Use per-capita evidence to identify density dependence

Mechanism before a label

Density-dependent effects vary with population density, such as competition for a limiting resource. Density-independent disturbances can affect populations without a consistent dependence on density, although their consequences may interact with density and other conditions.

Distinguish absolute deaths from per-capita mortality. A larger population can have more deaths with the same individual risk. To infer regulation, examine whether per-capita growth tends to decrease as density rises under comparable environments.

Correlation between density and mortality does not isolate the cause. Replicated comparisons that vary density while controlling resources, age composition and observation duration strengthen an explanation. Spatial and temporal samples can share conditions and are not automatically independent experiments.

Predict, compare and calculate

The following record is an original synthetic teaching dataset. Its assigned conditions are not a claim that an actual organism was measured. Write your prediction before revealing the changed condition.

Mortality comparison	Value (individuals)
Low-density initial N	100
Low-density deaths	10
High-density initial N	400
High-density deaths	40

Matched intervals in a fictional population study.

Your prediction before comparison

This scratch prediction is not saved automatically. Copy it to the lesson notebook to retain it.

Evidence conditionReference comparisonChanged condition / follow-up

Use per-capita evidence to identify density dependence

Supplied synthetic evidence: Matched intervals in a fictional population study.

Mortality comparison: reference condition

Quantity	Value	Unit
Low-density initial N	100	individuals
Low-density deaths	10	individuals
High-density initial N	400	individuals
High-density deaths	40	individuals

Task: Explain why a drought cannot automatically be classified from the word drought alone in every dataset.

Absolute death counts alone do not demonstrate density dependence.

Compare with the conditional interpretation

Mortality is 10% in both, despite four times as many high-density deaths.

Reference mortality ratios are10/100=0.10 and40/400=0.10. In the changed case120/400=0.30, a20-percentage-point increase relative to the low-density group.

Case index for this investigation

0. Distinguish directional responses, activity and fitness
1. Track energy flow and distinguish production from standing biomass
2. Estimate abundance with sampling assumptions and uncertainty
3. Compare exponential and density-limited growth models
4. Use per-capita evidence to identify density dependence
5. Infer direct and indirect interactions from controlled contrasts
6. Separate richness, evenness and sampling completeness
7. Connect disturbance, nutrient cycles and evidence-based intervention

No numerical confidence, laboratory credit or independent mastery is inferred from opening a supplied case. Save your first prediction and comparison in the notebook.

Reset this unsaved comparison

Worked reasoning

Reference mortality ratios are10/100=0.10 and40/400=0.10. In the changed case120/400=0.30, a20-percentage-point increase relative to the low-density group.

Mortality is 10% in both, despite four times as many high-density deaths.

Apply to a new case

Explain why a drought cannot automatically be classified from the word drought alone in every dataset.

Compare your reasoning with the worked response

The initiating abiotic event may be density independent, while mortality can depend on crowding or resources. Examine how per-capita effects vary with density and define the causal level being classified.

This public response supplies feedback, not a score for independent reasoning. Record which part of your prediction changed and why.

Evidence and investigation boundary

Absolute death counts alone do not demonstrate density dependence. A pattern alone does not identify competition, transmission or another mechanism.

For a supervised investigation, predeclare the question, biological unit of replication, treatment/comparison assignment, quantities and units, exclusion rules, graph and uncertainty method. Keep the raw records and the first prediction separate from later revisions. A simulated case does not supply laboratory hours or establish AP authorization. No physical manipulation is required by this lesson.

Source and continuation

Framework topics: 8.4. Current AP Biology framework; supporting scientific reference. Sources checked 30 October 2026; all teaching wording, questions and numbers here are original. Return to AP Biology to compare this mechanism with another scale of organization.