All organisms must obtain a supply of energy and nutrients from their environment in order to survive. The transformations of energy in an ecosystem begin first with the input of energy from the sun. Ecosystems maintain themselves by cycling energy and nutrients obtained from external sources. At the first trophic level, primary producers (plants, algae, and some bacteria) use solar energy to produce organic plant material throughphotosynthesis. Herbivores—animals that feed solely on plants—make up the second trophic level. Predators that eat herbivores comprise the third trophic level; if larger predators are present, they represent still higher trophic levels. Organisms that feed at several trophic levels (for example, grizzly bears that eat berries and salmon) are classified at the highest of the trophic levels at which they feed. Decomposers, which include bacteria, fungi, molds, worms, and insects, break down wastes and dead organisms and return nutrients to the soil.
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Last edited by maaat; 1st April 2013 at 02:48 PM.
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Last edited by ajaytopgun; 19th December 2015 at 08:53 PM.