The trophic level of an organism is the position it occupies in a food chain, which is a succession of organisms that eat other organisms and which may, in turn, be eaten themselves. A food chain starts at trophic level 1 with primary producers such as plants and algae, which manufacture their own food mostly by extracting nutrients from the soil or from water and using photosynthesis. An exception is deep-sea hydrothermal ecosystems, where there is no sunlight and the primary producers instead manufacture their food through chemosynthesis.
Trophic level 2 is herbivores, which consume the plants and algae, and trophic level 3 is carnivores, which consume the herbivores of level 2. Level 4 is carnivores that eat other carnivores. At the very top of a food chain are apex predators, which are predators that have no predators. Food chains end with decomposers, namely bacteria and fungi, which break down dead plant and animal material and wastes into inorganic chemicals that can be recycled as nutrients for plants to reuse.