Energy flow and productivity: food chains, food…

Core facts

Primary productivity: the rate of biomass production by plants, per unit area per unit time.
GPP (gross primary productivity): the total rate of photosynthesis.
NPP = GPP − R, where R is the energy plants use in respiration. NPP is the biomass available to heterotrophs (herbivores and decomposers).
Secondary productivity: the rate of formation of new organic matter by consumers.
Standing crop: the mass of living material at a trophic level at a given time.
Productivity units: g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Biomass is measured as dry weight.
Annual NPP of the whole biosphere is about 170 billion tonnes (dry weight) of organic matter. Oceans, despite covering about 70% of the surface, produce only about 55 billion tonnes.
The Sun is the energy source of all ecosystems, except deep-sea hydrothermal vent ecosystems (chemosynthesis).
Energy flow is unidirectional (one-way), not cyclic. Nutrients cycle; energy does not.
Food chains:
- Grazing food chain (GFC): starts with living green plants.
- Detritus food chain (DFC): starts with dead organic matter (detritus).
In terrestrial ecosystems, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.
Food web: interconnected food chains.

Explained

How much sunlight is used

Less than 50% of incident solar radiation is photosynthetically active radiation (PAR).
Plants capture only 2–10% of PAR. That small amount sustains the whole living world.
Ecosystems obey the second law of thermodynamics. They need a constant supply of energy to build complex molecules and to counter the natural tendency towards disorder (entropy).

Trophic levels

Trophic level	Organisms	Example
First	Producers	Grass, phytoplankton
Second	Primary consumers (herbivores)	Grasshopper, deer, zooplankton
Third	Secondary consumers (carnivores)	Frog, small fish
Fourth	Tertiary consumers (top carnivores)	Snake, large fish, eagle

A species can sit at more than one trophic level. A sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects. This is why a food web is more realistic than a food chain.
The number of trophic levels is limited (usually 4–5) because energy falls sharply at each step (about 10% passes on; see ENV-EC-04).

Decomposition: the five steps

Step	What happens
Fragmentation	Detritivores such as earthworms break detritus into small particles
Leaching	Water-soluble inorganic nutrients go down into the soil and are precipitated as unavailable salts
Catabolism	Bacterial and fungal enzymes break detritus into simpler inorganic substances
Humification	A dark, amorphous, colloidal substance called humus forms. It is highly resistant to microbial action and decomposes very slowly
Mineralisation	Humus is degraded by microbes, releasing inorganic nutrients

What speeds up or slows down decomposition

Faster: detritus rich in nitrogen and water-soluble substances such as sugars; warm and moist conditions.
Slower: detritus rich in lignin and chitin; low temperature and anaerobiosis (lack of oxygen). This is why organic matter builds up in cold or waterlogged places.

Deeper

Lindeman (1942) set out the trophic-dynamic view of ecosystems: energy is transferred between trophic levels with large losses at each step. This led to the "ten per cent law".
Why a larger fraction goes through the DFC on land: most plant matter (wood, leaf litter, roots) is not eaten while alive. It dies and enters the detritus pathway. In many aquatic systems, phytoplankton are grazed quickly, so the grazing chain carries a larger share.
Productivity varies widely. It depends on the plant species, the environment, nutrients and photosynthetic capacity. Tropical rain forests, estuaries, coral reefs and wetlands are among the most productive. Deserts and the open ocean are among the least, per unit area.
Food webs give stability. When a consumer has several food sources, the loss of one does not collapse the system. Simple chains (for example, in the Arctic tundra) are fragile.
Biomagnification: the rise in concentration of a non-degradable (persistent) substance at successive trophic levels. The substance is not broken down or excreted, so it builds up in fat.
- Classic example: DDT reached high levels in fish-eating birds, causing thin eggshells and breeding failure. Rachel Carson's Silent Spring (1962) drew attention to it.
- Bioaccumulation is the build-up within one organism over its lifetime; biomagnification is the increase across trophic levels.
Breaking a food chain has costs. In the 1980s India exported frog legs on a large scale. With fewer frogs eating insects, crop pests increased and farmers used more pesticides. The Government then banned the export of frog legs.

Common traps

✗ "Energy flow in an ecosystem is cyclic." ✓ It is unidirectional. Only nutrients cycle.
✗ "The grazing food chain begins with dead organic matter." ✓ The detritus food chain does. The grazing chain begins with living plants.
✗ "NPP = GPP + R." ✓ NPP = GPP − R.
✗ "Oceans produce most of the world's NPP because they cover 70% of Earth." ✓ Oceans produce only about 55 of the 170 billion tonnes.
✗ "Low temperature and anaerobic conditions speed up decomposition." ✓ They slow it. Warm, moist, oxygen-rich conditions speed it up.
✗ "Humification releases nutrients." ✓ Mineralisation releases nutrients; humification forms humus, which resists decay.

Quick revision

NPP = GPP − R; NPP feeds the heterotrophs.
Secondary productivity = new organic matter made by consumers.
Biosphere NPP about 170 billion t; oceans about 55 billion t.
PAR is under 50% of sunlight; plants capture 2–10% of PAR.
Energy: one-way; nutrients: cyclic.
On land, more energy flows through the DFC than the GFC.
Decomposition: fragmentation, leaching, catabolism, humification, mineralisation.
Lignin, chitin, cold and no oxygen slow decay.
Biomagnification: persistent chemicals (DDT) rise up the food chain.
Hydrothermal vents: the exception to sunlight-based ecosystems.

Go deeper: Mains writing (beyond what APPSC Group 1 usually asks)

Mains writing Mains writing

Frame: energy flow sets the limits of what an ecosystem can support. Every extra trophic step costs about 90% of the energy, which links ecology to food security and diet.
Analysis points:
1. Diet and land use: a plant-based diet feeds more people per hectare, because eating one level lower avoids the large loss at each transfer.
2. Detritus pathway and soil health: burning crop residue cuts off the detritus chain that builds soil organic carbon.
3. Persistent pollutants (DDT, mercury, PCBs) concentrate in top predators and in people who eat fish.
4. Removing one link (frogs, vultures, sharks) causes cascades through the web.
Way forward:
- in-field residue management and composting instead of burning
- phase out persistent organic pollutants (Stockholm Convention)
- integrated pest management that keeps natural predators
- protect detritus-based systems such as mangroves and wetlands
Example: the crash of vultures in India after cattle were treated with diclofenac shows a broken food chain: carcasses lay uneaten and feral dog numbers rose. Veterinary diclofenac was banned in 2006.

Medha AI writes its own notes and checks them against the sources listed. Medha AI is an independent preparation service. It is not affiliated with, endorsed by or connected to APPSC, TSPSC, MPSC, UPSC or any other commission or government body.

2.5.1.3 Energy flow and productivity: food chains, food webs, trophic levels and decomposition

Download this PDF