Summary of Chapter Concepts

Primary productivity provides energy to the ecosystem. Primary productivity is the process of capturing solar or chemical energy and converting it into chemical bonds by photosynthesis or chemosynthesis over a given amount of time. We can distinguish between gross primary productivity, which is the total amount of energy assimilated, and net primary productivity, which is the assimilated energy that is converted into producer biomass. Net primary productivity can be measured in numerous ways including measuring plant biomass, measuring CO₂ uptake and release in terrestrial ecosystems, measuring O₂ uptake and release in aquatic ecosystems, and using remote sensing. Across all ecosystems, the amount of net primary productivity has a direct positive relationship with the amount of net secondary productivity.

Net primary productivity differs among ecosystems. Net primary productivity differs a great deal among ecosystems around the world. In terrestrial ecosystems, major drivers of this productivity include temperature, precipitation, nitrogen, and phosphorus. In aquatic ecosystems, major drivers include temperature, light, nitrogen, and phosphorus.

The movement of energy depends on the efficiency of energy flow. The energy of ecosystems exists in different trophic levels and moves between these trophic levels with different efficiencies. Pyramids of energy exhibit similar distributions among ecosystems, with producers having the most energy and each higher trophic group possessing less energy. Pyramids of biomass show a similar trend in terrestrial ecosystems, but they are often inverted in aquatic ecosystems. To understand how energy moves between trophic levels, we can calculate the consumption efficiency, assimilation efficiency, and net production efficiency, all of which can be multiplied to determine the overall ecological efficiency of transferring energy between adjacent trophic groups. The efficiencies can be affected by the stoichiometry of consumer tissues relative to the consumer’s diet and can affect the number of links in an ecosystem’s food chain. Efficiencies can also affect the residence time of energy and biomass in ecosystems.