How does gross primary productivity differ from net primary productivity?

Gross primary productivity (GPP) is the total energy captured by photosynthesis, while net primary productivity (NPP) is the energy left after plant respiration.

Gross primary productivity and net primary productivity are both measures of energy flow in ecosystems, but they represent different stages of this process. Gross primary productivity refers to the total amount of energy that primary producers, such as plants, capture from sunlight through the process of photosynthesis. This energy is stored in the form of organic compounds, such as glucose, which can be used by the plants themselves or by other organisms in the ecosystem.

However, not all of this energy is available for use by other organisms. Plants also need to use some of this energy for their own metabolic processes, such as growth and reproduction. This is known as plant respiration. The energy used in plant respiration is subtracted from the gross primary productivity to give the net primary productivity. Therefore, net primary productivity represents the amount of energy that is actually available for consumption by other organisms, such as herbivores and decomposers.

In other words, while gross primary productivity is a measure of the total energy input into an ecosystem, net primary productivity is a measure of the energy that is actually available to support the rest of the food web. This distinction is important for understanding energy flow in ecosystems, as it helps to explain why energy availability decreases at higher trophic levels.

It's also worth noting that the ratio of net primary productivity to gross primary productivity can vary depending on factors such as the type of ecosystem and the specific species of plants involved. For example, in a highly productive ecosystem like a tropical rainforest, the ratio may be relatively high, indicating that a large proportion of the energy captured by photosynthesis is available for consumption by other organisms. On the other hand, in a less productive ecosystem like a desert, the ratio may be lower, indicating that a larger proportion of the energy is used in plant respiration.