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AP Biology · Unit 8: Ecology

8.4 Effect of Density of Populations

Density-dependent factors

Density-dependent factors have a stronger effect when a population is crowded. Examples include competition for resources, predation, and disease (which spreads faster in dense populations). These factors help regulate population size around the carrying capacity.

Density-independent factors

Density-independent factors affect a population regardless of its density. These are usually abiotic events like weather, natural disasters (floods, fires), and temperature changes — a hurricane hits a dense and sparse population alike.

Key terms

Density-dependent factor
A factor whose effect grows with population density (competition, predation, disease).
Density-independent factor
A factor that affects a population regardless of density (weather, disasters).
Competition
A density-dependent factor: organisms compete for limited resources.
Predation
A density-dependent factor that often increases as prey become crowded.
Population regulation
Density-dependent factors help hold populations near carrying capacity.

Try an AP-style question

Data

Logistic growth: dN/dt = rN(K − N)/K. A population has r = 0.2, K = 1,000.

At which population size is growth fastest?

  1. 100
  2. 500
  3. 900
  4. 1,000
Show the answer

B. Logistic growth is fastest at N = K/2, where rN(K − N)/K is largest.

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