Niche: the job in a habitat that a species performs.  In wetland environments green plants are producers converting the sun’s energy to chemical energy for their own use and other species that eat the plants.  Plants also filter water of nutrients and sediment, and provide nesting sites for other species, including herbivores that eat the plants.  Herbivores like snails may be eaten by predators such as small herons, which are then eaten by other predators like fox.  Decomposers, like bacteria metabolize the organic matter,  release elements to the system and are food for fresh water mussels and clams. Other decomposer bacteria metabolize inorganic solids making the minerals available to plants. The mussels and clams are then eaten by birds which are decomposed when they die to become nutrients for plants.  The shells of the snails, mussels and clams may also provide shelter for tiny fish or other species. 

Niche                           Species that fills it

producers                       green plants

decomposers                  bacteria, fungi

consumers                     snails, birds, fox, mussels, clams

In any habitat there will be at minimum: primary producers, consumers that eat the primary producers, and decomposers.  Different habitats will have different niches depending on the physical characteristics of the habitat.