AP Syllabus focus:
‘Density-dependent and density-independent factors limit population growth as population size increases.’
Population size changes because individuals are born, die, immigrate, or emigrate.
These graphs compare exponential growth (J-shaped curve) with logistic growth (S-shaped curve). Logistic growth levels off as the population approaches carrying capacity, illustrating how limiting factors constrain growth as density rises. Source
Ecologists classify the causes that slow or stop population growth as density-dependent or density-independent, based on whether their effects change with crowding.
Core idea: “limiting factors” and density
A limiting factor is any biotic or abiotic environmental variable that reduces population growth rate by lowering survival, reproduction, or both.
Density-dependent factors
Density-dependent factors become stronger as population density increases, often because individuals interact more frequently or compete more intensely.
Density-dependent factor: A limiting factor whose per capita effect on a population increases as population density increases.
Density dependence creates negative feedback: crowding reduces future growth, helping prevent indefinite increase.
This diagram highlights the S-shaped logistic growth curve as growth slows and levels off near carrying capacity. It visually reinforces negative feedback: increasing density reduces the growth rate until net growth approaches zero. Source
Density-independent factors
Density-independent factors affect populations regardless of density, typically through abiotic disturbance or broad-scale environmental change.
Density-independent factor: A limiting factor that affects population size or growth rate independent of population density.
Density-independent limits can cause rapid declines even when populations are small, and they may override density-dependent regulation.
Common density-dependent factors (mechanisms and outcomes)
Competition for resources
As density rises, individuals may face reduced access to food, water, space, nesting sites, light, or nutrients.
Less resource availability per individual can reduce growth, body condition, and fecundity (reproductive output).
Competition can be intraspecific (within a species), which is central to density dependence.
Predation and herbivory
Predators may remove a higher number of individuals where prey are abundant.
Predators can concentrate where prey density is high, increasing encounter rates.
Plants at high density may attract more herbivores or share pathogens more easily.
Disease and parasitism
Transmission often increases with crowding because individuals contact each other more or share contaminated environments.
Direct transmission (contact, aerosols) typically increases with density.
Vector-borne disease may show weaker density dependence if vectors, not hosts, are limiting.
Waste accumulation and stress
High density can elevate metabolic wastes and intensify physiological stress.
Stress hormones can suppress immune function, indirectly increasing disease susceptibility.
Territoriality and aggression can raise injury rates and reduce reproductive success.
Density-dependent changes across life stages
Density dependence may be strongest in a particular stage:
In many animals, juvenile survival is highly sensitive to food limitation.
In plants, seedling survival may decrease sharply due to shading or root competition.
Common density-independent factors (mechanisms and outcomes)
Weather and climate extremes
Abiotic variation can reduce survival or reproduction regardless of crowding.
Drought, heat waves, freezes, hurricanes, and floods can cause sudden mortality.
Timing matters: a late frost can reduce seed set even in low-density plant populations.
Natural disasters and disturbance
Events such as wildfire, volcanic eruption, or severe storms can reduce population size without regard to density.
Disturbance can remove individuals directly or eliminate habitat structure (shelter, nesting sites).
Human-caused (anthropogenic) impacts
Many anthropogenic pressures function largely as density-independent at the population level.
Pollution and toxins can reduce survival broadly.
Habitat destruction/fragmentation can reduce carrying space for all individuals.
Some harvesting can be density-independent if removal does not decrease at low density.
How to identify density dependence in data (what to look for)
To infer density dependence, focus on per capita rates rather than raw counts.
This figure shows density-dependent regulation via reproduction: fecundity (eggs per female) declines as the number of worms increases. It visualizes the key idea that per-capita rates often change with crowding, producing negative feedback on population growth. Source
If birth rate per individual declines as density increases, reproduction is density-limited.
If death rate per individual rises as density increases, mortality is density-limited.
If survival or fecundity changes track density across time or across sites, that supports density dependence.
Density-independent effects are suggested when:
Per capita birth/death rates shift with abiotic conditions (e.g., rainfall) more than with density.
Population crashes occur following extreme events across multiple sites, even where densities differ.
Interactions and combined limits
Real populations are often constrained by both factor types simultaneously.
A density-independent drought can reduce food supply; the remaining food then intensifies density-dependent competition among survivors.
Density-dependent disease may be minimal at low density but surge after a density-independent event concentrates individuals into smaller refuges.
FAQ
Frequency dependence depends on the proportion of phenotypes or behaviours rather than total density.
Evidence often comes from comparing outcomes when density is held constant but phenotype frequencies change.
Yes, if transmission is driven mainly by an external driver rather than host density, such as:
a toxin suppressing immunity broadly
a vector outbreak independent of host numbers
Patterns can shift seasonally.
A time lag occurs when the effect of crowding appears after a delay (e.g., resource depletion impacting reproduction next season).
Lags can cause overshoots and crashes because feedback is not immediate.
Small populations can be dominated by random events, and fragmentation can make “local density” differ from overall abundance.
Movement between patches can mask density signals.
They can adjust harvest to avoid pushing populations into low-density conditions where recovery is slow.
Approaches include reducing harvest when density indices fall and protecting key breeding habitats to maintain reproduction.
Practice Questions
Define a density-dependent factor and give one example. (2 marks)
Correct definition: effect increases with population density / crowding (1)
Valid example (e.g., competition, disease, predation) (1)
A fish population in a lake shows reduced average offspring per adult as the lake becomes crowded, and occasional sharp declines after severe winters. Explain how density-dependent and density-independent factors are limiting population growth as population size increases. (6 marks)
Identifies reduced offspring per adult with crowding as density-dependent (1)
Links mechanism to density (e.g., competition for food/space, stress) (1)
States density-dependent factors can reduce birth rate and/or increase death rate (1)
Identifies severe winter as density-independent (1)
Explains density-independent impact is unrelated to density (e.g., cold causes mortality across population) (1)
Notes both types can act together to limit growth as population size increases (1)
