The entire organism needs energy to survive. The source of energy is different for different living organism. Like, plants use the energy from sunlight to prepare food whereas animals eat plants as the source of energy. Likewise, we human eat plants as well as animals for the source of energy. Therefore, this process when occurs in step wise sequence constitute food chains.
If we go right from the beginning, plants use sunlight to prepare foods and hence are called autotrophs or primary producers. Then comes those organisms that eat the autotrophs, and they are called primary consumer. Some of the example here includes: deer, cow, rabbit, grasshopper etc. that eats plant or the plant product. Then comes the secondary consumers; these are those animals that eats primary consumers like that of birds that eats grasshopper. Further are the tertiary consumers that eat secondary consumers. Example includes snake that eats small birds and finally is the quaternary consumers that includes larger animal like hawk that prey on sneak and many such animals that have no further predators. Now, after this when any organism dies, they are eaten by detrivours and hence decomposers present in the soil decays the complete organism thus returning the energy back to the environment. Though the process is similar, but this chain is different for water that involves phytoplankton, zooplanktons, and so on. Therefore, food chain is the transfer of energy from the source organism to the series of other organism who eats them and is also eaten by other organisms.
Image reference: www.blueplanet.nsw.edu.au
Coming to the other point of the topic, food chain can be classified into two main types: grazing food chain and detritus food chain. Grazing food chain starts from green plants and passes to different level of consumers whereas detritus food chain starts from non- living matter that goes to small micro- organism, moving to the detritivores and all their predators.
In case of food chain, one thing must be noted, that is the energy when gets transferred from one level to the other goes on declining but the quality of that energy increases, which means that plants takes sunlight as the source of energy and stores it. When primary consumers feed on plants they use the fixed energy for growth and maintenance of the body. In this process, all the energy that primary consumer receive is not used up rather some amount of energy goes wasted. Likewise when the chain further proceeds, secondary consumers feed on primary consumers and they use only the energy taken in my primary consumers. Beside, this not all the energy they receive is used, in fact most part of it gets wasted. This way the chain continues by utilization of very less amount of energy on every increasing tropic level and returning most of the energy to the ecosystem. But the advantageous thing is that, the quality of energy every increasing level receives is always better than the preceding level.
On one hand, food chain is a linear event or step wise process that takes place in the ecosystem whereas food web is interconnecting network of many such food chains. Therefore, it can be said that there is lot more than eating and being eaten in the process of food web. Food web can be classified into two main groups the autotrophs and the heteroptrophs. Autotrophs are those organisms that produce organic substances from the inorganic one, like that of plant and heterotrophs are those who use this organic substance for development and survival. Beside this, the other important point here is that food web mostly depends on the biomass that is the energy source. As we go down the tropic level, the biomass decreases and this is the reason we have more producers than the consumers and more consumers than the decomposers. This maintains the balance in the ecosystem. But when any link of the food web is broken, like the extinction of an animal, then the complete web get disturbs. This can be clearly explained with an example. We suppose the numbers of plants have decreased, now due to less available food slowly deer’s, or the cows or any other animal who feed on plants also will decline, and further at every level the number gets narrowed down which results in imbalance in the ecosystem.
Image reference: www.cs.ubc.ca.com
Day by day, it can also be seen that human intervention and activities have been the major reason for imbalance in the ecosystem that have resulted in different kinds of natural calamities.