Energy flow

1. Foundations: energy, ecosystems and thermodynamics

An ecosystem consists of a biological community interacting with its physical environment. Its functioning requires both energy and matter. Energy supports maintenance, growth, reproduction and movement, while nutrients provide the materials needed to build organisms. Most ecosystems receive energy as solar radiation. Green plants, algae and cyanobacteria capture a fraction of this radiation through photosynthesis and store it as chemical energy in organic compounds. Consumers obtain chemical energy by feeding on other organisms or their products.

Photosynthetically active radiation, or PAR, broadly spans wavelengths of 400–700 nanometres. NCERT states that less than 50% of incident solar radiation is PAR and that plants capture only about 2–10% of PAR. These figures refer to different stages of energy capture and should not be confused with the 10% rule governing transfer between trophic levels. Radiation that is reflected, transmitted or unsuitable for photosynthesis does not enter the food web through photosynthetic production.

Energy flow follows the laws of thermodynamics. The first law states that energy is neither created nor destroyed, but changes form. Photosynthesis converts light energy into chemical energy. Under the second law, energy transformations increase overall entropy, and some energy becomes dispersed as heat unavailable for further biological work. Consequently, ecosystems require continuing energy input. Heat is not converted back into food by producers. Deep-sea hydrothermal-vent communities illustrate an important exception to direct solar dependence: chemoautotrophic microorganisms obtain energy by oxidising inorganic substances such as hydrogen sulphide.

Energy pathway: external source → chemical energy in biomass → biological work and heat.
Matter pathway: environmental nutrient pool → organisms → environmental nutrient pool.
An ecosystem is an open system, exchanging energy and often matter with its surroundings.

2. Primary productivity and the energy available to consumers

Primary production is the formation of organic matter by autotrophs. Productivity is its rate, commonly expressed as grams of dry matter per square metre per year or kilocalories per square metre per year. Gross primary productivity, or GPP, is the total rate at which producers fix energy through photosynthesis. Producers use some of this fixed energy in cellular respiration. The remainder is net primary productivity, or NPP: NPP = GPP − R, where R denotes producers’ respiratory losses.

NPP represents the rate of addition of producer biomass potentially available to herbivores and decomposers. It is not the amount necessarily consumed by herbivores: much plant material enters litter and detritus, remains uneaten or is exported elsewhere. Secondary productivity is the rate of formation of new organic matter by heterotrophic consumers. It must be distinguished from consumption because ingested food also supports respiration or leaves the organism as unassimilated material and excretory products.

Productivity depends on plant species, light, temperature, water, nutrient availability and photosynthetic capacity. Tropical forests and many wetlands have high annual productivity per unit area, whereas deserts and much of the open ocean have lower values. Nevertheless, the open ocean contributes substantially to global production because of its enormous area. Standing crop is the biomass present at a particular time; productivity measures formation over time. A small, rapidly renewed standing crop can therefore sustain substantial energy transfer, as in phytoplankton-based aquatic systems.

Example: if GPP is 2,000 g m−2 yr−1 and producer respiration is 1,200 g m−2 yr−1, NPP is 800 g m−2 yr−1.
Do not equate high standing biomass automatically with high current productivity.

Major energy pathways

1. Solar energy is captured by photosynthetic producers.
2. Gross primary production is partitioned between producer respiration and net primary production.
3. Living producer biomass supports grazing consumers.
4. Dead biomass and wastes from all trophic levels support detrital food webs.
5. Respiration across producers, consumers and decomposers ultimately dissipates energy as heat.

3. Food chains, food webs and detrital pathways

A trophic level is an organism’s feeding position in an energy-transfer sequence. Producers occupy the first trophic level, herbivores the second, primary carnivores the third and higher carnivores subsequent levels. A food chain is a simplified linear sequence, such as grass → grasshopper → frog → snake. In an energy-flow diagram, arrows point from the food resource towards the organism receiving its energy. They therefore indicate the direction of energy transfer rather than the direction in which a predator attacks.

Natural feeding relationships form food webs because most organisms use more than one food source. An omnivore may occupy different trophic levels depending on what it eats: a bird feeding on seeds acts as a primary consumer, but when eating herbivorous insects it acts as a secondary consumer. Alternative pathways may buffer some disturbances, although greater food-web complexity does not guarantee stability under every condition.

The grazing food chain begins with living producers eaten by herbivores. The detritus food chain begins with dead organic matter, including fallen leaves, carcasses and wastes. Detritivores such as earthworms fragment this material, while bacteria and fungi carry out much of its enzymatic breakdown. These pathways are interconnected. A large share of production in many terrestrial ecosystems enters the detrital pathway, whereas grazing commonly has a more prominent role in aquatic energy transfer. Decomposers process material originating from several trophic levels and cannot simply be assigned one universal final trophic level.

Decomposition releases mineral nutrients that producers can reuse.
Energy in detritus supports decomposers and detritivores before being dissipated through respiration.

Distinguishing ecological pyramids
Pyramid	Quantity measured	Possible shape	Illustration
Energy	Energy flow per area per time	Always upright	Progressively less production available at higher trophic levels
Biomass	Standing biomass per area or volume	Upright or inverted	Small phytoplankton standing crop sustaining larger consumer biomass
Numbers	Number of organisms	Upright, inverted or irregular	One tree supporting many herbivorous insects

4. Transfer efficiency and ecological pyramids

Trophic-transfer efficiency is the percentage of production at one trophic level converted into production at the next. The commonly taught 10% rule assumes that about one-tenth passes onward. If producer production contains 10,000 energy units, a simplified sequence gives 1,000 units to herbivores, 100 to primary carnivores and 10 to secondary carnivores. Actual efficiencies vary with food quality, digestibility, physiology, temperature and ecosystem conditions.

Transfer is incomplete because not all biomass is eaten, not all consumed food is assimilated, and assimilated energy is partly used in respiration rather than growth. Uneaten biomass and wastes may support detrital food webs, so they are not necessarily lost immediately from the whole ecosystem. However, energy dispersed as respiratory heat is unavailable for subsequent trophic transfer. Declining energy availability helps explain why long food chains and large populations of top predators are uncommon.

An energy pyramid represents energy flow per unit area per unit time and is always upright. A biomass pyramid records standing mass and can be inverted in aquatic ecosystems where rapidly reproducing phytoplankton sustain a larger standing biomass of consumers. A pyramid of numbers can also be inverted: one tree may support numerous insects. These alternatives do not violate thermodynamics because numbers and instantaneous biomass are not measures of energy throughput.

The 10% rule concerns trophic production, not the percentage of sunlight captured.
An inverted aquatic biomass pyramid does not imply an inverted energy pyramid.

5. Ecological significance and examination applications

Energy budgets link ecosystem productivity with carrying capacity, fisheries and wildlife conservation. Damage to producers or reductions in primary productivity can constrain the food web, while habitat loss can disconnect feeding and detrital pathways. Top predators often require extensive habitats because their food supply depends on production across several lower trophic levels. However, population size also depends on habitat quality, competition, disease and human pressures, not energy alone.

Human activities alter these pathways. Nutrient enrichment may initially increase algal production, but decomposition of excessive organic matter can consume dissolved oxygen and cause hypoxia. Harvesting, grazing and residue removal export organic matter that might otherwise support consumers or decomposers. Eating lower on a food chain generally involves fewer trophic-transfer losses, although real agricultural efficiency also depends on inputs, land suitability and whether livestock use biomass humans cannot eat.

For Prelims, distinguish decreasing energy availability from biomagnification. Biomagnification is an increase in the concentration of certain persistent pollutants at successive trophic levels; it is not an increase in available energy. Similarly, productivity, standing crop and organism abundance are separate variables. Identifying the quantity measured, its units and its time dimension is often sufficient to resolve questions on ecological pyramids.

Energy decreases along trophic transfers; some pollutant concentrations may increase.
Nutrient recycling cannot eliminate the need for fresh energy input.

Real-world case studies

Hydrothermal vents of the Galápagos Rift

Hydrothermal-vent communities discovered at the Galápagos Rift in 1977 demonstrated that abundant deep-sea life could be supported by chemosynthetic production. Sulphide-oxidising microorganisms support animals through symbiosis and feeding relationships. Giant tubeworms harbour symbiotic bacteria that fix carbon. The example shows that primary producers need not be green plants and that primary production need not use sunlight directly.

Leaf litter in Western Ghats forests

In Western Ghats forests, fallen leaves, wood and other organic residues support fungi, bacteria and soil fauna. Decomposition returns nutrients to soil while organic matter supplies energy to detrital communities. Warm, moist conditions generally favour decomposition, although litter chemistry and local conditions modify its rate. Nutrients can return to plants, but the energy used by decomposers is progressively dissipated as heat.

Previous year questions

No UPSC question has been asked directly on this micro-topic yet. Use the practice questions below.

Practice questions

Practice MCQ 1

An ecosystem has gross primary productivity of 2,400 kJ m−2 yr−1 and producer respiration of 900 kJ m−2 yr−1. Assuming 10% transfer of net primary production into herbivore production, what is the herbivore production?

A. 90 kJ m−2 yr−1
B. 150 kJ m−2 yr−1
C. 240 kJ m−2 yr−1
D. 330 kJ m−2 yr−1

Practice MCQ 2

Consider the following statements: 1. Rapid turnover of phytoplankton can support an inverted biomass pyramid. 2. An inverted biomass pyramid necessarily indicates an inverted energy pyramid. 3. Decomposers obtain organic matter from multiple trophic levels. Which statements are correct?

A. 1 and 2 only
B. 2 and 3 only
C. 1 and 3 only
D. 1, 2 and 3

Practice MCQ 3

Which statement best explains why ecosystems require a continuing external energy supply?

A. Energy is destroyed during cellular respiration.
B. Nutrients cannot be reused after decomposition.
C. Biological transformations progressively dissipate energy as heat.
D. Consumers convert all ingested energy into new biomass.

Mains practice · Explain why energy flow is unidirectional while nutrient movement is cyclic in ecosystems. How do grazing and detrital pathways contribute to ecosystem functioning? Answer in 150 words.

Introduce external energy capture and nutrient pools.
Use thermodynamics to explain heat dissipation and continuing energy requirements.
Distinguish GPP, producer respiration and NPP.
Explain living-producer and dead-organic-matter pathways with examples.
Show that decomposers recycle nutrients but not energy.
Conclude with the interconnected roles of producers, consumers and decomposers.