Questions: Density-Dependence: Mechanisms and Regulation

Question 1

A mule deer population in a national park crashes dramatically after a severe three-year drought. When the drought ends and normal rainfall returns, what does density-dependence predict will happen to the population, and why?

• A. The population will remain at its crashed level, because droughts do not create recovery mechanisms
• B. The population will recover, because reduced density lowers competition and disease transmission, raising per-capita birth rates and survival
• C. The population will overshoot its original size, because density-dependent factors become stronger after droughts
• D. The population will remain crashed, because density-independent factors like drought have permanent effects

Answer: (B)

A drought is a density-independent event — it crashes the population but provides no built-in recovery mechanism. However, once population density is low, density-dependent factors relax: competition for food and territory eases, disease spreads less readily, per-capita resource availability rises, and reproduction increases. This asymmetry is the key: density-independent factors can crash populations, but it is density-dependent negative feedback that drives recovery toward carrying capacity.

Question 2

Which of the following is the best example of a density-dependent limiting factor?

• A. A late spring frost that kills 40% of songbird chicks regardless of population size
• B. Intraspecific competition for nesting territories that intensifies as population density increases
• C. An oil spill that destroys 80% of a seabird colony's habitat
• D. A hurricane that reduces a lizard population by 70%

Answer: (B)

The defining feature of density-dependence is that the factor's intensity is proportional to population size — it creates negative feedback. Competition for nesting territories directly scales with density: as more birds compete for the same number of territories, fewer individuals successfully breed, lowering per-capita reproduction. Frost, oil spills, and hurricanes are density-independent — they kill a fixed proportion or absolute number regardless of how many organisms exist, and they provide no mechanism for population regulation.

Question 3

A blizzard that kills 30% of a deer population, regardless of whether the herd has 50 or 5,000 individuals, is a density-dependent regulating factor.

• T. True
• F. False

Answer: False

A blizzard that kills the same proportion regardless of population size is density-independent. Density-dependent factors must intensify *as a function of population density* — like disease spreading faster in a crowded herd, or food running out when too many animals compete for limited resources. Density-independent factors can crash a population but cannot regulate it, because they do not strengthen when the population is large or relax when it is small.

Question 4

Density-dependent factors create negative feedback loops that push populations toward a stable equilibrium near carrying capacity.

• T. True
• F. False

Answer: True

This is the defining property of density-dependence and why it constitutes regulation rather than mere limitation. When a population exceeds carrying capacity, density-dependent pressures intensify (more competition, more disease, more waste), mortality rises above birth rates, and the population declines. When density falls below K, pressures relax, birth rates exceed death rates, and the population recovers. This self-correcting dynamic is what distinguishes true population regulation.

Question 5

Why can density-independent factors like severe weather crash a population but not regulate it, while density-dependent factors do both?

Model answer: Density-independent factors affect populations with the same intensity regardless of how large or small the population is — a freeze kills the same proportion whether there are 100 or 10,000 individuals. Because their intensity doesn't scale with density, they cannot create the negative feedback needed to drive a population back to equilibrium after a crash. Density-dependent factors, by contrast, intensify when populations are large (more competition, more disease) and relax when they are small — creating the feedback loop that both limits population growth and drives recovery.

The practical implication is that conservation efforts must distinguish these two types. After a density-independent event (wildfire, oil spill), population recovery depends on whether density-dependent mechanisms can operate — a population that survives the crash will face reduced competition and may recover rapidly if habitat remains intact. But if a density-dependent regulator (like disease or a key predator) is disrupted, the population may fail to self-regulate even after the acute threat is removed.