Less energy is available at higher levels of a food chain because organisms use much of the energy they obtain for metabolism, movement, growth, repair, and other life processes. Only a fraction becomes new biomass that can be eaten by the next organism, so usable energy declines with each step up the food chain.
Energy enters the food chain through producers
Most food-chain energy begins with producers such as plants, algae, and some bacteria. These organisms capture energy from sunlight through photosynthesis and use it to make organic molecules from carbon dioxide and water.
When an animal eats a plant, it gains access to the chemical energy stored in that plant’s tissues. If a predator then eats the herbivore, some of that stored energy moves to the predator.
This creates a sequence of feeding levels called trophic levels. Producers occupy the first trophic level, herbivores generally occupy the second, and predators that eat those herbivores occupy higher levels.
Energy, however, does not move from one level to the next without losses.
Most energy is used before it can be passed on
An organism does not store all of the energy in the food it eats as new body tissue. Much of it is used to stay alive.
Cells use energy for processes such as maintaining internal conditions, transporting substances, growing, repairing damaged tissues, moving, and reproducing. Much of this energy ultimately leaves the organism as heat during cellular respiration.
Some of the food an organism consumes also remains undigested and leaves the body as waste. Other material may be parts of the food that the organism cannot use efficiently.
The portion of consumed food that is converted into new biological material is therefore much smaller than the total energy contained in the food.
That new biomass is what can potentially provide energy to the next trophic level.
Why energy decreases at every trophic level
Consider a simple food chain:
grass → grasshopper → frog → snake
The grass contains chemical energy captured by photosynthesis. A grasshopper eats some of the grass, but it does not turn all of that energy into grasshopper tissue. Energy is spent on the grasshopper’s metabolism and other activities, while some material is not digested or is otherwise lost.
A frog then eats the grasshopper. Again, only part of the energy in the grasshopper becomes new frog biomass. The frog uses much of the rest to maintain its body and carry out its activities.
The same process occurs when a snake eats the frog.
As a result, the amount of energy available to support organisms generally becomes smaller at each successive trophic level.
Heat is a major reason energy does not cycle back
Unlike nutrients, energy does not continually circulate through an ecosystem in the same way.
When organisms break down food molecules through cellular respiration, some of the chemical energy is transferred into forms that cells can use, but ultimately much of it is released as heat. That heat disperses into the surroundings and is no longer available to organisms as chemical energy for the food chain.
This is why ecosystems need a continuing input of energy, usually from sunlight.
Nutrients such as carbon, nitrogen, and phosphorus can be recycled through organisms and the environment. Energy moves through the ecosystem instead: it enters, is transferred between organisms, and is progressively dissipated as heat.
Why higher-level predators are less abundant
The decline in available energy helps explain why ecosystems generally support fewer organisms at higher trophic levels.
A large population of plants can capture and store enough energy to support herbivores. Those herbivores collectively contain less available energy than the plants they consumed. Predators feeding on them have access to still less energy.
This places limits on how much biomass and how many organisms can be supported at higher trophic levels.
It also helps explain why food chains are usually relatively short. By the time energy has passed through several feeding levels, the amount remaining to support another level becomes increasingly limited.
The energy loss is not a fixed amount
The idea that only a certain percentage of energy always moves from one trophic level to the next is a useful rule of thumb, but it is not a universal constant.
Energy transfer depends on the organisms and ecosystem involved. Different species digest food with different efficiencies, and organisms vary in how much energy they devote to growth, movement, maintenance, reproduction, and other activities. Some parts of an organism may also be eaten less often or be difficult for another organism to digest.
For this reason, the exact amount of energy transferred between trophic levels can vary substantially.
The important principle is not a particular percentage. It is that energy transfer is inefficient, so less usable energy is generally available at each higher trophic level.
Food chains also leave energy outside the chain
Not all energy follows a simple producer-to-herbivore-to-predator pathway.
Organisms may die without being eaten, and organisms produce waste. Decomposers and detritivores use organic material from these sources, creating additional pathways through an ecosystem.
This means a food web can contain many interconnected routes for energy transfer rather than one straight chain.
Even along these pathways, however, organisms use energy for their own life processes and release much of it as heat. The fundamental pattern remains the same: less usable energy is available as energy moves through successive trophic levels.
Why there are usually fewer trophic levels than you might expect
Because energy is lost during every transfer, an ecosystem cannot indefinitely support one predator feeding on another predator at higher and higher levels.
At the base of the food web, producers capture energy directly from an external source. Each subsequent trophic level receives only part of the energy stored at the level below it.
This declining energy supply is one of the basic reasons food chains tend to have only a limited number of trophic levels. Higher-level consumers depend on the relatively small amount of energy that remains after many biological processes have already used or dissipated it.
