Distinguish gross and net primary productivity, and apply Liebig's Law of the Minimum to identify the limiting factor in a described ecosystem.
Gross primary productivity (GPP) is the total energy a producer captures through photosynthesis. Net primary productivity (NPP) is what is left after the producer spends some of that energy on its own respiration (GPP minus respiration equals NPP) -- NPP is the energy actually available to feed the rest of the food web. Productivity varies enormously by ecosystem: tropical rainforests and estuaries (where rivers meet the sea) rank among the most productive places on Earth, while deserts and the open ocean are so unproductive per unit area that ecologists sometimes call the open ocean a "biological desert."
Liebig's Law of the Minimum states that growth is limited not by the total resources available, but by whichever necessary resource is scarcest relative to need -- like a wooden barrel built of staves of different heights, which can only hold water up to the height of its shortest stave. A field with abundant sunlight and water but very little nitrogen in the soil will still grow poorly, because nitrogen is the limiting factor, no matter how much sunlight or water is added.
Mastery looks like: They can compute NPP from GPP and respiration, and correctly identify the limiting factor in a data-based scenario using Liebig's Law.
Common stumbles: Assuming the resource present in the SMALLEST total amount is automatically limiting, without checking it against what the organism actually needs; adding more of a non-limiting resource and expecting it to help.