Note on Similarities and Differences of Terrestrial and Aquatic System

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  • Things to remember

Some similarities between Terrestrial (land) and Aquatic (water) Systems:

  • In both terrestrial (land) and aquatic (water) environments, the ecosystems include communities made up of a variety of species.
  • Within both terrestrial (land) and aquatic (water) communities, there are populations at the different trophic levels.
  • A great deal of mutual interdependence exists between species in both terrestrial and aquatic environments.
  • In undisturbed terrestrial (land) and aquatic (water) ecosystems equilibrium is reached, i.e. very rare foremost changes are observed over a dated of time.
  • In both ecosystems stratification occurs.

Some differences between Terrestrial (land) and Aquatic (water) systems

  • Because water environments are so ironic in nutrients, they support more live than equivalent land ecosystems. The small drifting photosynthetic organisms of the oceans, referred to collectivity as phytoplanktons are viewed as the major photosynthesizers or primary producers of the earth.
  • Aquatic (water) environments are much more stable than terrestrial (Land) environments, with smaller fluctuations in temperature and other variables.
  • Aquatic organisms (water living organisms) are seldom exposed to desiccation while terrestrial organisms are often exposed to desiccation and are usually relatively resistant to drying out.
  • Oxygen (because there is very much less present) is sometimes a limiting factor and aquatic habitats but this is seldom the case in terrestrial habitats.
  • Light can be a limiting factor in some aquatic habitats, but in most terrestrial environments there is hardly ever a shortage of lights.
  • Terrestrial animals (land-living animals) are influenced more by gravity, while water supports aquatic organisms.


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.

  1. Radiant energy: It is the light energy coming from the sun.
  2. Fixed energy: It is the potential energy which is found in the chemical energy of food stored in various organic substances which can be broken down in order to release their energy content.

Energy flows and Ecological System

Energy flow in any ecosystem is governed by two laws of thermodynamics.

  1. The first law of thermodynamics (law of conservation of energy): Energy may be transformed from one form to another form but is neither created nor destroyed.
  2. The second law of thermodynamics (law of entropy): No energy transformation is hundred percent efficient i.e. energy is always being transformed from a more useful to a less useful form. When energy is passed from one organism to another in the form of food; a large part of the energy is degraded or lost as heat through metabolic activity with a net increase in entropy and the remaining is stored as living tissue.

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.

P/R ratio:

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.


E.p., Odum. Fundamentals of Ecology. USA: W.B Saunters Company, n.d.

Jr., Miller G.T. Living in the Environment. Balmot, Californea,, USA: Wadsworth Publishing Company, 2003.

  1. 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.
  2. Living organisms use energy mainly into two forms.- radiant energy and fixed energy.
  3. Energy flow in any ecosystem is governed by two laws of thermodynamics- first law of thermodynamics and second law of thermodynamics.
  4. 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”. 

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