January 27, 2015

Junior Inter Zoology - Essay Question from Ecology and Environment

Q: Give an account of energy flow in ecosystem.
A: Except for the deep sea ecosystem, sun is the only source of energy
for all ecosystems on earth. Of the incident solar radiation, less
than 50% of it is photosynthetically active radiation. The producers
(i.e., green plants) fix the solar energy to synthesise food materials
from CO2 and H2O. They trap 2 to 10% of PAR and it sustains the entire
living world.

The heterotrophs depend on producers for their food either directly or
indirectly. The first law of thermodynamics (law of conservation of
energy) states that energy is neither created nor destroyed, but is
transformed from one form to another. Thus the solar energy that is
traped by the producers is converted into chemical energy. Further,
ecosystems also follow the second law of thermodynamics, which states
that no process involving energy transformation will spontaneously
occur unless there is degradation of energy.

As per this law energy dispersed is in the form of unavailable heat
energy, and constitutes the entropy. The organisms need a constant
supply of energy to synthesise molecules they need. The transfer of
energy through a food chain from one trophic level to the next is
called energy flow. The amount of energy available decreases at
successive trophic levels.

When organism dies, the energy in it becomes source of energy for
decomposers. According to 10% law (proposed by Lindeman), during the
transfer of energy from one trophic level to the next, only about 10%
of the energy is stored. The remaining is lost during transfer. The
energy transfer in a food chain can be illustrated as shown below.

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