Since some organisms feed at more than one trophic level, the food chain does not adequately
describe the passage of energy in an ecosystem. The more accurate representation is afood
web(Figure18.10). A food web recognizes that many organisms eat at multiple trophic
levels. Afoodwebincludestherelationshipsbetweenproducers,consumersanddecomposers.
Figure 18.10: An arctic food web. Besides the living organisms, some abiotic components
(nitrogen, mineral salts) and nonliving parts (dung) are included. ( 10 )
All organisms depend on two global food webs that are interconnected. The base of one is
phytoplankton, microscopic ocean producers. These tiny organisms are eaten by zooplank-
ton. The zooplankton are tiny animals which in turn are eaten by small fish and then larger
fish. Land plants form the base of the second food web. They are eaten by herbivores, that
are eaten by carnivores and so on. Birds or bears that live on land may eat fish, which
connects the two food webs. Humans are an important part of both of these food webs; we
are at the top of a food web since nothing eats us. That means that we are top predators.
Flow of Matter in Ecosystems
The flow of matter in an ecosystem is not like energy flow. Matter can enter an ecosystem at
any level and can leave at any level. It cycles freely between trophic levels and between the
ecosystem and the physical environment.Nutrientsare ions that are crucial to the growth
of living organisms. Nutrients, like nitrogen and phosphorous, are important for plant cell
growth. Animals use silica and calcium to build shells and skeletons. Cells need nitrates
and phosphates to create proteins and other biochemicals. From nutrients, organisms make
tissues and complex molecules like carbohydrates, lipids, proteins and nucleic acids.
Nutrients may enter an ecosystem from the breakdown of rocks and minerals. They enter