Energy Flow in Ecosystems

Energy Flow in Ecosystems:

Living organisms can use energy in several forms: radiant and fixed energy. Radiant energy is in the form of electromagnetic waves such as light. Fixed energy is potential chemical energy bound in various organic substances which can be broken down in order to release their energy content.

As the simultaneously occurring respiratory processes are energy consuming and use up the part of photosynthetic gain, the net capture of energy, i.e., net primary productivity is reduced to only 0.8 – 4 per cent of the incident total radiation, or 1.6 to 8 per cent of photosynthetically active process (PAR). Only the energy captured in net productivity is used by other trophic levels.

Trophic Level:

The producers and consumers in ecosystem can be arranged into several feeding groups each known as trophic level or feeding level. In any ecosystem, producers represent the first trophic level, herbivores represent the second trophic level, primary carnivores represent the third trophic level and top carnivores represent the last trophic level.

There are two main aspects of energy flow in ecosystem:

The energy flows one way, i.e., from producers through herbivores to carnivores. This cannot be transferred in reverse direction.

The amount of energy flow decreases with successive trophic levels.

Producers capture only a small fraction of solar energy, i.e., only 1-5 per cent of total solar radiation, and the bulk of unutilised energy is dispersed mostly as heat.

However, part of energy captured in gross production of producers is used for maintenance of their standing crop (respiration) and for providing food to herbivores, i.e., herbivory.

The unutilised net primary production is ultimately converted to detritus, which serves as source of energy to decomposers.

Thus, energy actually used by the herbivore trophic level is only a small fraction of the energy captured at the producer level.

On an average, in different ecosystems, the herbivore assimilation or productivity approximately comes to 10 per cent of gross productivity of producers.

The energy assimilated by herbivores is used in respiration and a fraction of unassimilated energy is transferred to decomposers, e.g., faecal matter.

The remaining herbivore level energy is either utilised by the carnivores, or gets transferred to decomposers after the death of herbivores.

The energy available at carnivore trophic level is again partitioned, leaving a very small fraction to support the next trophic level.

The species which occupy each trophic level in the food chain form a link, called the food – link. The links of one food chain are a flower, a butterfly, a dragonfly, a frog, a snake and a hawk. Here the flower nectar is eaten by butterfly; the butterfly is eaten by dragon fly, and so on through each successive link of the food chain.

The chain ends with the hawk. When a hawk dies, its cells and tissues are digested and used as food by bacteria and other organisms of decay. In this way, the organic materials of the food chain are returned to the soil and recycled through another chain.

However, the organisms that traps solar energy and act as producers also use up some energy for their own life processes. Animals being more active than plants use much of the energy acquired before they are consumed by the next trophic level.

The studies have shown that only about ten per cent of the food energy in green plants is utilized by themselves at the primary consumer level. Much of the other 90 per cent of the energy goes to heat and movement. The actual percentages of energy from food at one level converted to energy from food at the next level are more than 10 per cent or less than 10 per cent.

This general pattern forms an energy pyramid as each successive trophic level has less energy available to it. So, at each stage of energy transfer, a considerable energy is lost from the food chain. The decomposition of dead organisms also releases chemical energy. The chemical energy is transferred to living world through food.