Productivity, Nutrient Limitation and Ecosystem Stoichiometry
How much an ecosystem produces is usually set by whichever nutrient is scarcest relative to what organisms need, not by energy alone. Because tissues hold elements in fairly fixed ratios, adding one nutrient can shift what limits the system rather than raising production without end.
What a learner can do afterwards
- Identifies the limiting nutrient from supply ratios and requirement ratios.
- Predicts the effect of adding one nutrient, including a shift in what limits growth.
- Links element ratios in tissue to the nutrients a decomposer releases.
1 · Read
Ecosystem production is usually set by the scarcest nutrient, not by energy alone. To find it, compare two ratios: the supply ratio in the environment and the requirement ratio in organism tissue. The nutrient in shortest supply relative to demand is the limiting one. Phosphorus often plays that role, since it has no gas phase and cycles slowly through rock, water, and life.
Picture a lake where algae need nitrogen and phosphorus in a 16 to 1 ratio, but the water offers only 4 to 1. Phosphorus runs short first, so it limits growth even though nitrogen looks scarcer in absolute terms. Add phosphate and growth jumps until nitrogen runs out next, so the identity of the limiter shifts. The same jump from fertilizer runoff can feed algal blooms and dead zones downstream.
Decomposers are the recyclers that decide what comes back. They break dead tissue and keep the elements they need, releasing the surplus in mineral form. Tissue with a high nitrogen to phosphorus ratio locks up nitrogen, so phosphorus is released first. The ratio in the tissue therefore predicts the nutrients a decomposer hands back to the supply.
Never compare absolute amounts. Divide supply by demand for each nutrient in turn, and crown the smallest result the limiter.
Compare supply ratios with demand ratios, expect the limiter to shift, and read tissue ratios to predict recycling.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.