Ecological Pyramids
The steps of trophic levels in an ecosystem, when represented diagrammatically, are called ecological pyramids.
- The base of the pyramid is always formed by the producers, and the tip is occupied by the top carnivores.
- Intermediate levels are occupied by various consumers.
- Each horizontal bar in the pyramid represents a trophic level, with its length proportional to the number, biomass, or energy at that level.
- As we move up the pyramid, the number, biomass, and energy decrease progressively, giving it a characteristic pyramid shape.
Ecological pyramids are of three types:
- Pyramid of Numbers
- Pyramid of Biomass
- Pyramid of Energy
- The study of trophic level interactions helps explain how energy passes through the ecosystem, linking producers, consumers, and decomposers in a sequence of nutritional relationships.
1. Pyramid of Numbers
- This pyramid shows the number of individuals present at each trophic level in an ecosystem. It is a graphic representation of population count at each level.
(a) Upright Pyramid of Numbers
- Seen in grassland ecosystems.
- The number of individuals decreases progressively from producers to top carnivores.
- Example: Grass → Grasshopper → Rat → Snake → Hawk
(b) Inverted Pyramid of Numbers
- Seen in forests.
- A few large producers (trees) support a large number of herbivores (birds), which in turn support even more parasites and hyper-parasites.
- Hence, the pyramid becomes inverted.
2. Pyramid of Biomass
- To account for size differences among organisms, biomass (dry weight) is used instead of numbers. Biomass is expressed as g/m².
(a) Upright Pyramid of Biomass
- Seen in terrestrial ecosystems.
- Biomass decreases from producers → herbivores → carnivores → top carnivores.
(b) Inverted Pyramid of Biomass
- Seen in aquatic ecosystems.
- Here, tiny phytoplankton (producers) reproduce rapidly and sustain larger biomass of herbivores and carnivores, resulting in an inverted shape.
3. Pyramid of Energy
- This pyramid shows the flow of energy through different trophic levels and is always upright.
- Reflects the laws of thermodynamics — energy is lost as heat at each level, leaving less for the next level.
- Example: Out of 1000 calories of sunlight received by plants, only 100 calories are stored as plant biomass, 10 calories by herbivores, and 1 calorie by carnivores.
The energy pyramid also explains biological magnification — the tendency of toxic substances to accumulate at higher levels of the food chain.
