Define ecological pyramids and describe with examples, pyramids of number and biomass.
An ecological pyramid is defined as a graphical representation of various ecological parameters present at each trophic level. Ecological pyramids represent producers which are placed at the base, while the apex represents the consumers. There are three types of pyramids:
1. pyramid of number
2. pyramid of biomass
3. pyramid of energy
PYRAMID OF NUMBER: The pyramid of numbers deals with the relationship between the numbers of primary producers and consumers of different orders. In all cases, the base of such a pyramid always represents the numbers of primary producers and the subsequent structures on this base are represented by the number of consumers of successive levels, the top representing the number of top carnivores in that ecosystem. In the ecological pyramid so formed, the higher the step in the pyramid, the lower the number of individuals and the larger their size.
The shape of the pyramid of numbers may be upright or inverted.
EXAMPLE 1: UPRIGHT: In a grassland ecosystem, the pyramid of numbers is upright. In this type of a food chain, the number of producers (plants) is followed by the number of herbivores (mice), which in turn is followed by the number of secondary consumers (snakes) and tertiary carnivores (eagles). Hence, the number of individuals at the producer level will be the maximum, while the number of individuals present at top carnivores will be least.
EXAMPLE 2: INVERTED: In a parasitic food chain, the pyramid of numbers is inverted. In this type of a food chain, a single tree (producer) provides food to several fruit eating birds, which in turn support several insect species.
PYRAMID OF BIOMASS: A pyramid of biomass takes into account, for a given unit area, the biomass of the producers, the biomass of the herbivores, the biomass of the first-level carnivores, and so on. SO, a pyramid of biomass is a graphical representation of the total amount of living matter present at each trophic level of an ecosystem.
It can be upright or inverted.
EXAMPLE 1: UPRIGHT: It is upright in grasslands and forest ecosystems as the amount of biomass present at the producer level is higher than at the top carnivore level.
EXAMPLE 2: INVERTED: The pyramid of biomass is inverted in a pond ecosystem as the biomass of fishes far exceeds the biomass of zooplankton (upon which they feed).