Walk into any forest. Birds are eating insects. Insects are eating leaves. A fox might be watching the birds. A mushroom is slowly breaking down a fallen log nearby.
Everything living needs energy. And in every ecosystem on Earth, that energy follows a path. It moves from one organism to the next in a sequence that connects all living things in ways that are surprisingly orderly and predictable.
Understanding how energy transfers through living systems is one of the most fundamental concepts in biology.
Almost all energy entering living systems comes from the Sun.
Plants, algae, and some bacteria capture this light energy through photosynthesis and convert it into chemical energy stored in glucose molecules. These organisms are called producers because they produce their own food from inorganic sources.
The chemical equation for photosynthesis:
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
All other organisms in an ecosystem depend on this stored chemical energy — either directly by eating producers or indirectly by eating organisms that ate producers.
Every organism in an ecosystem plays one of three broad roles in energy transfer.
Producers are organisms that manufacture their own organic molecules from inorganic sources using an external energy supply, usually sunlight.
Consumers are organisms that obtain energy by eating other organisms.
Decomposers break down dead organic matter and waste products into inorganic substances that can be returned to the environment.
A food chain is a linear sequence showing how energy passes from one organism to the next through feeding relationships.
Each arrow in a food chain represents the transfer of energy and means "is eaten by." The arrow points in the direction energy flows.
Every food chain begins with a producer. The organisms that follow are consumers at increasing levels.
A trophic level is the position an organism occupies in a food chain.
| Trophic Level | Organism Type | Example |
|---|---|---|
| First | Producers | Grass, algae, phytoplankton |
| Second | Primary consumers | Rabbit, caterpillar, zooplankton |
| Third | Secondary consumers | Fox, frog, small fish |
| Fourth | Tertiary consumers | Eagle, shark |
| Fifth | Apex predators | Orca, tiger |
Note: An organism can occupy different trophic levels in different food chains. Humans, for example, eat plants (second trophic level) and animals (third or fourth trophic level).
In reality, most organisms eat more than one type of food and are eaten by more than one type of predator. A food web shows the complete network of feeding relationships in an ecosystem, connecting multiple food chains.
Food webs are far more realistic representations of energy flow in ecosystems than simple food chains. They show:
Here is something crucial about energy transfer. Not all the energy at one trophic level passes to the next. In fact, most of it is lost.
Only a small fraction is incorporated into the consumer's own body tissue.
The 10 Percent Rule: On average, only about 10 percent of the energy at one trophic level is passed on to the next trophic level. This is called ecological efficiency.
Ecological pyramids are diagrams that represent the quantitative relationships between trophic levels in an ecosystem.
Shows the number of organisms at each trophic level. Typically gets smaller at higher trophic levels because more organisms are needed at lower levels to support fewer at higher levels. However, this pyramid can be inverted — one oak tree can support thousands of caterpillars.
Shows the total mass of living material (dry mass) at each trophic level. This is almost always a true pyramid shape because biomass decreases at each successive trophic level due to energy losses.
Shows the amount of energy available at each trophic level. This is always a true pyramid shape. Energy always decreases moving up the trophic levels because energy is lost at every transfer. This is the most accurate and reliable of the three pyramid types.
The pattern of energy transfer through ecosystems has practical consequences.
Human relevance: Understanding energy transfer is also relevant to human food production. Eating plant foods directly is far more energy-efficient than eating animal products, because fewer trophic levels means less energy is lost before the food reaches humans.
Once energy enters an organism through feeding, it is released through cellular respiration, the process by which cells break down glucose to produce ATP.
Aerobic respiration:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
This occurs in mitochondria and releases the maximum amount of energy from glucose. Carbon dioxide and water are released as byproducts.
Anaerobic respiration occurs when oxygen is unavailable. It produces far less ATP and results in lactic acid in animals or ethanol and carbon dioxide in plants and yeast.
All the energy that organisms use for every life process ultimately traces back to the Sun, captured by producers through photosynthesis and transferred through ecosystems one feeding relationship at a time.