Energy flow in an ecosystem is a one-way process. We can identify the sequence or organisms through which the energy flows and this sequence is known as food chain. For example, plants are eaten by insects, which are eaten by frogs; these frogs are eaten by fish, which are eaten by human beings. In this food chain, there are five tropic or feeding levels. Several factors are important in determining an animal’s place in a food chain. Each species occupies a specific place and has special adaptations that make it fit for that place.
Flow of energy in ecosystems from sunlight through photosynthesis in autotrophic producers, to the tissues of herbivores, the primary consumers, to the tissue of carnivores, the secondary consumers, determines the number and biomass of organisms at each level in the ecosystem. Flow of energy is greatly reduced at each successive level of nutrition because of the energy utilization by the organisms and heat losses at each step in transformation of energy. This largely accounts for the decrease in biomass at each successive level. In addition, no predator is completely efficient at capturing its prey; some energy is lost in the hunt.
Some animals eat only one kind of food and therefore, are members of a single food chain. Other animals eat many different kinds of food, so they are not only members of different food chains but also may occupy different positions in different food chains thus ensuring the survival of their species. An animal may be primary consumer in one chain, eating plants but a secondary or tertiary consumer in other chains, eating herbivorous animals or other carnivores.
Humans are at the end of a number of food chains. For example, a man might eat a big fish, which in turn ate little fishes, which ate small invertebrates, which ate algae. The ultimate size of the population of any animal, is limited by the number of links in the food chain, the efficiency of energy transfer at each step in the chain, and ultimately by the amount of light energy falling on the earth in that region.
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Since man can do nothing about increasing the amount of light energy and very little about the efficiency of energy transfer, he can only shorten the food chain, to get energy i.e., by eating the primary producers – plants, rather than animals.
In highly populated countries, people tend to be vegetarians because then the food chain is shortest and a given area of land can in this way support larger number of people. Suppose that a farmer has a crop of wheat and vegetables. He can eat it directly or feed it to his cattle and then eat the cattle.
In nature, three types of food chains have been distinguished.
Grazing food chain:
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The consumers, which start the food chain, utilizing the plant or plant part as their food, constitute the grazing food chain. This food chain begins from green plans at the base and the primary consumer is herbivore, for example: grass -> grasshopper -> birds -> hawks or falcon.
Parasitic food chain:
It also begins from green plant base, and then goes to herbivores, which for example may be host of a huge number of lice, which live as ecotoparasites.
Detritus food chain:
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The food chain starts from dead organic matter of decaying animals and plant bodies to the microorganisms and then to detritus feeding organism derivers, and to other predators.
In a community of organisms in a shallow sea, about 30% of the total energy flows via detritus chains, but in a forest with large biomass of plants and a relatively small biomass of animals’ even larger portion of energy flow may be via detritus pathways.
Food chains are not always as simple as the one just described. Often, several different species may use the same item for food and one species may feed on different species of food organisms.