Every living thing needs to get energy from somewhere. This can be from making the energy out of sunlight, or by consuming another living thing. Ultimately, all energy comes from the sun. The energy then works it way up different trophic levels, radiating energy from the sun to each living thing in a biome. Scientists call this the food chain.
Each community of biological things, called a biosphere, has a food chain. This is the order of who eats whom. Most food chains have five basic trophic levels: primary producers, primary consumers, secondary consumers, tertiary consumers and quaternary consumers.
The first group, primary producers, is on the bottom. These are organisms that make their energy by using the sun or other non-living source. Through photosynthesis (or chemosynthesis), these organisms generate energy. Since they do not rely on other living things for energy, they can also be called autotrophs. A good example of a primary producer would be grass. It does not rely on living things for its energy.
The second group, primary consumers, is the next level. Primary consumers are an organism that only eats primary producers. Since most primary producers are plants, primary consumers are also called herbivores. A good example of a primary consumer would be a grasshopper. Grasshoppers only eat plant material, and so are only eating primary producers.
The third group, secondary consumers, comes next. Any organism that eats only primary consumers and primary producers. Remember that any thing on the food chain can eat anything from a lower trophic level. Secondary consumers are not limited to only organisms that eat primary consumers exclusively. Secondary consumers that eat from just the primary consumer level can also be called carnivores, because they eat meat. If a secondary consumer eats both primary consumers and primary producers, it can also be called an omnivore. A good example of a secondary consumer would be a rat. Rats eat both plants and primary consumers such as insects.
One level higher up is tertiary consumers, the third group. Tertiary consumers, in the same pattern, eat foods from trophic levels lower than themselves. Again, these can also be called carnivores or omnivores, as the case may be. A good example of a tertiary consumer would be a snake. Snakes eat rats, which are secondary consumers.
Lastly, we have the quaternary consumers. These are the top of the top that have no natural predators. They eat from all of the lower trophic levels. Like tertiary and secondary consumers, quaternary consumers can be either omnivores or carnivores. A good example of a quaternary consumer would be a hawk. Humans could also be considered quaternary consumers in the purest diet forms. Some modern day humans have made the choice to only eat vegetables, but humans are still biologically classified as quaternary consumers.
There is also a group of consumers who are partially outside of these classifications, but keep the circle moving: detritivores. Detritivores are organisms that consume dead matter and break it down into small minerals and organic materials that the primary producers can then use to build their energy. An example of this might be an earthworm. It is worth mentioning that in some situations, a detritivore can have a trophic level, because they can be eaten by other organisms. For example, many birds have diets that are dependent upon detritivores. But since they eat quaternary consumers and all the others, their trophic level is debated. Some ecologists place detritivores above quaternary consumers, others simply put them on the side of the food chain.
The food chain is how we live. In some ways, it is more accurate to think of it as a food circle, and not a food chain. This is why there are graphic representations that show it as a web. Because of this delicate balance of each trophic level, ecology works out. It is nature’s own miracle.