Ecological Pyramids

• An ecological pyramid is a graphic representation of the relationship between organisms of various successive trophic levels with respect to energy, biomass, and number.
• It has a broad base that gradually narrows upward, forming an upright cone-like structure.
• This concept was developed by C. Elton in 1927.
• The base of each pyramid represents the producers or first trophic level, while the apex represents the tertiary or top-level consumer.
• Any calculation of energy content, biomass, or number must include all organisms present at that trophic level.

The three ecological pyramids usually studied are:

1. Pyramid of numbers

• It represents the relative number of individuals per unit area at different trophic levels.
• In most well-balanced ecosystems, the pyramid of numbers is upright because producers are more numerous than herbivores, and herbivores are more numerous than carnivores.
• It can sometimes be inverted.
• Example: if the number of insects on a single tree, smaller birds feeding on those insects, and parasites on those birds are plotted, the result is an inverted pyramid of numbers.

(a) Pyramid of numbers in a grassland ecosystem. Only three top-carnivores are supported in an ecosystem based on the production of nearly 6 million plants

2. Pyramid of biomass

• It represents the biomass per unit area at different trophic levels.
• In most well-balanced ecosystems, the pyramid of biomass is upright.
• An exception occurs in oceanic ecosystems, which show an inverted biomass pyramid.
• A small standing crop of phytoplankton can support a large standing crop of zooplankton.

(b) Pyramid of biomass shows a sharp decrease in biomass at higher trophic levels

(c) Inverted pyramid of biomass-small standing crop of phytoplankton supports large standing crop of zooplankton

3. Pyramid of energy

• It represents the amount of accumulated energy per unit area at different trophic levels.
• The pyramid of energy is always upright.
• It can never be inverted because when energy flows from one trophic level to the next, only 10% passes on as net energy.
• A large amount of energy is always lost as heat at each step.
• In smaller food chains, more energy is available than in longer food chains.

 (d) An ideal pyramid of energy. Observe that primary producers convert only 1% of the energy in the sunlight available to them into NPP

• In most ecosystems, all three pyramids - number, biomass, and energy - are upright.
• This means producers are more numerous and have more biomass than herbivores, and herbivores are more numerous and have more biomass than carnivores.
• Also, energy at a lower trophic level is always more than at a higher trophic level.

• Ecological pyramids assume a simple food chain, which almost never exists in nature.
• They do not accommodate a food web.
• Saprophytes are not included in ecological pyramids, even though they play a vital role in the ecosystem.
• A given species may occupy more than one trophic level in the same ecosystem at the same time.
• Example: a sparrow is a primary consumer when it eats seeds, fruits, and peas, and a secondary consumer when it eats insects and worms.

• Ecological pyramids graphically display trophic levels, placing producers at the broad base and top consumers at the apex.
• The pyramid of numbers is typically upright, though it can become inverted in a single-tree ecosystem that supports many insects and parasites.
• The pyramid of biomass is generally upright, except in oceanic ecosystems where it is inverted (small phytoplankton supporting larger zooplankton).
• The energy pyramid is always upright because only 10% of energy is successfully transferred upward to the next trophic level.
• As a general rule in standard terrestrial ecosystems, producers naturally exceed consumers in both population count and total biomass.
• These models have major limitations: they assume simple linear food chains, ignore complex food webs, exclude decomposers, and cannot handle species that occupy multiple trophic levels.