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autotrophs or primary producers.
Next come organisms that eat the autotrophs; these organisms are called herbivores or primary consumers -- an example is a rabbit that eats grass.
The next link in the chain is animals that eat herbivores - these are called secondary consumers -- an example is a snake that eat rabbits.
In turn, these animals are eaten by larger predators -- an example is an owl that eats snakes.
The tertiary consumers are are eaten by quaternary consumers -- an example is a hawk that eats owls. Each food chain end with a top predator, and animal with no natural enemies (like an alligator, hawk, or polar bear).
The arrows in a food chain show the flow of energy, from the sun or hydrothermal vent to a top predator. As the energy flows from organism to organism, energy is lost at each step. A network of many food chains is called a food web.
The trophic level of an organism is the position it holds in a food chain.