Food webs and food chains are helpful tool that illustrate the relationships between different organisms in an ecosystem by indicating "who eats who." In a schematic that usually appears as a pyramid, organisms are divided based on their trophic level, or which level of consumer they are. These pyramids are broad at the bottom and small at the top, illustrating the movement of energy from the producers at the bottom through the consumers of various levels up to the top of the pyramid. Food webs illustrate the same information but use lines to connect each eater to what it eats.
First-level consumers are also known as primary consumers, herbivores or organisms that occupy the second trophic level. These organisms eat plants, or producers, which comprise the first trophic level. Primary consumers do not eat other consumers. The physical size of first-level consumers varies greatly, ranging from tiny zooplankton to large elephants.
Secondary or second-level consumers eat primary consumers. Tertiary, or third-level consumers eat lower-level consumers and are sometimes called final consumers. Some secondary and tertiary consumers eat plants as well as lower level consumers, making them omnivores. Humans are good example of omnivorous upper-level consumers; we eat primary producers -- plants, as well as other consumers -- animals.
General Trends and Differences
In a food web, the total energy or biomass is greatest among the producers, and generally diminishes with each subsequent trophic level. Take, for example, a food web that consists of plants, insects that eat the plants, chickens that eat the insects, and humans that consume the chickens. For the sake of simplicity, assume this is a closed web, without other food sources or consumers. The biomass and stored energy of the plants is greater than that of the insects in the next level. The biomass and energy of the insects is greater than that of the chickens, which is greater than that of the humans they support. This is because nothing in nature is 100 percent efficient; energy is lost with each transfer. Consequently, in a given ecosystem there are generally more producers than 1st-level consumers, and more 2nd-level consumers than 1st-level consumers, and so on.
Role of Decomposers
Other critical components of a food web include producers, or plants, which use photosynthesis to transform energy from the sun into sugars that consumers can use. Also important are decomposers, organisms that feed on and break down animal and plant waste and dead organisms. Decomposers, also known as detrivores, make use of energy stored in dead plant and animal tissue. In the process, they release nutrients stored in the plants and animals they break down, cycling the nutrients back into the ecosystem for plants and animals to use.