Why Only a Tenth of the Energy Passes Up the Food Chain

Every living thing needs energy, and almost all of it starts as sunlight captured by plants. As that energy passes from plants to herbivores to carnivores, most of it is lost at each step — which is why food chains are short and top predators are rare.

This part covers energy flow, food chains and food webs, ecological pyramids, and why some pyramids are inverted.

Energy flows in one direction, from the sun to producers and then to consumers, and cannot be recycled; plants capture only a small part of the photosynthetically active radiation reaching them, and on average only about 10 per cent of the energy at one trophic level passes to the next.

One-way flow:

- Energy passes from producers to herbivores to carnivores
- At every step much of it is lost as heat in respiration, so it cannot flow back

Photosynthetically active radiation (PAR):

- Less than 50 per cent of incident solar radiation is PAR
- Plants capture only 2 to 10 per cent of the PAR, yet this sustains the entire living world

The 10 per cent law. Only about 10 per cent of the energy at one trophic level is transferred to the next higher level.

Worked example. If producers in a grassland fix $10000$ J of energy:

$$\text{herbivores}=1000\text{ J},\text{primary carnivores}=100\text{ J},\text{secondary carnivores}=10\text{ J}$$

An everyday example. A meal of dal and rice passes on far more of the sun's original energy than meat from animals that were fed on the grain.

The substance. Energy is lost, not destroyed — it leaves each level mostly as heat, which living things cannot use again.

A grazing food chain begins with green plants eaten by herbivores, while a detritus food chain begins with dead organic matter used by decomposers; food chains interconnect into food webs, standing crop is the living biomass at a trophic level, and standing state is the amount of nutrients in the soil at a given time.

Grazing food chain (GFC):

- Begins with producers, as in grass, goat and human
- In aquatic ecosystems, the GFC is the main route for energy flow

Detritus food chain (DFC):

- Begins with dead organic matter, used by decomposers such as fungi and bacteria
- In terrestrial ecosystems, a much larger fraction of energy flows through the DFC than the GFC

Food web. Food chains interconnect naturally, because most organisms eat more than one kind of food.

Trophic levels:

- First — producers
- Second — herbivores, the primary consumers
- Third — primary carnivores, the secondary consumers
- Fourth — secondary carnivores, the tertiary consumers

Standing crop and standing state:

- Standing crop — the mass of living material at a trophic level at a particular time, as biomass or number per unit area
- Standing state — the amount of nutrients present in the soil at a given time

An everyday example. In a paddy field, a frog eats insects and a snake eats the frog, while birds also eat the insects — a small part of a food web.

The substance. An organism can occupy more than one trophic level — a sparrow eating seeds is a primary consumer, but eating insects makes it a secondary consumer.

Ecological pyramids show how trophic levels compare in number of individuals, biomass or energy, with producers at the base and top carnivores at the apex; most land ecosystems give upright pyramids.

Pyramid of number:

- Counts individuals at each level
- Upright in a grassland

Pyramid of biomass:

- Shows the dry weight of living material per unit area
- Upright in most land ecosystems such as forests and grasslands

Pyramid of energy:

- Shows the energy at each level per unit area per unit time
- Always upright, because energy is lost at every transfer

An everyday example. A grassland near a village, with lots of grass, fewer grasshoppers, fewer frogs and only a few snakes, gives a classic upright pyramid of number.

The substance. A pyramid of biomass records standing crop at one moment, not the rate at which biomass is produced.

A pyramid of number is inverted when one large producer, such as a tree, supports many small consumers, and a pyramid of biomass is inverted in water because tiny phytoplankton multiply and are eaten so fast that their standing crop stays small; the pyramid of energy is always upright because energy is lost at every trophic level.

Inverted pyramid of number:

- One large tree supports many insects, which support fewer birds

Inverted pyramid of biomass in the sea:

- At any moment, the biomass of fish exceeds that of phytoplankton
- Phytoplankton reproduce and are consumed so rapidly that their standing crop stays small, even though their productivity is high

Pyramid of energy — always upright:

- Part of the energy is lost as heat at every transfer, so it can never increase up the chain

Limitations of ecological pyramids:

- They ignore species that belong to two or more trophic levels
- They assume a simple food chain, which rarely exists in nature, and cannot show a food web
- Saprophytes are given no place, despite their vital role

An everyday example. A single mango tree in a courtyard, covered in caterpillars and visited by a few birds, forms an inverted pyramid of number.

The substance. An inverted biomass pyramid does not break the energy rule — the small phytoplankton biomass is replaced so quickly that it still passes enough energy upward.