Plants are the starting point for most food chains on land and in many aquatic ecosystems. They capture energy from sunlight and store it in chemical compounds made during photosynthesis. When an animal eats a plant, some of that stored energy enters the animal’s body. When a predator eats that animal, some of the energy moves again.
The basic path is simple: sunlight → plants → herbivores → predators. But energy does not pass from one organism to the next intact. At every step, much of it is used for life processes or released as heat. That is why food chains usually contain only a few major feeding levels.
Plants capture energy from sunlight
Plants make their own food through photosynthesis, a process that uses light energy to convert carbon dioxide and water into energy-rich organic compounds, primarily sugars.
The energy captured from sunlight becomes chemical energy stored in the bonds of these compounds. Plants then use that chemical energy to grow, repair tissues, reproduce, and carry out other cellular activities.
Because plants can produce organic matter from inorganic materials using an external energy source, they are called producers. Algae and some other photosynthetic organisms also serve as producers in aquatic ecosystems.
The energy stored in plant tissues is what makes the rest of the food chain possible. Animals cannot use sunlight directly as an energy source in the way plants do. They obtain chemical energy by consuming other organisms or organic matter.
Herbivores transfer plant energy into animal tissue
When a herbivore eats a plant, it takes in chemical energy stored in plant material. Herbivores such as deer, rabbits, caterpillars, and many fish occupy the primary consumer level of a food chain.
Digestion breaks food into smaller molecules that the animal can absorb and use. Some of the absorbed nutrients become part of the animal’s tissues. Others are broken down during cellular respiration to release usable energy for movement, growth, maintaining body functions, and other activities.
This means that only part of the energy contained in the plants becomes new herbivore biomass—the living material that can potentially be eaten by the next consumer.
Some plant material also passes through the digestive system without being fully digested and leaves the body as waste. Other energy is used to keep the herbivore alive rather than becoming tissue available to a predator.
Predators receive energy by eating consumers
When a predator eats a herbivore, it gains access to the chemical energy stored in the herbivore’s tissues. A hawk eating a rabbit, for example, is obtaining energy that originally came from plants eaten by the rabbit.
The predator is a secondary consumer when it feeds on a primary consumer. If another predator eats that predator, the next animal occupies an even higher trophic level, or feeding position, in the food chain.
Energy therefore moves through organisms rather than being created anew at each level. The original source may be sunlight, but the immediate source of energy for a predator is the food it consumes.
Most energy is lost between feeding levels
Energy transfer through a food chain is inefficient. A plant uses much of the energy it captures to maintain itself, and herbivores use much of the energy they obtain from plants for their own metabolism. Predators likewise spend much of their acquired energy on respiration, movement, maintaining body functions, and other activities.
Some material is not eaten, some is not digested, and some energy is ultimately released as heat during cellular respiration.
As a result, the amount of energy available to support new biomass generally decreases at each successive trophic level. A predator therefore has access to far less of the original captured energy than the plants at the beginning of the chain.
This declining energy supply helps explain why ecosystems can support many producers, fewer herbivores, and generally even fewer large predators.
Food chains are pathways, not closed loops
Unlike nutrients such as carbon and nitrogen, energy does not continually cycle through an ecosystem in the same way. Energy enters most ecosystems as sunlight, is converted into chemical energy by producers, and moves through consumers.
At each stage, some of that energy is dissipated as heat. Once released as heat, it is no longer available to organisms as the same usable chemical energy.
Matter behaves differently. Atoms and molecules from organisms can return to the environment through waste, decomposition, respiration, and other processes and can eventually become part of new organisms. Energy, by contrast, flows through the ecosystem in one general direction.
A food chain can contain several feeding levels
Consider a simple grassland chain:
Grass → grasshopper → frog → snake → hawk
Grass captures energy from sunlight. The grasshopper obtains some of that stored energy by eating grass. The frog receives some of the remaining energy when it eats the grasshopper. The snake gains energy by eating the frog, and the hawk gains energy by eating the snake.
At every step, however, the available energy becomes smaller because organisms use energy for their own survival and because not all consumed material becomes biomass available to the next consumer.
Real ecosystems are more complicated than this single chain. A hawk may eat several kinds of animals, and those animals may feed on many different plants or other organisms. These interconnected feeding relationships form a food web rather than a simple food chain.
Why predators are less numerous at the top
The loss of usable energy between trophic levels places limits on how much living biomass higher levels can support. Because predators receive energy only after it has passed through one or more lower levels, there is generally less energy available to support them.
This is one reason large populations of plants can support smaller populations of herbivores, which in turn can support still smaller populations of predators. The pattern is not identical in every ecosystem, but the basic energy constraint remains important.
A predator’s position in a food web therefore depends not only on what it eats but also on the amount of energy available through the organisms below it.
The same process connects plants to top predators
Whether the ecosystem is a forest, grassland, lake, or ocean, the central process is similar. Producers capture energy from an external source, usually sunlight. Consumers obtain chemical energy by eating producers or other consumers. As energy moves upward through trophic levels, much of it is used by organisms and eventually released as heat.
A wolf eating a deer is therefore receiving energy that can be traced backward through the food web to the plants the deer consumed, and ultimately to the sunlight captured by those plants.
The energy in a predator is not separate from the energy captured at the beginning of the chain. It is a portion of that original energy that has been transferred through successive organisms, with substantial losses at every step.
