Ecology
A community is all populations of different species that live in the same place. An ecosystem is a community and its environment. Ecosystem ecology studies how energy and nutrients move through an ecosystem. All living things need energy to survive, move, grow, and reproduce.  A trophic level is the number of times energy moves from one living thing to another, before reaching a particular living thing. The first trophic level, called producers or autotrophs, gets energy from the environment. They use the energy to make organic compounds. Most producers, such as plants, take in energy from sunlight, but some take it from inorganic compounds. Other trophic levels, called consumers or heterotrophs, get their energy by eating other living things. All animals are consumers, and there are three kinds: herbivores, carnivores, and omnivores. Herbivores eat only plants, carnivores eat only other animals, and omnivores eat both. Decomposers are living things which break down dead things. A food web shows the movement of energy in an ecosystem.