Producers (such as blue-green alga or water lilies) produce energy and oxygen to the ecosystems through the process of photosynthesis. Producers are also the food of first degree consumers. Alga-eating water fleas are an example of a first degree consumer. 

Second degree consumers are species that eat use first degree consumers. For example, small plankton-eating fish species, such as the roach, are second degree consumers. In addition to first and second degree consumers, many food chains also have third degree consumers. These species eat consumers of the lower degree. Predator fishes, such as the pike, are a good example of third degree consumers. Food chains can also accomodate consumers of even higher degrees, but in reality such species are very rare. The reason for this is the fact that only a part of the energy of the previous level of the food chain will transfer to the next level. 


When a eagle eats a large fish (which itself is a 3rd degree consumer), it is a 4th degree consumer.

Because the number of species and individuals that inhabit the top of the food chain is smaller than the number of producers or lower-degree consumers, the number of organisms in an ecosystem can be conceptualized as a pyramid. Such a pyramid describes how the quantity of individuals within the food chain decreases when moving from the bottom to the top. For example, a small pond can only accommodate a couple of third degree consumers (such as large pikes), whereas it accomodates a large number of producers and first degree consumers.