Food chains & food webs
- Producers, or autotrophs, make their own organic molecules. Consumers, or heterotrophs, get organic molecules by eating other organisms.
- A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another.
- In a food chain, each organism occupies a different trophic level, defined by how many energy transfers separate it from the basic input of the chain.
- Food webs consist of many interconnected food chains and are more realistic representation of consumption relationships in ecosystems.
- Energy transfer between trophic levels is inefficient—with a typical efficiency around 10%. This inefficiency limits the length of food chains.
Autotrophs vs. heterotrophs
- Photoautotrophs, such as plants, use energy from sunlight to make organic compounds—sugars—out of carbon dioxide in photosynthesis. Other examples of photoautotrophs include algae and cyanobacteria.
- Chemoautotrophs use energy from chemicals to build organic compounds out of carbon dioxide or similar molecules. This is called chemosynthesis. For instance, there are hydrogen sulfide-oxidizing chemoautotrophic bacteria found in undersea vent communities where no light can reach.
- At the base of the food chain lie the primary producers. The primary producers are autotrophs and are most often photosynthetic organisms such as plants, algae, or cyanobacteria.
- The organisms that eat the primary producers are called primary consumers. Primary consumers are usually herbivores, plant-eaters, though they may be algae eaters or bacteria eaters.
- The organisms that eat the primary consumers are called secondary consumers. Secondary consumers are generally meat-eaters—carnivores.
- The organisms that eat the secondary consumers are called tertiary consumers. These are carnivore-eating carnivores, like eagles or big fish.
- Some food chains have additional levels, such as quaternary consumers—carnivores that eat tertiary consumers. Organisms at the very top of a food chain are called apex consumers.
Grazing vs. detrital food webs
Energy transfer efficiency limits food chain lengths
- In each trophic level, a significant amount of energy is dissipated as heat as organisms carry out cellular respiration and go about their daily lives.
- Some of the organic molecules an organism eats cannot be digested and leave the body as feces, poop, rather than being used.
- Not all of the individual organisms in a trophic level will get eaten by organisms in the next level up. Some instead die without being eaten.