Energy is the capacity of doing work. Biological activities require energy which comes from the sun. The solar energy is transformed into chemical energy by the process of photosynthesis.
The energy flows from the sun to plants and then to all heterotrophic organisms such as microorganisms, animals and human beings.
Living organisms use energy mainly into two forms.
Energy flow in any ecosystem is governed by two laws of thermodynamics.
In the light of these two laws of thermodynamics, let us analyze the energy flow in an ecosystem.
The source of energy for all ecological system is the sun. The energy that enters the earth’s atmosphere as a heat and light is balanced by the energy that is absorbed by the biosphere plus the amount that leaves the earth’s surface as invisible heat radiation (first law of thermodynamics). When solar energy strikes the earth, it tends to be besmirched into heat energy. Only a very small part (about twenty percent) of this energy gets absorbed by the green plants and is subsequently transformed into food energy. The food energy then flows through a series of organisms in ecosystems. All organisms dead or alive are the potential source of food for other organisms. Grasshopper eats grass, a frog eats the grasshopper, a snake eats the frog and the snake is eaten by the eagle. When these organisms die, they all are consumed by decomposers (bacteria and fungi).
Ideally, the energy transformation from the sun to green plants to herbivores to carnivores should be hundred percent efficient. But in reality, this does not happen because at each link in a food chain, eighty percent to ninety percent of energy transferred is lost as heat (second law of thermodynamics). It is because of this loss that rarer individuals are found at each successive level of the food chain (Examples, fewer carnivores than herbivores). This also limits the number of levels in a food chain. All organisms are a part of food chain. Food chain consists of producers, primary consumers, secondary consumers, tertiary consumers, and decomposers.
Every organism in an ecosystem can be assigned a feeding level called trophic level. A trophic level consists of those organisms in food chains that share the same general types of food. All producers belong to first trophic levels because all producers utilize the similar energy during the process of photosynthesis. All consumers occupy the second trophic level because they all eat plants. All secondary consumers eat primary consumers and therefore occupy the third trophic level. Some organisms are omnivores which eat producers as well as herbivores. Such organism may lodge more than one trophic level in a food chain. Such organisms may occupy more than one trophic level in a food chain. In this way to run any ecosystems energy flow and nutrient cycling process takes place.
The Riverstrahler model of the ecological functioning of large drainage networks validated on the seine river system has been used for calculating the seasonal variation of production and respiration at various spatial scales. Based on the measurement of biological processes, the P/R ratio has led to an evaluation of the “ecological functioning”, beyond the notion of “good ecological status”. Furthermore, the effects, on the P/R ratio, of the geomorphological and climate factors characterizing the Hydro Eco-Regions (HER) of the seine watershed have been quantitatively explored with the model. Whereas one finds a distinctive upstream-downstream pattern of the P/R ratio variations under the traditional rural conditions that prevailed in the seine basin until the end of the 18th century, this pattern is strongly affected by the changes in built-up populations and the implementations of wastewater collection and treatment, more than by the specificity of the physical factors portraying the different HER. We have also found that autotrophy might prime to eutrophication symptoms when P exceeds 1-2 mg C m(-2) d(-1) and that heterotrophy of the system (P/R ratio<1) would reveal organic pollution when R exceeds 1-2mg C m(-2) d(-1), stocks and fluxes of organic matter being expressed in carbon unit. Accordingly, the P/R ratio appears as a respectable gauge of the perturbations caused by human activities in the watershed. The Riverstrahler model is able to quantify this effect.
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