D. Ecological Pyramids

Food and energy relationships can be shown using horizontal bar diagrams. Producers are always placed at the bottom, followed by primary consumers, then secondary consumers and finally the tertiary consumers are at the top.

Such diagrams form the ecological pyramids.

Types of ecological pyramids include (1) pyramid of numbers and (2) pyramid of biomass.

1. Pyramid of numbers

   • Enables comparison of the number of organisms present in each trophic level at a particular time.

   • May be upright or inverted. Inverted when many small organisms of one trophic level feed on a large organism of another trophic level.

   • Does not consider productivity of the individual species.

Consider the following food chain.

potato plant --> rat --> snake --> eagle

Phytoplankton are microscopic plant-like organisms that can make food by photosynthesis. Zooplankton are microscopic primary consumers that feed on phytoplankton. The pyramid of biomass may look like

Pyramid of biomass

From the above pyramid of biomass, there is an impression that the biomass of phytoplankton is smaller than that of zooplankton and this is not possible in reality. In actual fact, the rate of reproduction of phytoplankton is fast enough to replace the phytoplankton that were eaten by zooplankton.

3. Pyramid of energy

• • Compares the total energy in the various trophic levels of a food chain over a certain period of time.

  • Always upright.

  • Takes into consideration the rate at which the organisms in each trophic level reproduce.

Advantages of short food chains

• • Only about 10% of energy is transferred from one trophic level to the next. As we move along the food chain, less and less energy is available for the organisms at the next trophic level.

  • Thus, the shorter the food chain, the greater amount of energy is available to the final consumer because less energy is lost to the environment.