatically reduced at each step in a food chain, some toxic substances become more concentrated as the molecules are passed along. The concentration of molecules along the food chain was first noticed by the Atomic Energy Commission,which found that radioactive iodine and strontium released in the Columbia River were concentrated in tissue of birds and fish.
However, the pesticide DDT provided the most notorious example of biological magnification: DDT was found to be deposited in animal body fat in ever increasing concentrations as it moved up the food chain to ospreys, pelicans, and peregrine falcons. High levels of DDT in these birds broke down steroid hormones and interfered with eggshell formation.
Because humans are omnivores, able to feed at several levels on the food chain (that is, both plants and other animals), it has been suggested that a higher world population could be supported by humans moving down the food chain and becoming vegetarians.
A problem with this argument is that much grazing land worldwide is unfit for cultivation, and therefore the complete cessation of pig or cattle farming does not necessarily free up substantial land to grow crops.
While the food chain and food web concepts are convenient theoretical ways to summarize feeding interactions among organisms, real field situations have proved far more complex and difficult to measure.
Animals often switch diet between larval and adult stages, and they are often able to shift food sources widely. It is often difficult to draw the boundaries of food chains and food webs.
