Living things need energy to grow, breathe, reproduce, and move. Energy cannot be created from nothing, so it must be transferred through the ecosystem. The primary source of energy for almost every ecosystem on Earth is the sun. Primary producers use energy from the sun to produce their own food in the form of glucose, and then primary producers are eaten by primary consumers who are in turn eaten by secondary consumers, and so on, so that energy flows from one trophic level, or level of the food chain, to the next. The easiest way to demonstrate this energy flow is with a food chain. Each link in the chain represents a new trophic level, and the arrows show energy being passed along the chain. At the bottom of a food chain is always the primary producer. In terrestrial ecosystems most primary producers are plants, and in marine ecosystems, most primary producers are phytoplankton. Both produce most the nutrients and energy needed to support the rest of the food chain in their respective ecosystems.

All the biomass generated by primary producers is called gross primary productivity. Net primary productivity is what is left over after the primary producer has used the energy it needs for respiration. This is the portion that is available to be consumed by the primary consumers and passed up the food chain. In terrestrial ecosystems, primary productivity is highest in warm, wet places with plenty of sunlight, like tropical forest regions. In contrast, deserts have the lowest primary productivity. In marine ecosystems, primary productivity is highest in shallow, nutrient rich waters, such as coral reefs and algal beds.

To show the flow of energy through ecosystems, food chains are sometimes drawn as energy pyramids. Each step of the pyramid represents a different trophic level, starting with primary producers at the bottom. The width of each step represents the rate of energy flow through each trophic level. The steps get smaller further up the pyramid because some of that energy is changed to a form that cannot be consumed by organism at the next higher step in the food chain. This happens at every step of the pyramid.

Not all of the energy generated or consumed in one trophic level will be available to the organisms in the next higher trophic level. At each level, some of the biomass consumed is excreted as waste, some energy is changed to heat (and therefore unavailable for consumption) during respiration, and some plants and animals die without being eaten (meaning their biomass is not passed on to the next consumer). The waste and dead matter are broken down by decomposers and the nutrients are recycled into the soil to be taken up again by plants, but most of the energy is changed to heat during this process. On average, only about 10 percent of energy stored as biomass in a trophic level is passed from one level to the next. This is known as “the 10 percent rule” and it limits the number of trophic levels an ecosystem can support.

Energy Transfer in Ecosystems
Primary consumers, like the Giant African land snail (Achatina fulica), eat primary producers, like the plants the snail eats, taken energy from them. Like the primary producers, the primary consumers are in turn eaten, but by secondary consumers. This is how energy flows from one trophic level to the next.
biomass
Noun

living organisms, and the energy contained within them.

decomposer
Noun

organism that breaks down dead organic material; also sometimes referred to as detritivores

Noun

community and interactions of living and nonliving things in an area.

Noun

group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.

organism
Noun

living or once-living thing.

Noun

process by which plants turn water, sunlight, and carbon dioxide into water, oxygen, and simple sugars.

phytoplankton
Noun

microscopic organism that lives in the ocean and can convert light energy to chemical energy through photosynthesis.

primary consumer
Noun

organism that eats producers; herbivores.

primary producer
Noun

organisms, such as plants and phytoplankton, that can produce their own food through photosynthesis or chemosynthesis; also called autotrophs.

primary productivity
Noun

rate at which autotrophs such as plants use solar or chemical energy to grow and create new life.

respiration
Noun

breathing.

secondary consumer
Noun

organism that eats meat.

trophic level
Noun

one of three positions on the food chain: autotrophs (first), herbivores (second), and carnivores and omnivores (third).