The Living World - Environmental Science - page 39

piration in the balanced reaction C 6 H 12 O 6 + 6 O 2 → 6 CO 2 + 6 H 2 O + energy. This released energy is usable
by the plant or animal for a variety of other internal chemical and energy transfer reactions.
Food Webs and Trophic Levels
Food webs are models made up of multiple overlapping food chains. A food chain is a simple layer of energy
flow from the producer to various consumers. A food web represents more realistic and complex flow of en-
ergy from the producers to the consumers and, finally, the decomposers. Along the way, most of the energy
captured in the food web is not available to do work, but it is still retained by the ecosystem in the form of de-
graded heat energy (following the Second Law of Thermodynamics).
Food webs and food chains are composed of three primary groups: producers, consumers, and decomposers. In
a food chain or food web model, a trophic level is the layer an organism occupies. At the bottom level are the
producers, which are also called autotrophs . These are the plants that convert sunlight into energy through
photosynthesis, and the energy is then passed up through the different trophic levels. Consumers can include
herbivores, which are plant eaters only (primary consumers); omnivores , which eat both plants (primary con-
sumer) and animals (secondary or tertiary consumers); and carnivores (secondary or tertiary consumers),
which eat other animals. The consumers convert to their own use the stored energy in plants through cellular
respiration.
Detritivores and decomposers complete the breakdown of organic waste and dead organic material. Usually
both types of organisms are grouped together in one category and called decomposers. However, detritivores
are organisms that derive their energy from consuming nonliving organic matter such as dead plants and anim-
als. Decomposers are bacteria or fungi that absorb nutrients from nonliving organic matter, such as plant mater-
ial, the waste of living organisms, and dead organisms.
When diagramming food chains and food webs, the arrow that shows the energy flow always points from a
lower trophic level to a higher trophic level, as illustrated in the following simple food chain.
The two food webs shown below are simplified versions of actual food webs. The terrestrial food web is from
the Arctic, and the marine water food web can be found in oceans throughout the world. The direction of the
arrows shows the flow of energy. Energy flows from the lower trophic level to the higher level.