Energy Flow

Energy enters ecosystems from the Sun. Producers (plants, algae) capture only ~1% of incident light via photosynthesis — most light is reflected, transmitted or misses chlorophyll.

Energy flow is ONE-WAY. Energy passes from producer to consumer to higher consumer; at each step, most is lost as heat. Eventually all chemical energy is released as heat — a consequence of the second law of thermodynamics.

(In contrast, chemical NUTRIENTS cycle through ecosystems — see Carbon Cycling.)

The ~10% rule. Only about 10% of energy in one trophic level is converted to biomass at the next. The rest is lost as:

1. Heat from respiration (largest loss).
2. Uneaten or undigested material (faeces, bones).
3. Loss to decomposers (dead biomass).

Worked example. If grass receives 10 000 J of light energy and stores 100 J (1%), then a rabbit eating it incorporates ~10 J (10%), and a fox eating the rabbit gets ~1 J. By T4 there is only ~0.1 J — too little to sustain a viable population.

This is why top predators are scarce and food chains rarely exceed 4 levels.

Pyramid of energy. Bars represent the energy (J m⁻² yr⁻¹) at each trophic level. Always narrows upward — energy decreases.

Pyramid of biomass. Bars represent biomass (g m⁻² or kg ha⁻¹). Usually narrows upward — but can occasionally invert (e.g. in marine plankton ecosystems where producer biomass is small but rapidly turned over, supporting a larger consumer biomass at any instant).

Pyramid of numbers. Bars represent number of organisms — often pyramid-shaped but can invert when one large producer supports many consumers (e.g. one tree with many insects).

Implications for human diet. Eating lower on the food chain (more plant-based) is more energy-efficient. Producing 1 kg of beef requires ~10 kg of cereal (energy lost as heat in the cow's respiration). Plant-based diets feed more people per unit of arable land.

Bioaccumulation of toxins. Persistent toxins (e.g. DDT, mercury) move UP food chains and accumulate at higher levels because consumers eat many prey but excrete little of the toxin — top predators get the highest dose. Eagles laying thin-shelled eggs (DDT) is the classic case.

The same energy logic that limits trophic levels also concentrates pollutants.