The energy pyramid and the 10% rule

Of the 20,819 kcal that the plants of Silver Springs made on each square metre in a year, 21 kcal, about 0.1%, reached the top predators.

Most of the energy is not passed on

When a rabbit eats grass, it does not turn every calorie into rabbit. Some of the grass is never digested and leaves as droppings. Much of what is digested is burned to keep the rabbit alive, warm and moving, and that energy ends up as heat. Only what is left goes into new tissue, and only new tissue can feed the fox. OpenStax links this loss to the second law of thermodynamics: every time energy changes form, some of it becomes heat that living things cannot use again.

National Geographic Education sums this up as the 10 percent rule: on average only about 10% of the energy at one trophic level is passed to the next. Start with 10,000 kcal in the plants and the herbivores get about 1,000, the carnivores 100 and the top carnivores 10. Each step is ten times smaller. That is why the diagram is drawn as a pyramid, wide at the bottom and narrow at the top.

Measured numbers: Silver Springs

In the middle of the twentieth century Howard T. Odum measured the flow of energy through Silver Springs, a spring-fed river in Florida. OpenStax gives the yearly production for each level: 20,819 kcal per square metre for the producers, 3,368 for the primary consumers, 383 for the secondary consumers and 21 for the tertiary consumers. Work out the steps and you get about 16.2%, 11.4% and 5.5%. The real world does not follow a neat 10%, but the shape is the same: each level holds a small fraction of the one below.

OpenStax also defines the trophic level transfer efficiency, production at one level divided by production at the level below, times 100. For the first step in Silver Springs it quotes 1,103 out of 7,618 kcal, which is about 14.48%. Different books use slightly different bookkeeping, so it is worth checking which numbers a ratio is built from.

Why food chains are short

Because so little energy survives each step, OpenStax notes that after four to six transfers there is not enough left to support another level. There is no predator that lives only on eagles. Warm-blooded animals make the loss worse. A caterpillar eating leaves turns about 18% of the energy it absorbs into its own body, while a squirrel eating acorns may manage only 1.6%, because so much goes into body heat.

Pyramids that turn upside down

Ecologists draw three kinds of pyramid. A pyramid of numbers counts individuals; in a summer temperate forest a few trees support many insects, so the bottom is narrow. A pyramid of biomass weighs living matter; in the English Channel the phytoplankton weigh 4 g per square metre and the zooplankton that eat them 21 g, because the plankton are eaten almost as fast as they grow. A pyramid of energy measures flow over time, and it is always upright, since no level can pass on more energy than it received.