Energy Flow and Decomposition in Ecosystems
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Direct answer
Energy enters an ecosystem as sunlight fixed by producers and flows unidirectionally through herbivores and carnivores, with roughly 10 per cent passing to each successive trophic level and the rest dissipated as heat — which is why food chains stay short. Decomposition runs the parallel track: saprotrophic bacteria and fungi break down detritus through fragmentation, leaching, catabolism, humification and mineralisation, returning nutrients to the soil for reuse. Energy flow is a one-way street while nutrient movement is cyclic, and the rate of decomposition depends on detritus quality and climate.
What you must remember
- The energy source: the sun; green plants trap solar energy in food through photosynthesis, forming the base of every grazing food chain.
- Unidirectional flow: energy moves from sun to producers to consumers and is never recycled — it leaves the system as heat, in keeping with the laws of thermodynamics.
- Ten per cent law: only about 10 per cent of the energy at one trophic level transfers to the next; the rest is lost to respiration and heat, limiting chains to about four or five levels.
- Food chains and webs: the grazing food chain starts with living plants, the detritus food chain with dead organic matter; interlocking chains form food webs that buffer species loss.
- Decomposers: saprotrophs, mainly bacteria and fungi, secreting enzymes over dead matter.
- Steps of decomposition: fragmentation by detritivores such as earthworms; leaching of soluble nutrients into soil; catabolism by microbial enzymes; humification forming dark, resistant humus; and mineralisation, which releases inorganic nutrients from humus.
- Controls: decomposition is faster when detritus is nitrogen-rich (low carbon to nitrogen ratio) and slower when material is lignin-rich; warmth, moisture and good aeration speed it, while waterlogging and cold retard it.
Common confusion
Energy flow and nutrient cycling are opposites in direction — energy is unidirectional and dissipated, nutrients are recycled through decomposition. GPP is often confused with NPP: net primary productivity equals gross primary productivity minus the producers' respiration. Among decomposition steps, humification and mineralisation occur in the soil and are the slowest, while fragmentation is physical, not enzymatic.
Exam-focused takeaway
NEET-UG tests the 10 per cent law in numerical form — computing energy available at the third or fourth trophic level — and asks the order and definition of the five decomposition steps. Statement items contrast grazing and detritus food chains, energy flow with nutrient cycling, and the conditions that accelerate or retard decay. Assertion-reason stems link short food chains with energy loss and humus with nutrient storage.
Frequently asked questions
Why is energy flow called unidirectional?
Because energy passes from sun to producers to consumers and exits as heat; it is never returned to earlier trophic levels for reuse.
What does the ten per cent law state?
Only about 10 per cent of the energy stored at one trophic level is transferred to the next, the rest being lost as heat through respiration.
Which organisms carry out decomposition?
Saprotrophic bacteria and fungi, which secrete digestive enzymes onto dead organic matter and absorb the breakdown products.
What are the steps of decomposition?
Fragmentation of detritus, leaching of soluble nutrients, catabolism by microbial enzymes, humification into humus, and mineralisation releasing inorganic nutrients.
What factors speed up decomposition?
A warm, moist, well-aerated soil and nitrogen-rich detritus; cold, waterlogged conditions and high lignin content slow it down.