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

A village pond may look still, but it is full of activity: sunlight feeds tiny algae, small animals eat the algae, fish eat the small animals, and bacteria break down whatever dies. Energy flows through this system in one direction, while nutrients go round and round.

This lesson covers the pond ecosystem, productivity and decomposition, energy flow and food chains, and ecological pyramids.

A pond ecosystem is a self-sustaining unit made of abiotic components such as water, sunlight and dissolved nutrients and biotic components — producers, consumers and decomposers.

Abiotic components:

- Water with dissolved oxygen, carbon dioxide and inorganic and organic substances
- Sunlight, temperature, day length and other climatic conditions

Biotic components:

- Producers — phytoplankton, algae, and floating, submerged and marginal plants that photosynthesise
- Consumers — zooplankton, free-swimming and bottom-dwelling animals, and fish
- Decomposers — bacteria and fungi, especially abundant in the bottom mud, that break down dead matter

Functions:

- Producers convert inorganic materials into organic matter using sunlight
- Decomposers break down dead organisms and return minerals to the water
- Energy flows one way, from the sun through producers to consumers, while minerals are recycled

An everyday example. A temple tank or village pond in India holds all these components in a small, easy-to-observe space.

The substance. A pond is not a closed system — it exchanges energy, gases and materials with its surroundings, such as run-off from nearby land.

Gross primary productivity (GPP) is the rate at which producers make organic matter by photosynthesis, net primary productivity (NPP) is what remains after their own respiration, and decomposition breaks dead organic matter into inorganic nutrients through fragmentation, leaching, catabolism, humification and mineralisation.

Productivity:

- Primary production — the biomass produced per unit area over a time period by plants during photosynthesis
- NPP = GPP - R, where R is the energy producers lose in respiration

Worked example. If producers in a pond fix 20 000 kJ of energy per square metre over a season and use 8000 kJ in respiration,

$$\text{NPP}=20000-8000=12000\text{ kJ per square metre}$$

Steps of decomposition:

- Fragmentation — detritivores such as earthworms break detritus into smaller pieces
- Leaching — water-soluble nutrients sink into the soil and are precipitated as unavailable salts
- Catabolism — bacterial and fungal enzymes break detritus into simpler inorganic substances
- Humification — a dark, spongy layer of humus forms, resisting further breakdown
- Mineralisation — humus is slowly degraded, releasing inorganic nutrients

Factors affecting decomposition:

- Chemical composition — slow when detritus is rich in lignin and chitin; fast when rich in nitrogen and water-soluble sugars
- Climate — warm, moist conditions speed it up; low temperature and lack of oxygen slow it down

An everyday example. A compost pit of kitchen peels in a warm, damp corner turns into dark humus quickly, while dry fallen twigs rich in lignin rot slowly.

The substance. Productivity depends on more than light — plant species, nutrient availability and photosynthetic capacity also set how much organic matter an area produces.

Energy enters an ecosystem as sunlight captured by producers and passes one way through the trophic levels of food chains and food webs, with only about 10 per cent of the energy at one level reaching the next.

Energy capture. Plants capture only 2 to 10 per cent of the photosynthetically active radiation reaching them.

Trophic levels:

- Producers — the first trophic level
- Primary consumers, or herbivores — the second
- Secondary consumers, or carnivores — the third
- Tertiary consumers — the fourth
- The mass of living material at a level at a given time is its standing crop

Food chains and webs:

- Grazing food chain — begins with producers, as in grass, goat and human
- Detritus food chain — begins with dead organic matter and decomposers; on land, more energy flows through it than through the grazing chain
- Food webs — interconnected food chains, because most animals eat more than one kind of food

The 10 per cent law. Only about 10 per cent of the energy at one level passes to the next; the rest is lost mainly as heat in respiration.

Worked example. If producers hold 10 000 J of energy, about

$$10000\text{ J}\to 1000\text{ J}\to 100\text{ J}\to 10\text{ J}$$

reaches the herbivores, primary carnivores and secondary carnivores in turn.

An everyday example. A pond food chain of algae, water fleas, small fish and a kingfisher shows why kingfishers are far fewer than fish.

The substance. Food chains rarely go beyond four or five levels — too little energy is left to support another level.

Ecological pyramids show the number of individuals, biomass or energy at each trophic level with producers at the base; the pyramid of energy is always upright, while pyramids of number and biomass can be upright or inverted.

Pyramid of number:

- Upright in a grassland — many grasses, fewer herbivores, fewer carnivores
- Inverted in a tree ecosystem — one large tree supports many insects, which feed fewer birds

Pyramid of biomass:

- Inverted in the sea — the small standing crop of fast-reproducing phytoplankton weighs less than the fish that feed on it at any moment

Pyramid of energy:

- Shows the energy flowing through each trophic level over a period
- Always upright, because energy is lost at every transfer

An everyday example. A single mango tree in a courtyard supports many insects and only a few birds — an inverted pyramid of number.

The substance. An inverted pyramid of biomass does not break the 10 per cent law — phytoplankton reproduce so fast that their production over time is high, even when their standing crop is small.