Food Chains and Food Webs

Energy Flow Pyramids

Energy Capture And Plant Production

Photosynthesis

The interactions within ecosystem can be analyzed in terms of energy flows and materials cycle. Solar energy enters and cascades through the ecosystem; it is partittioned among physical processes such as longwave radiation exchanges and evapotranspiration, and among biological processes such as photosynthesis and the production of chemical energy. Nutrients and other materials are continually recycled within the system.

Ecologists view the flow of energy in an ecosystem as a pyramid composed of several trophic levels, in which energy is stored in an organism that serves as food for an organism at a higher trophic level. Each successively higher level supports a smaller number and mass of organism. There are various types of trophic levels.

At the base of the pyramid are the primary producers consisting principally of green plants that can convert solar energy to organic energy in the form of living tissue. This in turn is usable as an energy source by other organisms, none of which can manufacture their own food.

Certain bacteria are also primary producers, having the ability to use the chemical bonds of rock and soil minerals as energy sources. All other organisms are dependent directly or indirectly upon the primary producers.

These other organisms are either consumers, which ingest live plant material or the prey that they or others have killed, or decomposers, such as bacteria, molds, and funji, which make use of the energy stored in already dead plant and animal tissues,

The animals that injest plants are called herbivores, and these animals may in turn be ingested by other animals, called carnivores. Some animals, such as bears, blue jays, and humans, both herbivorous and carnivorous.

Food Chains and Food Webs

A food chain is a sequence of consumption and represents energy transfer through the environment. There are two basic types of food chains. The grazing food chain begins with green plants and goes on to herbivores, and then to carnivores. A grazing food chain can be symbolized as a linear relationship: Plant -> herbivore->carnivore-> second carnivore-> top carnivore.

The decay food chain begins with dead organic matter and goes on to microorganism, and then to their predators, bacteria ad fungi. Thes decomposers live within and on orgainc materials, especially dead tissues, breaking them down, and returning minerals to the soil. Ordinarily, much less than half of the plant material on the continents are consumed directly by animals; the is recycled by decpmposers.

Some food chains involve onley a few links. In an agricultural ecosystem, for example, cattle eat grass grain, and other agricultural edibles, and are in turn eaten by human carnivores. However, most feeding relationships in nature do not take the form of simple, isolated chains. Many food chains are interconnected, forming complex food webs.

For this reason, the tracing of feeding relationship is not a simple task. In midlatitude forest, there are numerous species of herbivores, each of which may feed on several plant species. Carnivores in the forest may also have a varied diet, feeding on herbivores and other carnivores.

One way to analyze a food chain is to take samples of all relevant animal species in the ecosystem, and examin the contents of their digestive tracts. An owl's recent diet will be revealed by the bits of bone and hair in the pellet that the bird spits up every few days. The droppings of both herbivores and carnivores contain clear indications of their recent meals.

The complexity of the food web is apparent when we consider that a fwe acres of grassland may harbor several hundred different species of insects and a dozen or more different kind of vertebrate animals.

Energy Flow Pyramids

Laboratory, and field studies have indicated that at each trophic level there is a significant loss of useful energy. Only a fraction of the organic production of one level becomes available as food at the next level. Food chains in a natural ecosystem exibit approximately the same fractional transfer of useful energy as the laboratory ecosystems show. In a grazing food chain, i.e., only about ten percent of the energy absorbed at one level becomes available to be transferred to the next feeding level. This is because no organism can convert the energy in the food it eats into an equal amount of stored energy.

An adult person uses most of the energy obtained from food for body heat, motion, and work. A small amount of energy is stored for growth, and only the stored energy is available to a predator. Reason for the low efficiency of the energy transfers along food chains is that the chemical reactions required for life are always accompanied by the transformation of energy into forms of heat that can't be utilized. In addition, at each step in the grazing food chain, energy in the form of waste products are lost to decay food chains.

Consider a simple plant-herbivore-carnivore food chain consisting of grass plants, mice, and snakes. The mice obtain about ten percent of the energy absorbed earlier by the grass plants, and the snake obtain approximately ten percent of the energy absorbed by the mice. Thus the carnivore receives only about one percent of the energy originally absorbed by the plants.

The fraction of the original energy available to a succeeding carnivore stage-a hawk, for instance, is still less. The rapid decrease of available energy along a food chain limits such chains to four or five links. Large carnivores, such as lions, which are the last natural link in a food chain, obtain only a small fraction of the energy absorbed by the primary producers in their habtat. Lions must roam over large areas to obtain their food, and one reigon cannot support many of them.

Energy Capture And Plant Production

A closer study of the energy flow in an ecosystem by looking at the factors that determine how a primary producer captures and utilize energy. We shall consider green plants, which support most terrestrial ecosystems. Of the solar radiation energy falling on a plant leaf, a small amount is immediately reflected, and approximately 80 percent is absorbed.

Some of the absorbed energy functions to warm the leaf and is then given off as longwave radiation, while much of the absorbed energy is used in the evaporation and transpiration of water stored in the plant. Perhaps only about one (1%)percent of the solar radiation is used in the process of photosynthesis to produce the chemical energy the plant requires for growth, and maintenance.

The chemical processes involved in photosynthesis are complex. In terms of energy, however, photosynthesis may be thought of as a process in which simple molecules, water (H20) and carbon dioxide (C02), are joined with the aid of solar radiant energy to form more complex carbohydrate (sugar or starch) molecules (CH2O): H2O + CO2 + solar energy -> CH2O + O2

The solar radiant energy is stored as chemical energy in carbohydrates. Further chemical reactions use the carbohydrates and nutrients from the soil to produce complex protein molecules that the plant's cells require for growth. In addition to synthesizing carbohydrates, photosynthesis produces free oxygen gas. The formation of oxygen is incidental to photosynthesis, but it is important to the ecosystem as a whole. Without phoposynthesis, there would be very little free oxygen in the atmosphere, both human and animal life as we know it could not exist.

Photosynthesis and Plant Growth

A significant question for agriculture is: How much chemical energy, or carbohydrate, can a plant produce photosynthetically for a given amount of incident solar energy? In 1926 Edgar Transeau measured the carbohydrate-producing efficiency of a corn field. He recognized that all the carbon in a plant comes from the carbohydrate produced during photosynthesis, and by mesauring the amount of carbon in corn plants, he was able to estimate the amount of carbohydrate produced during the growing season.

The amount of energy required to produce one kilogram of carbohydrate by photosynthesis had already been determined in laboratory studies, so Transeau was able to conclude that only (1.6%) percent of the total solar energy incident on a field of corn during a 100-day growing season became stored chemical energy through photosynthesis. This percentage is the energy conversion efficiencies of more than a few percent.

Photosynthesis is dependent on only the visibel light portions of the total solar spectrum, and the rate of photosynthesis in a leaf varies with the intensity of the incoming light. If the intensity of light is increased, the rate of carbohydrate production will also increase, up to the maximum value for each plant species. Further increases of light intensity beyond this point will not result in increased photosynthesis.

Leaves that are partially shaded, or that receive only indirect light, are able to carry on photosynthesis near the maximum rate. Plants adapted to the tropics receive more solar energy, they generally have higher maximum rates of photosynthesis than plants native to midlatitude regions, where less solar energy is available. The rate of photosynthesis depends also on the temperature of the leaf, which may differ from the air temperature. For plants in midlatitude regions, photosynthesis for a given light intensity reaches a maximum at a leaf temperature of about 25degrees centigrade (77 degrees F).

As for arctic plants, maximum photosynthesis occures at lower leaf temperatures. The rate of carbohydrate production decreases above and below a plant's optimum leaf temperature, and production stops if leaf temperatures rise above 40 degrees C (104 degrees F) approximately. In the event that there is no wind to cool the leaves, leaf temperatures may rise so high that photosynthesis stops during the middle of the day, when solar energy input is maximum.

Under such condition, carbohydrate production is limited to a period in the morning and to a brief period in the late afternoon. It is partly for this reason that the warmest regions of the tropics (Dominica, and the rest of the Caribbean) tend to have lower agricultural yields than midlatitude regions.

A number of other factures also influence the rate of photosynthesis. Adequate supplies of water and carbon dioxide are necessary for efficient photosynthesis. The availability of nutrients from the soil, particularly nitrogen and phosphorus, affects the rate of carbohydrate production.

Nitrogen is required for the synthesis of the plant proteins necessary for cell growth. Phosphorus, a comparatively rare element in the earth's crust, is requied in plants as they make chemical compounds important in the photosynthetic process.

Phosphorus deficiency is often the limiting factor for plant growth in moist climatic regions, lakes, and coastal areas.
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