Define ecological pyramids and describe with examples, pyramids of number and biomass.

An ecological pyramid is a graphical representation that shows the quantitative relationship between organisms at different trophic levels of a food chain. These pyramids depict the producers at the base and the successive trophic levels form the upper tiers. There are three types of ecological pyramids: pyramid of numbers, pyramid of biomass, and pyramid of energy.

Pyramid of Numbers:

This pyramid represents the number of organisms at each trophic level in a given ecosystem. It might not necessarily be pyramidal, as it directly reflects the number of organisms and not their size or contribution to the ecosystem.

Example: In a grassland ecosystem:

  • Base: Large number of grass plants (producers).
  • First level: Lesser number of herbivores (like grasshoppers) that eat the grass.
  • Second level: Even fewer small predators (like frogs) that eat the herbivores.
  • Third level: Even fewer top predators (like snakes) that eat the smaller predators.

Sometimes, however, this pyramid can be inverted. For instance, in a tree ecosystem, one oak tree (producer) might support many hundreds of caterpillars (primary consumers), which in turn could be preyed upon by fewer birds (secondary consumers).

Pyramid of Biomass:

This pyramid represents the total biomass (living material) present at each trophic level of an ecosystem. Biomass typically decreases as one moves up the trophic levels because energy is lost as one organism consumes another.

Example: In a forest ecosystem:

  • Base: Large biomass of trees and plants (producers).
  • First level: Lesser biomass of herbivores (like deer) that consume the plants.
  • Second level: Even smaller biomass of predators (like wolves) that eat the herbivores.
  • Third level: Very small biomass of apex predators (like bears).

In some aquatic ecosystems, the pyramid of biomass can be inverted. This happens when the biomass of phytoplankton (producers) is less than the biomass of zooplankton (primary consumers) at any given point because the fast turnover rate of phytoplankton means they are consumed faster than they accumulate as biomass.

Each pyramid gives its own perspective on the ecological relationships within an ecosystem. While pyramids of numbers might reveal the sheer quantity of organisms at each level, pyramids of biomass provide a clearer picture of the actual biological “weight” or productivity present at each level.

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