Trophic levels in an ecosystem are essentially the hierarchy of feeding and trophic interactions, including the transfer of energy, carbon, and nutrients from one organism to another.
The trophic pyramid (below) illustrates how energy is passed from one level to the next. While this pyramid only shows up to the level of tertiary consumers, it is possible for there to be a maximum of six trophic levels in an ecosystem.
Primary producers transform energy from sunlight into biologically-useful energy or sugars. Primary productivity is the rate at which producers can obtain energy from sunlight, and it is dependent on sunlight, temperature, and moisture.
The energy captured by primary producers is then moved up the trophic pyramid as higher-level consumers eat organisms from the levels below it.
Secondary productivity is the rate at which consumers convert organic material into biomass. In this sense, biomass describes the dry mass of organic material in an organism, and this can be used for energy. However, secondary productivity introduces no new energy into the system. It is entirely dependent on what was produced by primary productivity.
As shown in the percentages in the diagram above, as energy moves up the pyramid, only 10% of energy from the previous level is captured. The other 90% is lost as metabolic heat, due to inefficient energy transfer. This loss of energy going up the pyramid is what limits the maximum number of trophic levels to six. There is not enough energy to sustain any higher level.
Food chains are representations of who eats whom in an ecosystem. This diagram demonstrates how trophic levels help us distinguish between types of organisms.
In general, most predators consume prey that is smaller than they are, so as one goes up the food chain, organisms get larger and larger. Also, the larger the organism, the more space is required for that organism to find food, which results in lower population numbers at higher levels of the food chain.
Food webs, like the one shown below, are diagrams of trophic interactions. They essentially diagram ways that organisms obtain energy from other organisms. The majority of food webs are quite complex and involve many organisms and relationships.
This food web shows that the primary producers are at the first trophic level. The primary consumers represent the second trophic level, and the secondary consumers are part of the third trophic level.