13.1.1 Food Chains and Webs

. . Food Chains and Webs In Activity . we have formed a series of organisms feeding on one another.

This series or organisms taking part at various biotic levels form a food chain (Fig. . ). Each step or level of the food chain forms a trophic level.

The autotrophs or the producers are at the first trophic level. They fix up the solar energy and make it available for heterotrophs or the consumers. The herbivores or the primary consumers come at the second, small carnivores or the secondary consumers at the third and larger carnivores or the tertiary consumers form the fourth trophic level (Fig. .

). We know that the food we eat acts as a fuel to provide us energy to do work. Thus the interactions among various components of the environment involves flow of energy from one component of the system to another. As we have studied, the autotrophs capture the energy present in sunlight and convert it into chemical energy.

This energy supports all the activities of the living world. From autotrophs, the energy goes to the heterotrophs and decomposers. However, as we saw in the previous Chapter on ‘Sources of Energy’, when one form of energy is changed to another, some energy is lost to the environment in forms which cannot be used again. The flow of energy between various components of the environment has been extensively studied and it has been found that – The green plants in a terrestrial ecosystem capture about % of the energy of sunlight that falls on their leaves and convert it into food energy.

When green plants are eaten by primary consumers, a great deal of energy is lost as heat to the environment, some amount goes into digestion and in doing work and the rest goes towards growth and reproduction. An average of % of the food eaten is turned into its own body and made available for the next level of consumers. Therefore, % can be taken as the average value for the amount of organic matter that is present at each step and reaches the next level of consumers. Since so little energy is available for the next level of consumers, food chains generally consist of only three or four steps.

The loss of energy at each step is so great that very little usable energy remains after four trophic levels. There are generally a greater number of individuals at the lower trophic levels of an ecosystem, the greatest number is of the producers. The length and complexity of food chains vary greatly. Each organism is generally eaten by two or more other kinds of organisms which in turn are eaten by several other organisms.

So instead of a straight line food chain, the relationship can be shown as a series of branching lines called a food web (Fig. . ). Food chain in nature (a) in forest, (b) in grassland and (c) in a pond From the energy flow diagram (Fig.

. ), two things become clear. Firstly, the flow of energy is unidirectional. The energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs.

As it moves progressively through the various trophic levels it is no longer available to the previous level. Secondly, the energy available at each trophic level gets diminished progressively due to loss of energy at each level. Another interesting aspect of food chain is how unknowingly some harmful chemicals enter our bodies through the food chain. You have read in Class IX how water gets polluted.

One of the reasons is the use of several pesticides and other chemicals to protect our crops from diseases and pests. These chemicals are either washed down into the soil or into the water bodies. From the soil, these are absorbed by the plants along with water and minerals, and from the water bodies these are taken up by aquatic plants Figure . Diagram showing flow of energy in an ecosystem Food web, consisting of many food chains and animals.

This is one of the ways in which they enter the food chain. As these chemicals are not degradable, these get accumulated progressively at each trophic level. As human beings occupy the top level in any food chain, the maximum concentration of these chemicals get accumulated in our bodies. This phenomenon is known as biological magnification.

This is the reason why our food grains such as wheat and rice, vegetables and fruits, and even meat, contain varying amounts of pesticide residues. They cannot always be removed by washing or other means.