Trophic level

A trophic level is the position an organism occupies in a food chain, defined by its source of energy. The first level is made up of primary producers, mostly plants, algae and phytoplankton that capture sunlight by photosynthesis, plus some microbes that draw energy from chemicals. Primary consumers, the herbivores, form the second level; secondary consumers that eat herbivores form the third; tertiary consumers that eat other carnivores form the fourth; and apex predators sit at the top. Omnivores such as bears feed at more than one level.

Energy is lost at every step, because organisms burn most of the energy they take in for their own metabolism and give it off as heat, and some food is never eaten or digested. Typically only about 10 to 20 percent of the energy at one level is passed on to the next, although the figure varies between ecosystems. This is why energy pyramids always narrow toward the top and why most food chains end at the fourth or fifth level.

Decomposers and detritivores feed on dead material from every level and return its nutrients to the producers. Persistent toxins, by contrast, become more concentrated at each higher trophic level.