The number of trophic level in an ecosystem are each limited and are not more than 4-5. because the amount of energy flow decreases with successive trophic level as only $10 \%$ of energy is transferred from one trophic level to the next successive level.

So rest of the energy is lost in the form of respiration and other vital activities to maintain life. If more trophic levels are present, the residual energy will be limited and decreases to such an extent that it cannot further support any trophic level by the flow of energy.

So, the food chain is generally limited to $3-4$ trophic levels only.

$$ \text { e.g., } \quad \underset{(30,000 \mathrm{~J})}{\text { Sun }} \xrightarrow{1 \% \text { asborbed }} \underset{(300 \mathrm{~J})}{\text { Plants }} \xrightarrow{10 \% \text { transferred }} \underset{(30 \mathrm{~J})}{\text { Deer }} \xrightarrow{10 \% \text { transferred }} \underset{(3 \mathrm{~J})}{\text { Tiger }} $$

Aquarium is a man made ecosystem (artificial). If an ecosystem possess all physical and biological component, then it is said to be complete. Since aquarium has biotic (plants and fishes) and abiotic component (air, water) required for survival of fishes, so it is an complete ecosystem.

Tropics are characterised by high temperature and moisture and soil is rich in humus and minerals. As the decomposition rate is regulated by climatic factors, thus optinum temperature and humidity enhance the activity of decomposrs while the dead remain is also available in rich amount, which supports the faster rate of decomposition in the tropics.

Two human activities that interfere with carbon cycles are

(i) Rapid deforestation and

(ii) Massive burning of fossil fuel for energy and transport.

Flow of energy in an ecosystem is always unidirectional, i.e., energy flow in one way and is noncyclic, like

$$ \underset{\text { Ist Trophic level }}{\text { Plants }} \longrightarrow \underset{\text { Ilnd Trophic level }}{\text { Herbivore }} \longrightarrow \underset{\text { Ill rd Trophic level }}{\text { Carnivore }} \longrightarrow \underset{\text { IVth Trophic level }}{\text { Top carnivore }} $$

As the energy content is decreasing from lst trophic level to next trophic level and so on, thus the energy cannot pass in reverse direction.