A food web describes the passage of nutrients and energy between different organisms. Essentially, the trophic level describes an organism's position in the web, or what eats what. Food chains are a more familiar description of this process; however, in nature the process is rarely as linear as described in a food chain.

Primary Producers

The food web begins with primary producers. This commonly involves photosynthetic organisms (such as plants or phytoplankton) using sunlight to synthesize glucose molecules. However, some food webs begin in the absence of sunlight using chemosynthetic processes (deep sea vents are one example).

Herbivores

The primary producers are consumed by organisms known as herbivores. Consumption of the primary producers provides herbivores with energy and nutrients. This process is often more complicated than appears, as many herbivorous animals use microbes to aid in digestion (for example, termites require a symbiotic relationship with bacteria and protozoa to digest the cellulose of wood).

Carnivores

Many herbivores are in turn the prey for predators. The predators use the herbivores as a source of nutrients and energy. Some carnivores are prey for other predators.

Omnivores

The term omnivore applies to organisms that consume both primary producers and higher trophic organisms. For example, many songbirds consume a mixture of insects and plant material in their diet.

Decomposers

A number of bacteria, fungi and protists break down nutrients discarded in the food web. These discarded nutrients include dung and dead organisms. The action of decomposers often facilitates the availability of nutrients for primary producers.

Source:

Food Chains and Food Webs

Ecosystems

Marine Food Webs