Feeding Relationships
Energy flows through ecosystems in one direction, typically from the Sun, through photosynthetic organisms including green plants and algae, to herbivores to carnivores and decomposers.  Green plants and algae are called autotrophs or producer organisms, as they capture solar energy to make sugars in the process of photosynthesis.   Herbivores or primary consumers use the producer organisms to provide them with their food.   Carnivores are secondary consumers as they eat the primary consumers as their source of food.    Some organisms are capable of functioning as primary consumers (eating plant material) and as secondary consumers (eating animal material).   These organisms are called omnivores.  Humans are examples of omnivores.   All consumers are examples of heterotrophic organisms, as they can not make their own food using the sun, but depend upon the ingestion of other organisms for their nutrition.

Food Chains
If an ecosystem is to be self-sustaining it must contain a flow of energy.   One way of  representing the flow of energy through the living components of an ecosystem is through the use of a food chain.   A food chain indicates the transfer of energy from producers through a series of organisms which feed upon each other.

A Food Chain
Note that the arrows in the food chain point to the organisms which are doing the eating.   Thus the arrows in the food chain represent the flow of energy through the ecosystem.	The algae and floating plants are the producers in this food chain.   The aquatic crustaceans are the primary consumers which eat the producers. Fish are secondary consumers eating the primary consumers. A food chain may also contain third level or other consumers as indicated by the raccoons in this food chain.

Food Webs
In a natural community, the flow of energy and materials is much more complicated than illustrated by any one food chain.   A food web is a series of interrelated food chains which provides a more accurate picture of the feeding relationships in an ecosystem, as more than one thing will usually eat a particular species.

A Food Web
Energy flow in a food web also starts with the producer organisms through the various levels of consumer organisms as in a food chain.

Energy Pyramids
An energy pyramid provides a means of describing the feeding and energy relationships within a food chain or web.    Each step of an energy pyramid shows that some energy is stored in newly made structures of the organism which eats the preceding one.   The pyramid also shows that much of the energy is lost when one organism in a food chain eats another.   Most of this energy which is lost goes into the environment as heat energy.   While a continuous input of energy from sunlight keeps the process going, the height of energy pyramids (and therefore the length of food chains) is limited by this loss of energy.