Energy flow in an ecosystem
Sunlight fixed by producers flows one way to consumers. Only about 10 per cent of energy passes to each next trophic level; most of the rest is lost as heat.
The source: the Sun
Except for deep-sea hydrothermal ecosystems, the Sun is the only source of energy for all ecosystems on Earth. Of the sunlight falling on Earth, less than 50 per cent is photosynthetically active radiation (PAR): the part plants can use. Plants and photosynthetic bacteria (autotrophs) fix the Sun's radiant energy to make food from simple inorganic materials. Plants capture only 2 to 10 per cent of the PAR, yet this small amount sustains the entire living world.
One-way flow of energy
At every step, much of the energy is used in respiration and lost as heat. Heat cannot be passed back, so energy flow is unidirectional.
Why energy flow is one-way
- Energy moves from the Sun to producers and then to consumers. It never returns to the Sun or to the producers.
- First law of thermodynamics: energy is neither created nor destroyed, only changed from one form to another (light to chemical to heat). An ecosystem only converts energy.
- Second law of thermodynamics: in every transfer, some energy becomes unusable heat. So ecosystems need a constant supply of energy to build their molecules and resist the natural drift towards disorder.
- No energy stays in an organism forever. It is either passed on to a consumer, or the organism dies and its energy enters the detritus food chain.
The 10 per cent law
Only about 10 per cent of the energy at one trophic level reaches the next. The rest is used in the organism's own life processes (respiration) and lost as heat, or stays in uneaten and dead parts. (Extra reading, not in your textbook: the law is usually credited to the ecologist Lindeman, 1942.) It is why the number of trophic levels in a grazing food chain is limited: producer, herbivore, primary carnivore and secondary carnivore. A higher level would get too little energy to survive.
Worked example: the 10 per cent law
The plants in a grassland fix 50,000 J of energy. Using the 10 per cent law, how much energy reaches (a) grasshoppers, (b) frogs that eat them, and (c) snakes that eat the frogs?
- Each step passes on about 10%, so multiply by 0.1 at each level.
- (a) Grasshoppers (herbivores): 50,000 × 0.1 = 5,000 J.
- (b) Frogs (primary carnivores): 5,000 × 0.1 = 500 J.
- (c) Snakes (secondary carnivores): 500 × 0.1 = 50 J.
5,000 J, 500 J and 50 J. The snakes get only 0.1% of the energy fixed by the plants.
A vegetarian meal of rice and dal eats at the first trophic level. It uses the Sun's energy far better than meat, which has already lost about 90% at each step. That is one reason plant-based diets can feed more people from the same farmland.
1 marks: “What share of incident solar radiation is photosynthetically active radiation?” Less than 50 per cent.
2 marks: “If producers trap 20,000 J of energy, how much will reach a secondary consumer by the 10 per cent law?” Primary consumer: 20,000 × 0.1 = 2,000 J. Secondary consumer: 2,000 × 0.1 = 200 J.
3 marks: “Why is the flow of energy in an ecosystem said to be unidirectional? Explain with the 10 per cent law.” Energy passes from the Sun to producers to consumers and never back. At each transfer much is lost as heat (second law of thermodynamics), so only about 10% reaches the next level. Heat cannot be reused by producers.
Ten per cent passes on, the rest is heat: every step up the chain, knock off a zero.