Anything which we eat to live is called food. Food contains energy. The food (or energy) can be transferred from one organism to the other through food chains. The starting point of a food chain is a category of organisms called producers. Producers are, in fact, plants. So, we can say that all the food chains begin with a green plant (or grass) which is the original source of all food. Let us take an example to understand the meaning of food chain.
Suppose there is a field having a lot of green plants (or producers of food). Now, plants can be eaten up by a rat. The rat, in turn, can be eaten up by a cat. And finally, the cat can be eaten up by a dog.
So, we find that there is a sequence (or order) in which one organism eats up the other organism (or consumes the other organism) to fill its belly. The sequence of living organisms in a community, in which one organism consumes another organism to transfer food energy, is called a food chain.
In simple words, a list of organisms (living beings) showing “who eats whom” is called a food chain. Let us make this point more clearly by taking the example of a simple food chain operating in a grassland or forest. In a grassland or forest, there is a lot of grass (which are green plants). This grass is eaten up by animals like deer. And this deer is then consumed (eaten up) by a lion. This simple food chain operating in a grassland or forest can be represented as:
Grass –> Deer –>Lion
(Producer) (Herbivore) (Carnivore)
This food chain tells us that grass is the starting point of this food chain. The grass is eaten up by deer and the deer is then eaten up by a lion. In this food chain, grass is the producer organism which uses sunlight energy to prepare food like carbohydrates by the process of photosynthesis.
This grass is then consumed by a herbivore called deer. And the deer is consumed by a carnivore called lion. The above food chain can be represented more clearly with the help of a diagram as follows.
In the food chain that we have discussed above, there are three organisms involved in it: grass, deer and lion, so it is said to be a food chain having three steps or three links. The same grassland has many other food chains operating in it which can have different number of steps.
Let us take the example of a grassland food chain having four steps or four links. In a grassland ecosystem, grass is eaten by insects; the insects are eaten by frog; and the frog is then eaten by birds. This is a grassland food chain involving four organisms (or four steps) which can be represented as follows:
Grass–>Insect –>Frog –> Bird
(Producer) (Herbivore) (Carnivore) (Top carnivore or large carnivore)
The above food chain can be represented more clearly with the help of pictures of all the organisms involved as follows:
Please note that in this food chain grass is the producer. The insect (herbivore) is the primary consumer; the frog (small carnivore) is the secondary consumer whereas the bird (top carnivore or large carnivore) is the tertiary consumer.
Another four-step food chain operating in grassland is:
Plants –>Worms –>Birds –> Cat
(Producer) (Herbivore) (Carnivore) (Top carnivore)
We will now discuss the food chain operating in an aquatic ecosystem (water ecosystem) like a pond, lake, or sea (ocean). In a pond, lake or sea ecosystem, the algae are eaten up by protozoa; the protozoa are eaten up by small fish; and the small fish is eaten up by big fish. This aquatic food chain can be represented as:
Algae –>Protozoa –> Small Fish –> Big Fish
(Phytoplankton) (Zooplankton) (Carnivore) (Large carnivore)
Please note that in a pond, lake or ocean ecosystem, the producer is a minute organism called algae and protozoa is the minute herbivore.
Each organism (or living being) occupies a specific position in the food chain. For example, grass, deer and lion occupy specific positions in the food chain:
Grass –>Deer –>Lion
Another point to be noted is that one organism (or same organism) can occur in more than one food chains. For example, in the forest food chains, a deer may be consumed by a lion as well as by a jackal:
Grass –> Deer –>Jackal
So, the same organism, deer, occurs in the food chains of lion as well as that of jackal. The organisms representing producers and consumers in a food chain give a definite structure to an ecosystem.
Food Web :
A large number of food chains exist in a community of living organisms in an ecosystem such as grassland, a forest, a pond or a crop-field. Many of these food chains are inter-connected by species (organisms) which occur in more than one food chain.
The inter-connected food chains operating in an ecosystem which establish a network of relationships between various species, is called a food web. In simple words, the network of a large number of food chains existing in an ecosystem is called a food web. The food web has many intercrosses and linkages among the various species (producers and consumers) present in it. This means that the various food chains in an ecosystem do not operate in isolation (or alone).
In this food web, we can see a network of numerous pathways along which the food (or energy) flows within grassland community. This food web starts from the plants which is a producer and ends in top carnivore hawk (baaz). There are as many as six food chains operating in the food web shown above which have been marked 1, 2, 3, 4, 5 and 6.
1. In the 1st food chain, plants are eaten by rabbit and then rabbit is eaten by hawk:
Plants –> Rabbit –>Hawk
2. In the 2nd food chain, plants are eaten by mice (or rats) and the mice are eaten by hawks:
Plants –> Mice –> Hawk
3. In the 3rd food chain, plants are eaten by mice; mice are eaten by snakes and then snakes are consumed by hawks:
Plants –> Mice –> Snake –> Hawk
4. In the 4th food chain, plants are eaten by seed-eating birds and the seed-eating birds are consumed by hawks:
Plants –> Seed-eating Bird –> Hawk
5. In the 5th food chain, plants are eaten up by grasshopper and the grasshopper is consumed by hawks:
Plants –> Grasshopper –> Hawk
6. In the 6th food chain, plants are eaten by grasshopper, grasshopper is eaten by frog, frog is eaten by snake and then snake is consumed by hawk:
Plants –> Grasshopper –> Frog –> Snake –> Hawk