Questions: Population Regulation: Density-Dependent and Density-Independent Factors

Question 1

A respiratory virus sweeps through a seabird colony, killing roughly 30% of individuals regardless of whether the colony has 500 birds or 5,000 birds. What type of factor is this, and what is its effect on population regulation?

• A. Density-dependent; it acts as a negative feedback that stabilizes the population near carrying capacity
• B. Density-independent; it perturbs the population but provides no restoring force toward any particular size
• C. Density-independent; it acts as a negative feedback because it consistently reduces population size
• D. Density-dependent; it causes Allee effects by reducing mate-finding success at low density

Answer: (B)

Because the mortality rate is the same regardless of population size, this is density-independent. It can cause dramatic population crashes but cannot regulate the population — regulation requires a restoring force that intensifies at high density and relaxes at low density. A density-independent factor does not push the population toward any equilibrium; it simply reduces the population by a fixed fraction regardless of where it is relative to carrying capacity.

Question 2

After a severe winter kills 40% of a deer herd, the surviving population recovers rapidly over the next few years. What is the primary mechanism driving this recovery?

• A. The severe winter was a density-dependent factor that temporarily increased per-capita food availability
• B. At low post-winter density, density-dependent competition is relaxed, increasing per-capita food, survival, and reproduction
• C. Density-independent factors intensify at low density, providing additional resources that accelerate growth
• D. The Allee effect kicks in below a threshold density, causing rapid population expansion through positive feedback

Answer: (B)

The recovery is driven by density-dependent regulation. At low density, competition for food and nesting sites is relaxed, predators have fewer targets per unit area, and disease spreads less readily — all of which increase per-capita birth rates and survival. The winter crash (density-independent) created the low-density condition; the density-dependent mechanisms then provide the restoring force that drives recovery toward carrying capacity. Option D is wrong because the Allee effect is the opposite: positive density dependence at very low populations that causes further decline, not growth.

Question 3

Density-dependent factors provide the regulatory mechanism that can return a perturbed population toward its carrying capacity, while density-independent factors cannot.

• T. True
• F. False

Answer: True

Regulation requires a restoring force — something that becomes stronger when the population is above carrying capacity (pushing it down) and weaker when the population is below (allowing growth). Density-dependent factors have this property by definition: their effect intensifies with density. Density-independent factors hit equally hard regardless of population size, so they can crash a population but cannot provide the negative feedback that constitutes regulation.

Question 4

A regulated population maintains a constant size equal to the carrying capacity K.

• T. True
• F. False

Answer: False

Regulated populations fluctuate around a mean set by density-dependent processes — they are not locked to a constant K. Density-independent events (weather, fire, disease) continuously perturb the population above and below K, and density-dependent mechanisms provide the restoring force that keeps fluctuations from becoming permanent. The hallmark of a regulated population is not constancy but rather that deviations from K trigger corrective forces, like a thermostat that maintains a set point despite temperature fluctuations.

Question 5

Why can density-independent factors cause dramatic population crashes but cannot regulate a population, while density-dependent factors can?

Model answer: Regulation requires negative feedback — a force that intensifies when the population is above a set point and weakens when it is below, so that the population is always pushed back toward equilibrium. Density-dependent factors (competition, predation, disease, parasitism) have this property: they become stronger as density increases and weaker as it falls. Density-independent factors (drought, storms, temperature extremes) hit populations with a fixed intensity regardless of size. They can reduce a population dramatically, but they provide no tendency to restore any particular size — whether the population is at 100 or 10,000, the factor has the same per-capita impact. Without a density-sensitive restoring force, there is no regulation, only perturbation.

The key insight is that regulation is a dynamical property requiring feedback, not just any factor that reduces population size. A flood that kills 40% is not regulatory even if it prevents overpopulation — because the same flood would kill 40% of a tiny, vulnerable population too. Only factors that change their intensity with density can create the stabilizing feedback loop that defines population regulation.