How Does Energy Flow Through Ecosystems?

Energy flows one way through an ecosystem: sunlight is captured by producers, passed to consumers when they feed, and eventually dissipated to the surroundings, mainly as heat. At each transfer, less energy is available to the next trophic level.

Food chains and trophic levels

A food chain shows who eats whom. Each feeding position is a trophic level. For example, grass is a producer at the first trophic level; a rabbit eating the grass is a primary consumer at the second; and a fox eating the rabbit is a secondary consumer at the third.

Producers use photosynthesis to convert light energy into chemical energy stored in organic molecules. When a consumer eats a producer or another consumer, some of that stored energy enters its body. Decomposers obtain energy from dead organisms and waste material. Decomposers also respire, releasing energy as heat. They help recycle nutrients, not energy.

Why does less energy pass on?

Gross primary production ($\mathrm{GPP}$) is the chemical energy stored by producers. Producers use some of it in respiration ($R$), leaving net primary production ($\mathrm{NPP}$) as new biomass:

$$\mathrm{NPP}=\mathrm{GPP}-R$$

For instance, if $\mathrm{GPP}=1200$ and respiratory loss is $500$, then $\mathrm{NPP}=700$ energy units. Only some of that biomass is eaten. Some parts of organisms are not eaten; others are eaten but cannot be digested and leave as faeces. Respiration by each organism releases energy, so less remains stored in biomass for the next trophic level.

Of the energy a consumer ingests ($I$), some is lost in faeces ($F$) and some is used in respiration ($R$). Its net production, available as new biomass, is:

$$N=I-(F+R)$$

In an exam calculation, keep all energy values in the same units. The key distinction is that energy is not recycled through a food chain. Nutrients can be recycled, but energy continually enters, passes between organisms and leaves the ecosystem as heat.