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The Food Web and Other Vital Cycles
The energy necessary for all life processes reaches the earth in the form of sunlight. By photosynthesis green plants convert the light energy into chemical energy, and carbon dioxide and water are transformed into sugar and stored in the plant. Herbivorous animals acquire some of the stored energy by eating the plants; those animals in turn serve as food for, and so pass the energy to, predatory animals. Such sequences, called food chains, overlap at many points, forming so-called food webs. For example, insects are food for reptiles, which are food for hawks. But hawks also feed directly on insects and on other birds that feed on insects, while some reptiles prey on birds. Since a severe loss of the original energy occurs with each transfer from species to species, the ecologist views the food (energy) structure as a pyramid: Each level supports a smaller number and mass of organisms. Thus in a year's time it would take millions of plants weighing tons to feed the several steer weighing a few tons that could support one or two people. The ecological conclusion is that if human beings would eat more plants and fewer animals, food resources would stretch much further. Once the energy for life is spent, it cannot be replenished except by the further exposure of green plants to sunlight.
The chemical materials extracted from the environment and elaborated into living tissue by plants and animals are continually recycled within the ecosystem by such processes as photosynthesis, respiration, nitrogen fixation, and nitrification. These natural processes of withdrawing and returning materials are variously called the carbon cycle, the oxygen cycle, and the nitrogen cycle. Water is also cycled. Evaporation from lakes and oceans forms clouds; the clouds release rain that is taken up by the soil, absorbed by plants, and passed on to feeding animals—which also drink directly from pools and lakes that catch the rain. The water in plant and animal wastes and dead tissue then evaporates and can be recycled. Interference with these vital cycles by disturbance of the environment—for example, by pollution of the air and water—may disrupt the workings of the entire ecosystem. The cycles are facilitated when an ecosystem has a sufficient biological diversity of species to fill its so-called ecological niches, the different functional sites in the environment where organisms can act as producers of energy, consumers of energy, or decomposers of wastes. Such diversity tends to make a community stable and self-perpetuating.
A climax community is one that has reached the stable stage. When extensive and well defined, the climax community is called a biome. Examples are tundra, grassland, desert, and the deciduous, coniferous, and tropical rain forests. Stability is attained through a process known as succession, whereby relatively simple communities are replaced by those more complex. Thus, on a lakefront, grass may invade a build-up of sand. Humus formed by the grass then gives root to oaks and pines and lesser vegetation, which displaces the grass and forms a further altered humus. That soil eventually nourishes maple and beech trees, which gradually crowd out the pines and oaks and form a climax community. In addition to trees, each successive community harbors many other life forms, with the greatest diversity populating the climax community.
Similar ecological zonings occur among marine flora and fauna, dependent on such environmental factors as bottom composition, availability of light, and degree of salinity. In other respects, the capture by aquatic plants of solar energy and inorganic materials, as well as their transfer through food chains and cycling by means of microorganisms, parallels those processes on land.
The early 20th-century belief that the climax community could endure indefinitely is now rejected because climatic stability cannot be assumed over long periods of time. In addition nonclimatic factors, such as soil limitation, can influence the rate of development. It is clear that stable climax communities in most areas can coexist with human pressures on the ecosystem, such as deforestation, grazing, and urbanization. Polyclimax theories stress that plant development does not follow predictable outlines and that the evolution of ecosystems is subject to many variables.
See E. P. Odum, Fundamentals of Ecology (3d ed. 1971); R. L. Smith, ed., The Ecology of Man: An Ecosystem Approach (1971); P. A. Colinvaux, Introduction to Ecology (1973); R. M. Darnell, Ecology and Man (1973); T. C. Emmel, An Introduction to Ecology and Population Biology (1973); D. B. Sutton and N. P. Harman, Ecology: Selected Concepts (1973); K. E. F. Watt, Principles of Environmental Science (1973); D. Worster, Nature's Economy (1977); R. Brewer, The Science of Ecology (1988).
the interrelation of various species of plants, animals, fungi, and microorganisms that are connected to each other as food and food consumers. Each of the successive feeding links consists of organisms that consume those of the preceding link in the chain; the resulting transfer of energy and matter lies at the foundation of the cycle of matter in nature. As much as 80 or 90 percent of the potential energy is lost in each such transfer, being dispersed in the form of heat. Hence the number of links, or sets of species, in a food chain is usually no more than four or five.
At the base of every food chain are the producer species—the autotrophic organisms that synthesize organic matter. These are primarily green plants, consisting of water, inorganic salts, and carbon dioxide, that synthesize organic matter by assimilating the energy of sunlight; also included in this category is a group of bacteria, such as sulfur and hydrogen bacteria, that use the energy obtained from the oxidation of chemical compounds to synthesize organic matter.
The next link in a food chain consists of the consumer species—the heterotrophic organisms that consume organic matter. The primary consumers are herbivorous animals that feed on grass, seeds, fruit, the underground portion of plants (roots, tubers, and bulbs), and even—in the case of some insects—wood. Carnivorous animals are classified as secondary consumers and are subdivided into (1) those that subsist on great quantities of small prey and (2) active predators, which frequently attack prey larger than themselves. The great majority of these consumers subsist on a variety of foods, including a certain amount of plant food. Thus, for example, the size of the marten and sable populations depends not only on the plentifulness of small mammals and birds but also on fruit and seed crops, and particularly the available yield of pine nuts. At the same time, herbivorous animals also consume a certain amount of animal food, thereby obtaining essential and irreplaceable amino acids of animal origin.
Finally, the saprophytic organisms—chiefly fungi and bacteria —obtain their essential energy by decomposing dead organic matter.
In the case of animals that develop by metamorphosis, the larvae and the adult individuals require different types of food and occupy different positions in the food chain. The ecological niche of a given species (or of a given phase in its development) is determined by its position in the food chain and its relation to its partners—the organisms at higher and lower levels of the food chain. The various populations or age groups of a single species may be part of several different food chains, thus forming a more complex set of relationships.
Two major types of food chains are found in biocenoses, or biotic communities—the “pasture” and the “detritus” type. The pasture type of food chain begins with photosynthesizing green plants and usually forms the basis of the biocenosis. The detritus type begins with saprophytic organisms, which utilize the energy liberated by the decomposition of dead organic matter (fungi and many microorganisms). Together, these two types of food chains ensure the functioning of the three principal stages in the cycle of matter, as reflected in three trophic levels: the producers— plants—are at the first level; the consumers—primary consumers (herbivorous animals) and secondary consumers (carnivores)—at the second; and the reducers—the saprophages that break down organic matter—at the third.
The designation of trophic levels groups together types of activity rather than species; the population of one species may occupy one or more trophic levels, depending on its energy source. The flow of energy through a trophic level equals the total amount of energy assimilated at this level; in its turn, the total energy assimilated equals the total biomass produced plus respiration.
Biotic communities usually contain a number of parallel food chains—for example, the chain of herbaceous vegetation-rodents-small predators together with the chain of herbaceous vegetation-ungulates-large predators. While the inhabitants of different strata (soil, grass, and trees) often belong to parallel food chains, relationships may also be found to exist between them. The complex structure of food chains ensures both the integrity and the dynamic nature of the biotic community. Any reduction in the number of individuals in a species constituting a link in a food chain—whether due to human activity or other factors—inevitably brings injury to the integrity of the biotic community.
REFERENCESNaumov, N. P. Ekologiia zhivotnykh, 2nd ed. Moscow, 1963.
Odum, E. Osnovy ekologii. Moscow, 1975. (Translated from English.)
Williamson, M. Analiz biologicheskikh populiatsii. Moscow, 1975. (Translated from English.)
N. P. NAUMOV