Food Chain and Food Web

 

Energy is the most essential requirement of all living organisms which help in their metabolic activities. The main source of energy for an ecosystem is the solar radiation or solar energy.

The functional aspect of ecosystem throws light on the food chain and energy flow. Various food levels exist in every ecosystem. All organisms with similar feeding habits are grouped together to be known as Trophic level.

Green plants (producers) trap this radiation and with the help of minerals taken form the soil and aerial environment they build up complex organic matter i.e., food materials – carbohydrates, proteins, lipids, etc) that flows from one tropic level to another. Thus, in ecosystem, energy is transferred from one trophic level to the other trophic level. During this process good amount of energy is lost or dispersed as heat and some of it is converted to other forms of energy. Energy transfer is always unidirectional.

The transfer of food energy from the producers, through a series of organisms – herbivores to carnivores to decomposers with repeated eating and being eating, is known as food chain.

 Green plants, the producers utilise the radian energy of sun which is transformed to chemical form, ATP during photosynthesis.  Thus, green plants occupy ‘first trophic level’ (T₁) – producers level  and are called ‘primary trophic level’ or primary producers.

The energy, as stored in food material manufactured by green plants, is then utilised by the plant eaters- the herbivores, which constitute the second trophic level- the primary consumers level and are called primary consumers.

Herbivores in turn are eaten by carnivores, which constitute the third trophic level- the secondary consumers levels, and are called the secondary consumers.

These in turn may be eaten still by other carnivores at tertiary consumers level i.e., by the tertiary consumers. The ultimate carnivores not eaten by others are ‘top carnivores’. Man occupies several trophic levels as he can consume plants as well as various types of animals. Lion also has no fixed trophic level. They are called omniovers.

In any food chain energy flows from primary producers to primary consumers (herbivores), form primary consumers to secondary consumers (carnivores), and from secondary consumers to tertiary consumers (carnivores/omnivores) and so on. This simple chain of eating and being eaten away is known as food chain.

In nature, we generally distinguish two general types of food chains.

1.     Grazing food chain: this type of food chain starts from the living green plants, goes to grazing herbivores , and to to carnivores or animal eaters.

Ecosystem with such type of food chains are directly dependent on an influx of solar radiation. This type of chian thus depends on autotropohic energy capture and the movement of this captured energy to herbivores.

Most of the ecosystems in nature follow this type of food chain.

The phytoplanktons – zooplanktons – fish

Grasses – rabbit - fox

Grass – Insects – Frogs – Birds – Hawks or Falcons.

 

 2.     Detritus food chain:

This type of food chain goes from dead organic matter into microorganisms and then to organisms feed on detritus (detritivores) and their predators.

Such ecosystem is thus less deplendent on direct solary energy.

Detritus food chain occurs in estuarine and coastal ecosystems. Heald and Odum described the , copepods, insect larvae, example of a red mangrove – Rhizophora mangle, whose leaves fall into the warm, shallow waters. The fallen leaf fragments are eaten by small animals like crabs, copepods, insect larvae, grass shrimps. All these animals are detritus consumers.

These detritivores are eaten by some minnows and small game fish, i.e., the small carnivores. These animals are consumed by large game fish and fish eating birds which are the top carnivores.

Thus, mangroves contribute to the food chain that supports the fisheries, an important economy in that region.

Dead organic matter – Earthworms – Frogs – Birds – Hawks or Falcons

Dead organic matter – Bacteria or Fungi – Small fishes – Birds

Food Webs:                                                                  

In an ecosystem an organism operates at more than one trophic level, and obtain its food requirements from different sources. Similarly, it may be eaten by different types of organisms.

Thus, Food chains in natural conditions never operate as isolated sequences, but are interconnected with each other forming some sort of interlocking pattern, known as ‘food web’.

A food web, unlike a food chain shows several alternate pathways for the flow of energy. It also al lows an organism to obtain food from tow or more types of organisms of the lower trophic level.

For example, in grazing food chain of a grassland, in the absence of rabbit, grass may also be eaten by mouse. The mouse in turn may be eaten directly by hawk or by snake first which is then eaten by hawk.

Thus, in nature there are found alternatives, which all together constitute some sort of interlocking pattern – the food web.

Food webs are very important in maintaining the stability of an ecosystem in nature. For example, decrease in the population of rabbit would naturally cause in increase in the population of alternative herbivore, the mouse. This may decrease the population of the consumer or carnivore that prefers to eat rabbit.

Thus, alternatives serve for maintenance of stability of the ecosystem. Moreover, a balanced ecosystem is essential for the survival of all living organisms of the system. For instance, had primary consumers or herbivores not been in nature, the producers would have perished due to overcrowding and competition.

Thus, each species of any ecosystem is indeed kept under some sort of a natural check so that the system may remain balanced.