An organisms trophic level is it's position within a food chain. There are the producers, the primary consumers, the secondary consumers, etc... A lot of the time you can determine an organisms trophic level by looking at some of their characteristics, including dentition (teeth), claws or talons, genetic makeup, coloration, and size.
Producers are also known as autotrophs. These are the plants and autotrophic bacteria and algae that are capable of producing their own food. All producers contain chlorophyll which they use to complete the photosynthesis process.
Primary consumers are the orgnisms which eat the producers. These include herbivores, and omnivores. Because the primary consumers are getting their food directly from the producer, they will gain much more usable energy then any of the trophic levels above them.
Secondary consumers are the organisms that eat the primary consumer. This means that all secondary consumers are eating other organisms (i.e. predators) Because secondary consumers are getting their energy indirectly from the producers there is more energy lost, meaning that they have to consume more.
Tertiary consumers, as you may have guessed by now, are the consumers that eat the secondary consumers. There are many predators that can fit into this category, but that may also fit into the secondary consumer category depending on their prey. For example a snake may eat a mouse, which is a primary consumer, or it may eat a bird that eats insects making it the tertiary consumer. Typically making a food web will help you determine which is which.
Trophic Levels and Biomass:
As a rule of thumb, biomass levels peak in the producer category, and will decrease as you climb the trophic pyramid. This is because as you climb the pyramid, more energy is lost in every level at a rate of about 10%, meaning that the higher up you go in the trophic pyramid, the more food you have to consumer in order to maintain energy levels. The more food you have to consume in the wild world to stay alive, the smaller your population will be (in order to conserve food sources.)
As we discussed earlier, as you move up the trophic pyramid, the flow of energy is reduced about 10% per level. This is due to energy lost at each trophic level as heat and chemical energy absorbed by the tissues of the trophic level below it. The amount of energy transferred from one trophic level to the next is known as ecological efficiency. The fact that so much energy is lost as you go up the pyramid accounts for the smaller amount of biomass, and the larger amounts of food that organisms at each higher level must consume.
So where do decomposers fit into all of this? Technically decomposers would fit into the 4th or 5th level since they are located at the end of the food chain (they eat anything and everything they can.) The decomposer class consists of bacteria, fungi, flies, earthworms, and many others. These are the species which break down wastes and deceases organisms in order to release their nutrients and energies back into the soils/ecosystems.