First-level consumers are animals or organisms that feed exclusively on producers or plants. First-level consumers can also be referred to as herbivores, and usually come at the second stage of the food chain.
The lowest stage of the food chain consists of producers, which are basically organisms that are able to manufacture their own food. Plants are the only organisms that can make their own food and therefore all plants are producers.
Right above plants are the first-level consumers, or herbivores, which are basically animals that feed on producers. Examples of first level consumers include grasshoppers, antelopes, rabbits and insects that feed on plants.
Second-level consumers are those animals that feed on herbivores. They are sometimes referred to as carnivores, since they feed exclusively on flesh. Examples of second level consumers include cheetahs, leopards, jackals, lions and tigers.
Third-level consumers, or tertiary consumers, are sometimes referred to as omnivores. These not only eat first-level consumers, but they also feed on producers. Examples of third-level consumers include pigs, human beings and bears.
At the top of the food chain are decomposers, which are basically organisms that aid in decomposing other organisms. Examples of decomposers include fungi and bacteria.