AP Syllabus focus:
‘Population growth dynamics depend on birth rate, death rate, and current population size.’
Population size changes over time because individuals are added and removed. In AP Biology, the key drivers are birth rate, death rate, and current population size, which together determine how quickly populations grow or shrink.
Core drivers of population growth dynamics
Population growth dynamics describe how a population’s size changes across time as conditions and the population itself change.
These paired graphs compare exponential growth (J-shaped curve) with logistic growth (S-shaped curve) as population size changes over time. The logistic curve levels off at a carrying capacity, illustrating how resource limitation alters the balance between births and deaths as a population becomes large. Source
Population growth dynamics: Patterns and causes of change in population size over time, determined primarily by births, deaths, and current population size.
A population increases when additions via births exceed losses via deaths, and it decreases when deaths exceed births.
This figure depicts rapid exponential (J-shaped) population growth that overshoots carrying capacity, followed by a die-off as deaths rise above births. It reinforces that when population size becomes very large, the same basic processes (births and deaths) can shift dramatically due to resource depletion and other density-dependent pressures. Source
The same birth and death rates can have very different outcomes depending on how many individuals are already present.
Birth rate (natality)
Birth rate is the contribution of new individuals to the population per unit time. Birth rate is shaped by biological capacity to reproduce and environmental conditions that affect reproduction.
Birth rate: The number of new individuals produced in a population per unit time.
Key factors that influence birth rate include:
Age structure: Populations with many individuals in reproductive age classes tend to have higher birth rates than populations dominated by juveniles or older, post-reproductive individuals.
Time/seasonality: Many species reproduce during specific seasons; birth rate can vary substantially across the year.
Resource availability: Limited food, water, or nesting sites can reduce reproductive output by lowering fertility or increasing failure of offspring to survive to be counted as new individuals.
Physiological stress and health: Stress responses, malnutrition, and disease can lower fertility, reduce mating success, or increase embryo/offspring loss.
Mate availability and reproductive behaviour: If individuals cannot find mates or signals/courtship are disrupted, births may decrease even if the environment could otherwise support reproduction.
Because birth rate is a rate, it is sensitive to how births are measured (e.g., counting births, counting recruits that survive to a census point, or counting offspring per breeding female). These choices can change the apparent magnitude of birth rate without changing the underlying biology.
Death rate (mortality)
Death rate is the removal of individuals per unit time. It is influenced by both internal factors (such as age-related decline) and external pressures.
Major factors that influence death rate include:
Predation and herbivory pressure: Increased predator efficiency or predator abundance can raise mortality, especially in juveniles.
Disease and parasitism: Outbreaks can sharply increase death rates and can be amplified when individuals are stressed or in poor condition.
Resource limitation: Insufficient resources can cause starvation and weaken immune function, increasing mortality.
Abiotic conditions: Temperature extremes, drought, storms, and other physical conditions can increase deaths directly or indirectly by reducing habitat quality.
Life stage vulnerability: Mortality often differs across life stages; for many organisms, early life stages have the highest death rates, strongly affecting overall population change.
Birth and death rates interact: conditions that reduce reproduction may also increase mortality, producing a compounded negative effect on growth dynamics.
Why current population size matters
The current population size affects growth dynamics because each birth or death represents a different fraction of the total population depending on how large the population is.
This diagram shows logistic (sigmoid) growth of population size over time, with the curve approaching a carrying capacity. It visually emphasizes that as population size increases, growth slows because density-dependent limits reduce the net effect of births relative to deaths. Source
In small populations, a few deaths (or a short reproductive failure) can cause a large proportional decline, making population size more variable and recovery less certain.
In large populations, the same absolute number of births or deaths produces a smaller proportional change, so population size tends to change more smoothly.
Current size also influences how often individuals encounter mates, competitors, pathogens, or predators, which can feed back to change future birth and death rates. Even without naming specific models, AP Biology emphasizes this central idea: birth rate, death rate, and present population size jointly determine population growth dynamics.
FAQ
Common approaches include repeated censuses of known individuals and tracking age classes over time.
Methods may involve:
Marking individuals and recording survival and recruitment
Nest/offspring monitoring to estimate successful recruitment, not just births
Per-capita rates standardise reproduction and mortality relative to population size, making comparisons across populations and time periods more meaningful.
They help separate “more births because there are more individuals” from “higher reproductive output per individual”.
Short intervals can capture brief pulses (e.g., a breeding season), while long intervals average over highs and lows.
Longer intervals may hide:
Seasonal reproduction
Short disease outbreaks
Episodic extreme weather mortality
Age-specific rates break birth and death rates down by age class (e.g., juvenile vs adult). Overall rates can be misleading if most individuals are too young to reproduce or concentrated in high-mortality stages.
If deaths exceed births over the same time period, total size falls even when many births occur.
This can happen when:
Juvenile mortality is extremely high
A disease or stressor increases deaths across multiple life stages
Practice Questions
State how birth rate and death rate determine whether a population increases or decreases in size. (2 marks)
Population increases when birth rate is greater than death rate. (1)
Population decreases when death rate is greater than birth rate (or is stable when equal). (1)
Describe two factors that can reduce birth rate and two factors that can increase death rate in a population. Explain why the same number of deaths can have a larger impact on a small population than on a large population. (5 marks)
Any two valid birth-rate-reducing factors (e.g., low mate availability, limited resources, seasonality, disease/stress reducing fertility). (2)
Any two valid death-rate-increasing factors (e.g., predation, disease, abiotic extremes, starvation/resource limitation). (2)
Explanation that in a small population, a fixed number of deaths is a larger proportion of the total, so population size changes more dramatically. (1)
