Energy flow in ecosystems

Why so little sunlight reaches the top predator.

The idea

Producers — plants, algae, and cyanobacteria — capture roughly 1% of the sunlight that reaches them and store it as chemical energy in their tissues. That energy passes along the food chain to primary consumers (herbivores), secondary consumers, and tertiary consumers, but at each step only about 10% is built into new biomass at the next level; the rest is lost as heat from respiration, in undigested feces, and in parts that are never eaten. Because of this trophic efficiency of roughly 5–20%, a pyramid of energy is always upright, whereas a pyramid of numbers can be inverted (one oak tree feeding thousands of caterpillars) and a pyramid of biomass can be inverted in the sea, where fast-breeding phytoplankton support a heavier standing crop of zooplankton. Energy dwindles so quickly that food chains rarely run past four or five links, and unlike carbon, nitrogen, and water, energy flows through an ecosystem once and never cycles back.

Work through an example

A meadow’s grasses store 10,000 kJ per square meter per year. Grasshoppers get about 1,000 kJ, the frogs that eat them 100 kJ, the snakes 10 kJ, and a hawk at the fifth level just 1 kJ — one ten-thousandth of what the plants captured, which is why top predators need large territories. The same arithmetic applies to farming: a hectare of grain feeds about ten times as many people when eaten directly as it does after being fed to cattle first.

What to watch for

Energy is not recycled — decomposers return nutrients to the soil, but the energy in every organism ultimately leaves as heat, and only a fresh supply of sunlight keeps the system running. And 10% is a rule of thumb, not a law: mammals and birds, which burn most of their food keeping warm, convert only 1–3% of what they assimilate into new tissue, while insects can manage around 40%.