The entire dynamics of an ecosystem revolves round the plants and animals in that system. These plants and animals are intimately related to each other in their behavior and interactions in the work of assimilating energy from the non-living or abiotic environment and storing food for biotic community and then finally decomposing the dead and useless into elements, returning them to the atmosphere (abiotic environment) to be used in the next cycle. This is a beautifully perfected cycle ordained for the preservation of species in the biosphere.
Productivity: In the ecological system, productivity within is of prime importance, as this decides living, nourishment and reproduction in all biotic ecosystem as a whole; or any community or part thereof is the rate at which the radiant energy from the sun is stored by means of photosynthetic and chemosynthetic activities in the form of organic substances by the producer organisms This is the food for the community.
This production process has four distinct steps. They are: Gross Primary Productivity, Net Primary Productivity, Net Community Productivity, and Secondary productivities.
(i) Gross Primary Productivity: The entire vegetative community produces food by means of photosynthesis and they spend some organic matter in the process of respiration. Gross primary Productivity is the total rate of photosynthesis, including the organic matter used up in respiration by plants.
(ii) Net Primary Productivity: This is the rate of storage of organic matter in plant tissues over and above what had been utilized by plants in respiration.
(iii) Net Community Productivity: This is the rate of storage of organic matter not used by heterotropics. Organisms whose nutritional needs are met by other organisms are called Heterotropics; while the food producing organisms happen to be self-feeding, they are called Autotropics. Net community productivity is net primary production minus heterotropic consumption.
(iv) Secondary Productivity: This is the final rate of energy storage at consumer levels.
Energy Flow in Ecosystem :
Food Chain The primary source of energy is sun and all energies come from solar energy. But, this solar energy cannot be made use of by all organisms. Only green plants are able to trap this solar energy and convert them into food energy by means of photosynthesis. The ecosystem is characterized by the energy flow and circulation of material through its members. Thus, transfer of food energy from the source in plants through a series of organisms with repeated eating and being eaten is called FOOD CHAIN.
The plants are eaten by herbivores and the latter are eaten by carnivores. Thus one becomes the food for another and the energy flows from one group of species to another group in the form of food. Animals and plants after death become food for bacteria. Thus the energy flows back to the atmosphere where from it has come. In the flow of energy, i.e., at each transfer of food in the food chain in the process of eating and being eaten, a large proportion (about 90 per cent) of potential energy is lost as heat.
Therefore the number of steps in the food chain is limited to four or five. Shorter the food chain, greater the energy available.
There are three types of food chains:
(a) The Predator Food Chain: This starts with green plants which are eaten by grazing herbivores as primary consumers and predators as secondary consumers.
(b) Parasite Food Chain: In this case, food energy passes from larger to smaller organisms. It also starts with herbivores. Large animals become hosts and small animals thrive on them.
(c) Saprophytic Food Chain: This is also called Detritus Food Chain. In this, energy is transferred from dead organic matter into micro-organisms and then to detritus-feeding organisms and their predators.
The food chains are not isolated sequences, but are inter- connected with one another. This interlinking pattern is known as Food Web. Depending upon the availability and choice of food, different organisms at each level have food relationship with more than one organism at lower levels. Thus, in a food web, a predator eats several types of food and every kind of food is eaten by many different organisms and it is the outcome of interactions between types of food chains.