A limiting factor is the factor in shortest supply, which controls the rate or the size. In a data question, the factor limits growth while the response rises as it increases, and stops limiting once the line levels off.
This lesson follows interpreting growth phases and adds the cause. It draws on decomposition and recycling, because nutrients reach plants from decomposed matter. The module overview is nutrient cycles and populations.
How does the idea build up?
- Find the variable you changed, such as nitrate concentration.
- Find the response, such as algae per mL.
- Look for a rising section. Here the changed factor is limiting, because more of it gives more response.
- Look for a plateau. Here the changed factor is no longer limiting, because extra does not help.
- Name what limits the plateau, using other conditions stated in the question, such as light, carbon dioxide, space or temperature.
Worked example
Algae are grown in identical flasks with different nitrate concentrations, same light and temperature, for one week. The data are invented for practice.
| Nitrate (mg per litre) | 0 | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|---|
| Algae (×1000 cells per mL) | 5 | 20 | 40 | 60 | 62 | 61 |
Question: Describe the effect of nitrate, calculate the rate of increase from 2 to 6 mg per litre, and explain the plateau.
Step 1, describe: algae increase from 5 to 60 thousand as nitrate rises from 0 to 6 mg per litre, then stay at about 60 to 62 thousand from 6 to 10.
Step 2, rate: from 2 to 6 mg per litre, the increase is 60 − 20 = 40 thousand over 4 mg per litre. 40 ÷ 4 = 10 thousand cells per mL for each 1 mg per litre.
Step 3, limiting up to 6: nitrate is limiting because algae increase as nitrate increases. Nitrate is needed to make proteins for new cells.
Step 4, plateau: from 6 to 10 mg per litre the count no longer rises, so nitrate is no longer limiting. Another factor is now limiting. Since light and temperature were kept constant, light, carbon dioxide or space are possible limits.
The data support “another factor limits”, not which one. A careful answer names a likely factor and says more data would be needed to be sure.
What mistake should I watch for?
Mistaken answer: “At 8 mg per litre the algae stop growing because nitrate runs out.”
This reverses the logic. At the plateau there is plenty of nitrate, since the amount was increased and the response did not change. Running out would be the explanation for the rising part of the line.
Corrected: “At 8 mg per litre nitrate is no longer limiting. Another factor, such as light or carbon dioxide, now limits growth.”
Ask yourself whether the graph is rising or flat at that point before you name the cause.
Check yourself
1. In the table, at which nitrate concentration does the plateau begin?
Show answer
About 6 mg per litre, where the count reaches 60 thousand and then stays at about 60 to 62 thousand.
2. In a different data set, growth of duckweed fronds rises from 10 to 50 as phosphate goes from 0 to 10 mg per litre. Calculate the rise per 1 mg per litre.
Show answer
Increase = 50 − 10 = 40 over 10 mg per litre. 40 ÷ 10 = 4 fronds per 1 mg per litre.
3. A plant in a dark cupboard is given extra carbon dioxide and its photosynthesis does not increase. Which factor is limiting?
Show answer
Light intensity. Extra carbon dioxide does not help, so light is in shortest supply.
Where does this lead next?
Next, separate causes that depend on crowding from one-off events, in density dependence versus a one-off event. The mixed practice set gives more data to try.
If you often name a limiting factor but struggle to explain how it limits growth, a teacher in online one-to-one Biology tuition can rehearse those explanations with you.