Energy Flow in Ecosystems: Food Chains, the 10% Rule and Ecological Pyramids (Animated)

Every living thing needs energy, and almost all of it comes from the Sun. Plants and algae capture sunlight and store it in chemical bonds. Animals get their energy by eating plants or by eating other animals. At every step, most of the energy is lost as heat. This one-way flow of energy explains some of the most basic patterns in nature: why food chains rarely have more than four or five links, why big predators are rare, and why a vegetarian diet uses less land than a meat-based one.

What you will learn

How energy enters ecosystems through primary production
How trophic levels and food chains are organised
The 10 percent rule of energy transfer, and why energy is lost
The three types of ecological pyramid

Energy and the laws of thermodynamics

Two physical laws govern energy in ecosystems:

The first law of thermodynamics: energy cannot be created or destroyed, only converted from one form to another. Plants convert light energy into chemical energy; animals convert chemical energy into movement and heat.
The second law of thermodynamics: every energy conversion loses some energy as heat, which cannot be used again by living things.

Together, they mean that energy flows through an ecosystem in one direction and must be continually replaced by sunlight. Nutrients, by contrast, cycle (see Module 5).

Primary production

Primary producers, mainly plants, algae and cyanobacteria, capture energy through photosynthesis. Only about 1 to 2 percent of the sunlight reaching a typical ecosystem is converted into plant tissue.

Gross primary production (GPP) is the total energy captured by photosynthesis.
Net primary production (NPP) is what is left after producers use some energy for their own respiration. NPP is the energy available to the rest of the food chain.

NPP varies enormously between ecosystems. Tropical rainforests, coral reefs, estuaries and wetlands are among the most productive; deserts, tundra and the open ocean are among the least productive per square metre. The open ocean is large enough, however, to contribute about half of global primary production.

A few ecosystems run on a different source: deep-sea hydrothermal vents support communities of tubeworms, clams and shrimp powered by bacteria that use chemical energy from hydrogen sulphide, a process called chemosynthesis.

Trophic levels and food chains

Each step in a food chain is a trophic level:

Trophic level	Role	Savanna example	Ocean example
1. Primary producers	Make food from sunlight	Grasses	Phytoplankton
2. Primary consumers	Herbivores	Zebra, wildebeest	Krill, copepods
3. Secondary consumers	Carnivores that eat herbivores	Lion, cheetah	Fish, penguins
4. Tertiary consumers	Carnivores that eat carnivores	(Lions sometimes kill cheetahs)	Leopard seal, orca

Decomposers and detritivores, such as fungi, bacteria, earthworms, termites and dung beetles, feed on dead material and waste at every level. They release nutrients back into the soil and water. In many ecosystems, more energy flows through the decomposer food chain than through grazing food chains.

The 10 percent rule

On average, only about 10 percent of the energy at one trophic level is converted into new tissue at the next level. The figure varies, from about 5 to 20 percent depending on the organisms, but the 10 percent rule, first described from Raymond Lindeman’s work on a Minnesota lake in 1942, is a useful average.

Where does the other 90 percent go?

Respiration. Most energy is burned to fuel movement, growth and body heat, and lost as heat. Warm-blooded animals such as mammals and birds lose far more energy this way than cold-blooded animals.
Not eaten. Roots, bark and bones are often left behind.
Not digested. Much of what is eaten passes out as waste and goes to decomposers.

The consequences are dramatic. If grasses on a patch of savanna capture 100,000 units of energy, zebras might get 10,000, lions 1,000, and a hypothetical predator that ate lions just 100. This is why food chains are short and why top predators need huge areas.

Ecological pyramids

Ecologists use three types of pyramid to show the structure of an ecosystem:

Pyramid of numbers shows how many individuals are at each level. It can be inverted: a single oak tree can support thousands of caterpillars.
Pyramid of biomass shows the total mass of living material at each level. It can also be inverted in the ocean, where fast-reproducing phytoplankton are eaten so quickly that at any moment their biomass can be smaller than that of the zooplankton feeding on them.
Pyramid of energy shows the energy flowing through each level over time. Because of the second law of thermodynamics, it can never be inverted.

Why eating lower on the food chain uses less land

Because energy is lost at each step, producing food from animals takes far more land, water and crops than producing the same food from plants. Around three-quarters of the world’s agricultural land is used for livestock and animal feed, yet animal products supply less than a fifth of the world’s calories. This is one of the main links between diet, land use and wildlife habitat.

Case study: Silver Springs

In the 1950s, the ecologist Howard T. Odum measured the energy flowing through Silver Springs, a clear spring-fed river in Florida. He tracked sunlight, plant production, herbivores, carnivores, top carnivores and decomposers, producing one of the first complete energy budgets of an ecosystem. It showed exactly the pattern predicted by thermodynamics: a steep drop in energy at each trophic level, with most energy lost as heat through respiration.

Try it yourself

Write out a food chain from your own meals today, starting with the Sun. How many trophic levels are there? Estimate how much sunlight energy was needed to produce 100 units of energy in your food, using the 10 percent rule.

Common misconceptions

“Energy is recycled in ecosystems.” Nutrients are recycled; energy flows one way and is lost as heat.
“Predators get all the energy of their prey.” Only a small fraction is converted into predator tissue.
“All pyramids are upright.” Pyramids of numbers and biomass can be inverted; only pyramids of energy cannot.

Key terms

Primary production: the creation of organic matter by producers.
Trophic level: a feeding position in a food chain.
Ecological efficiency: the percentage of energy transferred from one trophic level to the next.
Detritivore: an animal that feeds on dead organic matter.
Chemosynthesis: producing food using chemical energy instead of light.

Quick quiz

Test what you have learned. Scoring 60 percent or more marks this module as complete.

Energy Flow and Ecological Pyramids