Producers, consumers and decomposers

In Lake Ontario the energy in a Chinook salmon has passed through only three meals since it was captured from sunlight by green algae.

Where the energy comes from

Every living thing needs energy to build its body and keep it running, and almost all of that energy starts as sunlight. Plants, algae and some bacteria capture it by photosynthesis and store it in sugars. Ecologists call them primary producers, or autotrophs, because they make their own food from carbon dioxide and water. Everything else, from a grasshopper to a blue whale, is a heterotroph: it gets energy by eating producers, eating something that ate producers, or breaking down the dead.

There is one important exception to the sunlight rule. At hydrothermal vents on the deep sea floor, where no light reaches, bacteria use chemical energy from compounds such as hydrogen sulfide that pour out of the vents. This is chemosynthesis. OpenStax describes a NOAA photograph from the NW Eifuku volcano off Japan, 1,535 m below the surface, where shrimp, squat lobsters and hundreds of mussels live on these bacteria and on food sinking from above.

Trophic levels

A food chain lists who eats whom in a single line, and each step is called a trophic level. Producers are the first level. Herbivores that eat them are primary consumers. Carnivores that eat herbivores are secondary consumers, and carnivores that eat other carnivores are tertiary consumers. The animal at the top, with no regular predators of its own, is the apex consumer. In the Lake Ontario chain in OpenStax Biology, energy runs from green algae at the bottom to the Chinook salmon at the top, with only three transfers between them.

Notice which way the arrows point. In ecology an arrow runs from the food to the eater, because it shows the direction energy and matter travel. A drawing with an arrow from the fox to the rabbit would mean the rabbit eats the fox. This convention trips up many students, so it is worth checking every time you read a food web.

The clean-up crew

Leaves fall, animals die and droppings pile up. If nothing broke them down, the nutrients locked inside would never return to the soil or the water. Decomposers, mostly bacteria and fungi, and detritivores such as earthworms and woodlice, feed on this dead material. OpenStax notes that most grazing food webs, which start with living plants, come with a detrital food web that starts with dead matter. In a meadow the same plants feed rabbits while they are alive and fungi once they die.

Many animals do not fit neatly on one level. A fox eats rabbits, which makes it a secondary consumer, but it also eats berries, which makes it a primary consumer at the same time. Such animals are omnivores. That is one reason a single chain is too simple, and the next unit replaces it with a web.