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Ecosystem - Ecological Pyramids

Grade 12CBSEBiology

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Ecological pyramids are graphical representations of the relationship between different organisms in an ecosystem at various trophic levels, starting with producers at the base (T1T_1) and top carnivores at the apex.

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The Pyramid of Number represents the total number of individuals at each trophic level. It is usually upright in a grassland ecosystem but can be inverted in a tree ecosystem where one large producer supports many herbivores (e.g., 11 tree supporting 10001000 insects).

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The Pyramid of Biomass represents the total fresh or dry weight of living matter. In terrestrial ecosystems, it is typically upright. However, in aquatic ecosystems, it is often inverted because the biomass of phytoplanktons (T1T_1) is smaller than the biomass of zooplanktons and fishes (T2,T3T_2, T_3) due to higher turnover rates.

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The Pyramid of Energy is always upright and can never be inverted. This follows the laws of thermodynamics, where energy is lost as heat at every transfer. Only about 10%10\% of energy is transferred from one trophic level to the next.

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Limitations of ecological pyramids: They do not take into account the same species belonging to two or more trophic levels, they assume a simple food chain (which rarely exists), and they do not include saprophytes/decomposers in the model.

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Net Primary Productivity (NPPNPP) is the rate of storage of organic matter in excess of the respiratory utilization by plants: NPP=GPP−RNPP = GPP - R.

📐Formulae

10% Law: En+1=En×0.1010\%\text{ Law: } E_{n+1} = E_n \times 0.10

NPP=GPP−RNPP = GPP - R

Ecological Efficiency=Energy converted to biomass at TnEnergy consumed from Tn−1×100\text{Ecological Efficiency} = \frac{\text{Energy converted to biomass at } T_n}{\text{Energy consumed from } T_{n-1}} \times 100

💡Examples

Problem 1:

In a food chain consisting of Grass →\rightarrow Grasshopper →\rightarrow Frog →\rightarrow Snake, if the energy available at the producer level (T1T_1) is 2,000 J2,000\text{ J}, how much energy will be available to the Snake (T4T_4)?

Solution:

According to the 10%10\% law:

  1. Energy at T1T_1 (Grass) = 2,000 J2,000\text{ J}
  2. Energy at T2T_2 (Grasshopper) = 2,000×0.10=200 J2,000 \times 0.10 = 200\text{ J}
  3. Energy at T3T_3 (Frog) = 200×0.10=20 J200 \times 0.10 = 20\text{ J}
  4. Energy at T4T_4 (Snake) = 20×0.10=2 J20 \times 0.10 = 2\text{ J}

Explanation:

Lindeman's 10%10\% Law states that only 10%10\% of the energy from one trophic level is transferred to the next, while 90%90\% is lost as heat during respiration and other metabolic processes.

Problem 2:

Explain why the pyramid of biomass in a sea is generally inverted.

Solution:

In a marine ecosystem, the biomass of producers (phytoplankton) is often much lower than the biomass of consumers (zooplankton and fish) at any given point in time. The relationship can be expressed as: Biomass(T1)<Biomass(T2)Biomass(T_1) < Biomass(T_2).

Explanation:

Even though the phytoplankton produce energy rapidly, they have a very short life span and high turnover rate, meaning their standing crop biomass is small compared to the longer-lived consumers that feed on them.

Ecological Pyramids - Revision Notes & Key Diagrams | CBSE Class 12 Biology