Chapter 13 · Question 5

Explain the flow of energy in an ecosystem. State the 10 percent law and explain why food chains rarely have more than four trophic levels.

Answer Revealed

Direct Answer:

Energy flows unidirectionally through an ecosystem: sunlight

\to

producers

\to

herbivores

\to

carnivores

\to

top carnivores. It never flows backwards — energy captured by producers does not return to the sun, and energy passed to consumers does not flow back to previous levels. At each trophic level, energy is lost as heat through respiration, metabolism, and undigested matter. According to the 10 percent law, only about 10% of the food energy at one trophic level becomes available as body mass for the next level. Since energy diminishes so sharply — after the fourth level, negligible usable energy remains — food chains rarely exceed three or four trophic levels.

Simple Explanation

Energy in an ecosystem is like money that gets spent at each step. Sunlight hits green plants, which capture only about 1% of it to make food. When a deer eats plants, only 10% of the plant's energy goes into building the deer's body — the rest is used for living or lost as heat. When a tiger eats the deer, again only 10% of the deer's energy reaches the tiger. By the fourth step, there is barely any energy left. That is why you will not see very long food chains in nature — the energy runs out.

Exam-Ready Structure

Energy flow in an ecosystem is governed by two fundamental principles: it is unidirectional, and it decreases at each successive trophic level — both explained through the 10 percent law. 1. Unidirectional flow: Energy flows in one direction only — from the Sun to producers, then to herbivores, carnivores, and finally top carnivores (NCERT Figure 13.4). Energy captured by autotrophs from sunlight does not revert back to the solar input. Similarly, energy that passes from herbivores to carnivores does not flow back. Once energy is transferred, it is no longer available to the previous trophic level. 2. Energy loss at each trophic level: At every transfer from one trophic level to the next, a large fraction of energy is lost — used for respiration, metabolic activities, maintenance of body temperature, locomotion, and as undigested material in faeces. Only a small fraction is stored as body tissue (biomass) and made available to the next trophic level. 3. The 10 percent law: On average, only about 10% of the food eaten at a trophic level is turned into the consumer's body mass and becomes available for the next level of consumers. This means: if producers fix 1000 kJ of solar energy, primary consumers get about 100 kJ, secondary consumers get about 10 kJ, tertiary consumers get about 1 kJ, and top carnivores get only about 0.1 kJ. 4. Why food chains rarely exceed three or four trophic levels: Because of the progressive energy loss at each step — after four trophic levels, the amount of usable energy has diminished so drastically that it can no longer support another level of consumers. Any organism trying to occupy a fifth trophic level would receive too little energy to survive and reproduce. 5. Starting point — how much energy do producers capture? Green plants in a terrestrial ecosystem capture only about 1% of the solar energy falling on their leaves and convert it into food energy. The remaining 99% is reflected, transmitted, or unused. This is why the population is largest at the lowest trophic levels (producers are most numerous) and decreases at each higher level.

Key Points

Energy flow is unidirectional: Sun $\rightarrow$ Producers $\rightarrow$ Herbivores $\rightarrow$ Carnivores $\rightarrow$ Top carnivores — no reversal
Energy is lost at each trophic level as heat, respiration, and undigested matter
10 percent law: only about 10% of food energy at one trophic level becomes available as body mass for the next level
Producers capture only about 1% of incident solar energy
Food chains rarely exceed 3-4 trophic levels because negligible usable energy remains after the fourth level
Population size is largest at the lowest trophic level and decreases at each higher level due to energy limitation

Common Mistakes

Thinking that energy cycles in the ecosystem like nutrients do — energy flow is strictly unidirectional
Confusing the 10% law with '10% lost' — actually 90% is lost at each level; only 10% is transferred

Explain the flow of energy in an ecosystem. State the 10 percent law and explain why food chains rarely have more than four trophic levels.