Energy Flow and Trophic Levels

Energy flow in ecosystems is UNIDIRECTIONAL (sun → producers → consumers → decomposers); it never flows back. It obeys the laws of thermodynamics: (1st) energy is neither created nor destroyed, only transformed; (2nd) every transfer involves heat loss, increasing entropy. Lindeman's 10% Law (1942): only ~10% of energy at one trophic level passes to the next; ~90% is lost as metabolic heat. Hence food chains rarely exceed 4-5 trophic levels. PYRAMID OF ENERGY is ALWAYS UPRIGHT (energy declines upward). Memory aid: 'Energy = always upright, never inverted.' Productivity-based numbers always shrink upward. The amount of energy decreasing at successive trophic levels is the reason carnivores are fewer than herbivores.

Three types (Charles Elton): pyramids of NUMBER, BIOMASS, and ENERGY. Pyramid of ENERGY is ALWAYS upright. Pyramid of NUMBER can be inverted (e.g., a single large tree supporting many insects and birds — tree ecosystem). Pyramid of BIOMASS can be inverted in aquatic ecosystems: phytoplankton biomass at any instant is less than that of zooplankton/fish because phytoplankton reproduce and are consumed rapidly (high turnover). Shortcut: 'ENERGY always upright; NUMBER and BIOMASS may invert.' Limitations of pyramids: they assume simple food chains, ignore the same species at multiple levels, and exclude saprophytes/decomposers though they are vital. Standing crop = biomass present at a given time, measured as fresh/dry weight or energy.

Cover the answer, recall, then check. 12 cards on energy flow and ecological pyramids for UPSC Prelims.

Q1. State the 10% law of energy transfer and who proposed it.
A1. Only about 10% of energy at one trophic level passes to the next; ~90% is lost as heat through respiration and metabolism. Proposed by Raymond Lindeman (1942).

Q2. Is energy flow in an ecosystem cyclic or one-way?
A2. Unidirectional and non-cyclic (Sun → producers → consumers → decomposers, lost as heat). Nutrients, by contrast, are recycled.

Q3. What is the ultimate energy source, and how much solar radiation do producers fix?
A3. The Sun. Producers fix only about 1–5% of the incident photosynthetically active radiation (PAR) into chemical energy.

Q4. Name the three types of ecological pyramids.
A4. Pyramid of numbers, pyramid of biomass, and pyramid of energy.

Q5. Which ecological pyramid is ALWAYS upright, and why?
A5. The pyramid of energy — because usable energy always decreases at each successive trophic level (10% law/heat loss), so it can never be inverted.

Q6. Give an example of an inverted pyramid of biomass.
A6. A pond/sea: phytoplankton (producers) have small standing biomass but reproduce rapidly, while zooplankton and fish above them carry more biomass — so the biomass pyramid is inverted.

Q7. Give an example of an inverted pyramid of numbers.
A7. A single big tree (1 producer) supports many herbivorous insects and even more parasites on them — the number pyramid takes a spindle/inverted shape.

Q8. Why do food chains rarely exceed 4–5 trophic levels?
A8. Because ~90% of energy is lost at each transfer, leaving too little energy to sustain higher levels.

Q9. Define trophic level with the standard sequence.
A9. Feeding position in a food chain: T1 producers → T2 primary consumers (herbivores) → T3 secondary consumers → T4 tertiary consumers, plus decomposers acting across levels.

Q10. What does NPP represent in energy terms?
A10. Net primary productivity = energy stored by producers after their own respiration; it is the energy available to be passed to herbivores and higher levels.

Q11. In most terrestrial ecosystems, which food chain carries more energy — grazing or detritus?
A11. The detritus (decomposer) pathway usually carries more energy than the grazing food chain in terrestrial and shallow-water ecosystems.

Q12. What limits productivity in open oceans despite their size?
A12. Nutrient scarcity (especially nitrogen, phosphorus, iron) and light penetration limits, giving low NPP per unit area even though total ocean NPP is large.

Worked Example

Problem/Question: With reference to energy flow in ecosystems, consider the following statements:

1. Energy flow in an ecosystem is unidirectional.
2. According to the 10% law of Lindeman, only about 10% of energy is transferred from one trophic level to the next.
3. The pyramid of energy in any ecosystem is always inverted.
   Which of the statements given above are correct?
   (a) 1 and 2 only (b) 2 and 3 only (c) 1 and 3 only (d) 1, 2 and 3

Solution/Model answer:

• Statement 1: Energy flow is unidirectional (sun → producers → consumers; it is not recycled). Correct.
• Statement 2: Lindeman's 10% law states ~10% of energy passes to the next trophic level. Correct.
• Statement 3: The pyramid of energy is always upright (never inverted), because energy decreases at each higher level. Statement 3 is wrong.
  Eliminate options with 3: (b), (c), (d) drop. Only (a) remains.

Answer/Takeaway: (a) 1 and 2 only.