Final answer:
The largest value in an ecosystem is the gross primary productivity (GPP), which is the total energy photoautotrophs incorporate from the sun before any losses due to respiration or growth.
Step-by-step explanation:
In any ecosystem, the highest value is always the gross primary productivity (GPP). GPP refers to the total rate at which the photosynthetic primary producers, such as plants and algae, incorporate energy from the sun. However, some of this energy is used by the producers for their own growth, metabolism, and reproduction. The remaining energy after these losses is known as the net primary productivity (NPP), which is available to the next trophic levels, namely the primary consumers. As you move up the trophic levels to secondary and tertiary productivity, the available energy decreases due to metabolic energy losses at each level.
Therefore, to measure the gross primary productivity in a terrestrial ecosystem, you would measure the total rate of energy incorporation by plants from the sun, before any energy is consumed for their biological functions. After accounting for the used energy, the leftover is the net primary productivity, which is always less than the gross primary productivity. In summary, GPP is always the largest value in the ecosystem due to the energy losses at each subsequent trophic level.