Patterns of Flow of Energy through the Ecosystems | Essay

Essay on Patterns of Flow of Energy through the Ecosystems !

In an ecosystem one can observe the transfer or flow of energy from one trophic level to other in succession. A trophic level can be defined as the number of links by which it is separated from the producer, or as the nth position of the organism in the food chain.

The patterns of eating and being eaten forms a linear chain calked food chain which can always be traced back to the producers. Thus, primary producers trap radiant energy of sun and transfer that to chemical or potential energy of organic com­pounds such as carbohydrates, proteins and fats.

When an herbi­vore animal eats a plant (or when bacteria decompose it) and these organic compounds are oxidized, the energy liberated is just equal the amount of energy used in synthesizing the substances (first law of thermodynamics), but some of the energy is heat and not useful energy (second law of thermodynamics).

If this animal, in turn, is eaten by another one, along with transfer of energy from a herbivore to carnivore a further decrease in useful energy occurs as the second animal (carnivore) oxidizes the organic substances of the first (herbivore or omnivore) to liberate energy to synthesize its own cellular constituents.

Such transfer of energy from organism to organism sustains the ecosystem and when energy is transferred from individual to individual in a particular com­munity, as in a pond or a lake or a river, we come across the food chains. The number of steps in a food chain is always restricted to four or five, since the energy avai­lable decreases with each step.

In other words, the energy fixed in organisms such as in aquatic plants in a pond passes through the ecosystem in the form of plant → herbivore → carnivore, where only three steps are encountered in the chain.

But very often, the chains are very complicated with several steps being involved. For example, the food chain of shore ecosystem includes plant → herbivore → carnivore 1 → carnivore 2 → carnivore n. Basically two types of food chains are recognized—grazing food chain and detritus food chain.

Grazing Food Chain:

The grazing food chain starts from green plants and ends to carnivores by passing through herbivores. Thus, gross production (if a plant in an ecosystem may meet three fates-it may be oxidized in respiration, it may die and decay and it may be eaten by plant eating animals or herbivores.

In herbivores the assimilated food can be stored as carbohydrates, protein or fat, transformed into relatively simple substances or rebuilt by the animal into much more complex organic molecules.

The energy to perform these transformations is supplied by respiration. Like autotrophs, the ultimate disposition of energy in herbivores occurs by three routes: respiration, decay of organic matter by bacteria and other decom­poser organisms and consumption by carnivores.

Fig. 18.3. Diagrammatic representation of a grazing food chain, showing inputs and losses of energy at each trophic level trophic levels are numbered, and used as subscripts to letters indicating energy transfer. A-assimilation of food by the organisms at the trophic level; F-energy lost in the form of faeces and other excretory products; C-energy lost through decay; and R-energy lost to respiration (after Clapham, Jr., 1973).

The primary carnivores or secondary consumers eat herbivores or primary consumers of the ecosystems. And likewise, secondary carnivores or tertiary consumers eat primary carnivores. The total energy assimilated by primary carnivores, or gross tertiary produc­tion, is derived entirely from the tissues of herbivores, and its dis­position into respiration, decay and further consumption by other carnivores is entirely analogous with that of the herbivores.

It seems clear that much of the energy flow through grazing food chain can be described in terms of the categories or trophic levels, outlined as follows:

As a general model for the direction and extent of energy flow within communities, the food chain is very useful. It demonstrates the amount of energy found at any trophic level, the transfer of energy from one trophic level to the next, and the amount of energy lost from the grazing food chain, either through respiration or decay.

Further the producer herbivore carnivore chain is a pre­dator chain. Parasitic chains also exist wherein smaller organisms consume larger ones without outright killing as the case of the predators.

Detritus Food Chain:

The organic wastes, exudates and dead matter derived from the grazing food chain are generally termed detritus. The energy contained in this detritus is not lost to the ecosystem as a whole; rather it serves as the source of energy for a group of organisms (detritivores) that are separate from the grazing food chain, and generally termed the detritus food chain.

The detritus food chain represents an exceedingly important component in the energy flow of an ecosystem. Indeed in some ecosystems, consi­derably more energy flows through the detritus food chain than through the grazing food chain. In the detritus food chain the energy flow remains as a continuous passage rather than as a step­wise flow between discrete entities.

The organisms of the detritus food chain are many and include algae, bacteria, slime molds, actinomycetes, fungi. Protozoa, insects, mites, Crustacean, centipedes, moll uses, rotifers, annelid worms, nematodes and some vertebrates some species are highly specific in their food requirements and ^some can eat almost anything.

Many Protozoa, for instance, need certain specific organic acids, vitamins, and other nutrients before they can thrive; on the other hand, the guts of small Collembola group of tiny soil insects) have been reported to contain decaying plant material, fungal fragments, spores, fly pupae, other Collembola, parts of decaying earthworms.

In contrast to the grazing food chain, in which energy storage is entirely within the tissues of living organism, energy storage for the detritus food chain may be largely external to the organisms, and is the detritus itself.

Detritus organisms ingest pieces of partially decomposed orga­nic matter, digest them partially, after extracting some of the chemical energy in the food to run their metabolism, excrete the remainder in the form of slightly simpler organic molecules.

The wastes from one organism can immediately be utilized by a second which repeats the process. Gradually, the complex organic mole­cules present in the original wastes or dead tissue is broken down to much simpler compounds, sometimes all the way to carbon dioxide and water.

In most instances, however, the organic mate­rial is broken down until all the easily biodegradable material has been degraded, and all that is left are some refractory organic sub­stances turned humic acids, or simply, humus. In a normal environ­ment the humus is quite stable and will form an essential part of the soil.

Links between Grazing and Detritus Food Chains:

Just as energy enters the detritus food chain from the grazing food chain as leaf litter, or dead organisms, energy can re-enter the grazing food chain from the detritus food chain if organisms from the latter are consumed by grazers. Considering the diver­sity of the detritus community, it is not surprising that many of these serve as prey for carnivores in the grazing” food chain.

A common example is robins citing earthworms. In addition, many insects, among them beetles and flies, spend their larval period in the detritus food chain and their adulthood in the graz­ing food chain. But through such links through which energy pas­ses from detritus food chain basic into grazing food chain, very little amount of energy is passed in comparison to the amount of energy which flow from grazing food chain to detritus food chain.

Food web:

In nature simple food chains occur rarely. The same organism may operate in the ecosystem at more than one trophic level, i.e., it may drive its food from more than one source. Even the same organism may be eaten by several organisms of a higher trophic level or an organism may feed upon several different organisms of a lower trophic level.

Usually the kind of food chan­ges with the age of the organism and the food availability. Thus, in a given ecosystem various food chains are linked together and intersect, each other to form a complex network called food web.