The correct option is
D Only III
An ecological pyramid is the graphic representation of ecological parameters (number, biomass and energy) in various trophic levels of a food chain.
The ecological pyramids always have the producers at the base and the top carnivores at the apex. Based on the type of relationship between these trophic levels represented by an ecological pyramid, it can be of 3 types
- Pyramid of number - representing the number of individuals at each trophic level
- Pyramid of biomass - representing the total mass of all living organisms at a particular trophic level in a given area at a given time
- Pyramid of energy - representing the amount of energy present at each trophic level.
An ecological pyramid can be upright (broad base and narrow apex), inverted (narrow base and broad apex) or spindle shaped (broad in the centre and tapering towards the ends).
In most ecosystems, the ecological pyramids are upright as the number, biomass and energy levels decrease as we go from the lower trophic levels to the higher.
However, in aquatic ecosystems, the pyramid of biomass is inverted as the biomass of phytoplankton (producers) supporting the higher trophic levels is much less. The biomass increases successively at higher trophic levels, giving the pyramid an inverted shape.
The pyramid of numbers for a tree ecosystem can be spindle-shaped. This is because a big tree (producer) supports a large number of insects (primary consumers). But the numbers of the secondary consumers (small birds) and tertiary consumers (large birds) decrease progressively, giving the pyramid a spindle shape.
Thus, only statement III is correct.