USA Biology Olympiad (USABO) Practice Exam

Net primary productivity in ecosystems is important because it represents the:

1. Rate of photosynthesis

2. Storage of chemical energy available to heterotrophs

3. Amount of biomass in an ecosystem

4. Level of species diversity

The correct answer is: Storage of chemical energy available to heterotrophs

Net primary productivity (NPP) is a crucial concept in ecology as it quantifies the amount of organic material produced by photosynthetic organisms, primarily plants, that is available for consumption by other organisms, particularly heterotrophs, which rely on organic compounds for their energy and growth. NPP can be defined as the difference between the rate of photosynthesis and the rate of respiration in these primary producers. Since photosynthesis converts sunlight into chemical energy and respiration uses that energy, NPP reflects the net gain of chemical energy stored as biomass in plant matter after accounting for the energy consumed by plant respiration. This stored energy becomes accessible to heterotrophs, including herbivores and carnivores, establishing an essential link in food webs. Therefore, the importance of NPP lies in its role in delineating the quantity of energy that is potentially available to support the entire ecosystem. While the rate of photosynthesis, the amount of biomass in an ecosystem, and species diversity are significant ecological factors, they do not directly illustrate the availability of energy for heterotrophs in the same way that NPP does. Thus, understanding NPP is vital for grasping energy flow and nutrient cycling in ecosystems.