AI Tutor
Why do massive grasslands support millions of zebras, but only a handful of lions?
The answer comes down to trophic energetics — the strict accounting rules that govern how chemical energy flows through living systems.

Primary Productivity

Gross primary productivity (GPP) is the total rate at which autotrophs (producers like plants and algae) convert solar energy into chemical energy through photosynthesis. It is typically measured in energy per unit area per unit time, such as kJ⋅m−2⋅yr−1\text{kJ}\cdot\text{m}^{-2}\cdot\text{yr}^{-1}.
Autotrophic Respiration (RR) represents the portion of captured chemical energy that primary producers consume to fuel their own metabolic processes and cellular maintenance, released as heat.
The energy that remains after metabolic costs is stored as new plant tissue and is called net primary productivity (NPP).
NPP=GPP−R\text{NPP} = \text{GPP} - R
Let's work through an example: A temperate forest ecosystem has a measured GPP of 28,000 kJ⋅m−2⋅yr−128{,}000\text{ kJ}\cdot\text{m}^{-2}\cdot\text{yr}^{-1}. If autotrophs consume 16,000 kJ⋅m−2⋅yr−116{,}000\text{ kJ}\cdot\text{m}^{-2}\cdot\text{yr}^{-1} for respiration, what is the NPP?
Applying our formula gives:
NPP=28,000−16,000=12,000 kJ⋅m−2⋅yr−1\text{NPP} = 28{,}000 - 16{,}000 = 12{,}000\text{ kJ}\cdot\text{m}^{-2}\cdot\text{yr}^{-1}
Only this 12,000 kJ⋅m−2⋅yr−112{,}000\text{ kJ}\cdot\text{m}^{-2}\cdot\text{yr}^{-1} represents actual plant growth and reproduction available to herbivores.
Once herbivores eat this biomass, how much of that energy actually makes it into the next predator?