The flow of nutrients in ecosystem takes place in cyclic pathways through food chains, food webs and bio-geo-chemical cycles.
Flow of materials in an ecosystem in a cyclic pathway is called as food-chain.
In fact the food synthesized by green plants passes through different trophic levels to the top consumer after the death of which it is decomposed in simpler form to get mixed with the soil. From there plant roots again absorb them through water solution.
Thus the cycle goes on.
Food Chains are of diverse types. It depends on the structure of ecosystem. For example food-chains of a pond, a desert, and grassland remain highly diverse. There exists great diversity in terms of trophic levels in food chains of different places.
In grassland or a crop field plants are the only producers. Numerous insects get their food from these plants. Insects are Primary Consumers.
Frogs eat away insects and so they are Secondary Consumers. There may be snakes in the grassland or crop field as they live in the holes dug out by rats. Snakes eat away frogs. So snakes are Tertiary consumers.
Since snakes too are eaten by some animals say Peacock or Vulchers, these snake eaters may be called as Top Consumers. When top consumer dies, its body is decomposed by numerous decomposers. The decomposed simple organic substances get mixed into soil and absorbed by plan roots. Thus the cycle completes.
Thus we can trace out food-chains of other places as well.
In an aquatic food chain, aquatic plants like lemna, Sagitaria, vallisneria, Echornia etc. are producers. Small aquatic insects eat aquatic plants and in turn are eaten up by fish. Fish is eaten up by water bird. Thus the aquatic food chain is completed after the death and decomposition of the top consumer. We may trace many other food chains too that run in a water body.
If we take the example of a forest we can very easily guess that the lion or a tiger may be the top consumer. Producers like grasses or plants that grow on the forest floor are eaten by deer and deer is eaten by the lion. Many other food chains may be traced out from a forest ecosystem.
It is food –chain which transfers nutrients and energy through different trophic levels in the ecosystem. But a single food-chain is not sufficient to transfer food and energy in the whole ecosystem.
As producers are of many types so are consumers. Different consumers have different food preferences due to which a single consumer often eats many different types of food. Rabbit, rat and insects can eat grass in an ecosystem.
In the same way a Vulchers can eat both rat and rabbit. Again a snake can eat many types of animals like frogs, eggs of birds and rats and many more. A Vulchers can eat rabbit and snake. Thus many food-chains can run at the same time in an ecosystem. This is called as Food-Web. The network of food-chains is called as food-web.
It so happens sometimes in an ecosystem that the number of organisms in trophic levels becomes too much as compared to the other trophic level.
Let us consider it through an example- There are numerous insects in a forest but the number of birds that feed upon these insects is very less. The number of predator birds is very less than insectivorous birds.
If we demonstrate the number of individuals of different trophic levels on a vertical line, we get a Pyramid which is called as Food-Pyramid. The concept of food pyramid was presented for the first time in 1972 by Charles Elton.
The Food-pyramid may be drawn on the basis of number and their biomass. The first one is called as Pyramid of Number and the second one is called as Pyramid of Biomass.