The transfer of energy in an ecosystem between trophic levels can be termed as energy flow. It is the key function in an ecosystem. Energy flow is always unidirectional in an ecosystem.

The amount of living materials present in a population at any given time is known as standing crop.

Biotic (living) components includes all living organisms like plants, animals, fungi and bacteria. They form the trophic structures of any ecosystem.

Plants \(\rightarrow\) Grasshopper \(\rightarrow\) Frog \(\rightarrow\) Snake \(\rightarrow\) Hawk

There is a gradual decrease in energy transfer at successive tropic levels from producers to higher levels, hence the pyramid of energy is always upright.

Because there is no effective light penetration & predominance of heterotrophs.

Primary productivity depends upon the plant species of an area, their photosynthetic capacity, availability of nutrients, solar radiation, precipitation, soil type, topographic factors (altitude, latitude and direction), and other environmental factors.

Pyramid of number
(i) In a forest ecosystem the pyramid of number is different in shape, it is because the base (T ₁) of the pyramid occupies large sized trees (producer) which are lesser in number.
(ii) Herbivores (T₂) occupying second trophic level, are more in number than the producers (T₁).
(iii) In final trophic level (T₄) tertiary consumers (lion) are lesser in number than the secondary consumers (T₃).
(iv) Therefore, the pyramid of numbers in forest ecosystem looks spindle-shaped.
Pyramid of biomass
(i) In forest ecosystem, there is a gradual decrease in biomass of organisms at successive trophic levels from producers to top carnivores (Tertiary consumer). Therefore, the pyramid of biomass looks upright.

Mechanism of decomposition
Decomposition is a step wise process of degradation mediated by enzymatic reactions.
Detritus acts as a raw material for decomposition.
It occurs in the following steps.
(a) Fragmentation - The breaking down of detritus into smaller particles by detritivores like bacteria, fungi and earth worm is known as fragmentation. These detritivores secrete certain substances to enhance the fragmentation process and increase the surface area of detritus particles.
(b) Catabolism - The decomposers produce some extracellular enzymes in their surroundings to break down complex organic and inorganic compounds in to simpler ones. This is called catabolism.
(c) Leaching or Eluviation - The movement of decomposed, water soluble organic and inorganic compounds from the surface to the lower layer of soil or the carrying away of the same by water is called leaching or eluviation.
(d) Humification - It is a process by which simplified detritus is changed into dark coloured amorphous substance called humus. It is highly resistant to microbial action, therefore decomposition is very slow. It is the reservoir of nutrients.
(e) Mineralisation - Some microbes are involved in the release of inorganic nutrients from the humus of the soil, such process is called mineralisation.

Grassland ecosystem : There is a gradual decrease in the number of organisms in each trophic level from producers to primary consumers and then to secondary consumers, and finally to tertiary consumers. Therefore pyramids of number in grassland ecosystem are always upright.