A change in population size is density dependent if its effect grows as the population gets more crowded, and a one-off event if it strikes whatever the size. Questions often give a graph or table and ask you to choose, then justify with evidence.
This follows explaining a limiting factor. Together they build the causes behind population growth phases. The wider module is nutrient cycles and populations.
How does the idea build up?
Ask one question: would this effect be stronger if the population were bigger?
- Yes: density dependent. Examples are competition for food or space, disease that spreads more easily when individuals are close, and predators that catch more of a large prey population.
- No: a one-off event, such as a flood, fire, cyclone or pollution spill. It affects the population regardless of how crowded it was.
The data test is the percentage affected. If the percentage rises with population size, the cause is density dependent. If the percentage is about the same, or the cause is clearly an outside event, it is not.
Worked example
A biologist records the percentage of fish dying from food shortage in ponds of different population size. The data are invented for practice.
| Population | 200 | 800 | 1600 |
|---|---|---|---|
| Fish dying from food shortage (%) | 10 | 25 | 50 |
Question: Calculate the number dying in each pond and explain whether food shortage is density dependent.
Step 1, numbers: 200 × 10% = 20. 800 × 25% = 200. 1600 × 50% = 800.
Step 2, compare: the population went up 8 times (200 to 1600), but deaths went up 40 times (20 to 800). The percentage rising from 10% to 50% shows the effect strengthens with crowding.
Step 3, conclude: food shortage is density dependent. More fish compete for the same food, so a greater share go without.
Now compare a second pond whose count was 500, 500, 100, 160 and 300 over five years. A flood in year 3 killed most fish.
Step 4, event: 500 to 100 is a fall of 400, which is 400 ÷ 500 × 100 = 80%. The flood caused this, and it did not depend on crowding.
Step 5, recovery: the population then rises to 160 and 300. Survivors have more food each and breed, so numbers recover.
What mistake should I watch for?
Mistaken answer: “The population fell sharply, so it must be competition.”
A sharp fall is not enough evidence. Competition usually builds gradually as numbers climb. A sudden crash with a named event, and recovery afterwards, is better explained by the event.
Corrected: “The population fell by 80% in one year, matching the flood. This is a one-off event, not density dependent. Numbers rose again afterwards as survivors had more food.”
Match the cause to the timing and size of the change.
Check yourself
1. Is a fungal disease that spreads faster in crowded fish ponds density dependent or a one-off event?
Show answer
Density dependent. Its effect is stronger when the population is more crowded.
2. A population of 1200 falls to 300 after a fire. Calculate the percentage fall.
Show answer
Fall = 1200 − 300 = 900. 900 ÷ 1200 × 100 = 75%.
3. Why might small populations be hit hard by chance events? (A tool like the probability tree and counting board can help you practise chance reasoning.)
Show answer
Few individuals means that a chance loss can remove a large share or all of the group, so random events matter more.
Where does this lead next?
Put all five lessons together in the mixed practice set. Later, human influence on ecosystems uses the same ideas for pollution and habitat change.
If you want feedback on whether your causes match your evidence, online one-to-one Biology tuition can give you that, using your own answers.