Feeding relationships
Every ecosystem has hierarchical feeding relationships called feeding levels or trophic
interactions. Trophic interactions show the flow of materials and energy through the
ecosystem. An organism’s trophic classification is based on what it feeds upon. Each level of
consumption is equal to a trophic level.
Food chains
Trophic interactions can be presented graphically by drawing a food chain. Food chains
show what organisms are eating. The arrows between organisms mean ‘eaten by’.
In the food chain shown, the plant is eaten by the herbivore and the herbivore is
eaten by the carnivore. Eventually all of the organisms in the food chain will die and be
broken down into simpler compounds by decomposers.
Food chains begin with a producer or autotroph. This is because producers are the
support layer for all food chains. Producers are almost always plants-sulfur-fixing bacteria
are an exception. Producers do not consume other organisms but produce their own food.
The next level in a food chain is occupied by a consumer, or heterotrophy.
Consumers are ranked as first-order (primary), second-order (secondary) or third-order
(tertiary) consumers, depending on where they occur along a food chain. First-order
consumers are usually herbivores, as they eat plants (producers). Further steps of the food
chain are occupied by carnivores.
Food webs
Within an ecosystem there are many feeding interactions that can be represented by food
chains. Some of these food chains interact to form a food web. Food webs are more
complex than food chains because they show that predators are not limited to one prey
species and they indicate that some organisms are consumed by a variety of predators. In
many cases a species may be a predator to one species and the prey to another.
Food webs often include decomposers. This can make food webs look very complex,
as all discarded organic matter and dead organism are broken down by bacteria and fungi.
Collectively, all discarded decaying matter in the ecosystem is called detritus.