4. Energy Flow in Ecosystems

Kids will commonly get the erroneous notion that ecosystems recycle energy as well as nutrients. Emphasize that energy is not recycled. Rather, it flows through ecosystems entering as sunlight and exiting as the low-temperature heat (LTH) given off by each of the organisms in the food chain. The LTH will only flow toward a cooler place — eventually outer space.

Going into more detail regarding this concept.

1. Energy enters the ecosystem as sunlight, but at best, only about 3% of the energy contained in sunlight is captured in photosynthesis and ends up in the energy rich compounds of biomass. 97% exits as LTH

2. Animals feed on the plant biomass and thereby convert the plant biomass into biomass of their own body’s. However, 90% of what they consume is broken down in their cell respiration to supply the energy needs of their body. That energy exits as exits as LTH. Only 10% of what is consumed gets converted to biomass of the consumers. 

3. The same goes for each step of the food chain regardless of its design.  Ninety percent of the energy is given off as LTH at each step. Thus, the energy contained in biomass declines:

  Producers:           primary                       secondary                     tertiary                                                                                                                                                      =                            Consumers                   consumers                   consumers                                                                                     –       100         90% reduction = 10.      90% reduc = 1.          90% reduc = 0.1  

4. It obviously does not take many steps of a food chain before the amount of biomass is reduced to nil. Four or five steps is generally the maximum.  

Conclusion: An ecosystem requires the constant input of new biomass. The biomass is produced via the photosynthesis of producers. The photosynthesis requires thin constant input of solar energy (sunlight). Thus, all ecosystems depend on the input of solar energy, energy which flows through the system exiting as low temperature heat making its way to outer space. Without the input of solar energy the ecosystem is doomed to extinction.

The concept of biomass depletion through a food chain or food web is  commonly depicted as a pyramid as seen in the following video: