Population Dynamics and Sampling - TCE Biology (Tasmania)
“Explain the factors that change population size and describe methods used to estimate populations.”
A population changes size through births, deaths, immigration, and emigration. Growth is limited by the carrying capacity of the environment, set by density-dependent factors (such as food and disease) and density-independent factors (such as drought and fire). Ecologists estimate population size using quadrats for non-mobile organisms and capture-mark-recapture for mobile animals.
Population dynamics and field sampling are not part of TASC's Biology Level 3 course (BIO315124, current for 2026), so this page is not examined in the Level 3 external exam. They are part of TASC's Biology Level 2 course (BIO215123). Level 3 examines changes in the gene pool of populations instead: see population genetics.
What this dot point is asking
What changes a population's size
A population is all the individuals of one species in an area. Its size changes through four processes:
- Births and immigration (individuals arriving) increase the population.
- Deaths and emigration (individuals leaving) decrease it.
When births plus immigration exceed deaths plus emigration, the population grows. The balance of these four factors over time produces the population's growth pattern.
Carrying capacity is the maximum population size of a species that an environment can support indefinitely, given the available resources such as food, water, and space.
Patterns of population growth
Two idealised growth patterns are commonly described:
- Exponential growth: when resources are unlimited, a population grows faster and faster, producing a J-shaped curve. This cannot continue for long in nature.
- Logistic growth: as the population approaches carrying capacity, growth slows and levels off, producing an S-shaped curve. Resource limits and rising competition cause this slowing.
In reality, populations often fluctuate around the carrying capacity rather than holding perfectly steady, rising when conditions are good and falling when they worsen.
Density-dependent factors
Density-dependent factors have a stronger effect as the population becomes more crowded. They include:
- Competition for food, water, light, and space.
- Predation, which often increases when prey are abundant.
- Disease and parasites, which spread more easily at high density.
- Accumulation of waste.
These factors act as negative feedback, pushing a population back toward its carrying capacity. As density rises, they intensify and slow growth; as density falls, they ease and allow recovery.
Density-dependent factors such as competition, predation, and disease grow stronger as a population becomes more crowded. This means they tend to hold a population near its carrying capacity, acting as a self-correcting brake on unlimited growth.
Density-independent factors
Density-independent factors affect a population regardless of how crowded it is. They are usually abiotic events such as:
- Drought, flood, or fire.
- Extreme temperatures.
- Natural disasters such as storms or volcanic eruptions.
A bushfire kills a similar proportion of a population whether that population is large or small, so its effect does not depend on density. These factors can cause sudden, large changes in population size.
A rabbit population grows rapidly in a good season (low density-dependent pressure). As numbers rise, food runs short and a disease spreads more easily, both density-dependent factors that slow growth near carrying capacity. Then a drought, a density-independent factor, kills many rabbits regardless of density, crashing the population. The combination produces the rise-and-fall pattern seen in real populations.
Estimating population size
It is rarely possible to count every individual, so ecologists sample and estimate. For organisms that do not move much (plants, slow invertebrates), quadrats are placed, ideally at random, and counts are scaled up to the whole area. For mobile animals, the capture-mark-recapture method is used:
- Capture a sample of animals, count and mark them harmlessly, then release them.
- Allow time for them to mix back into the population.
- Capture a second sample and count how many are marked.
If marked animals make up a small fraction of the second sample, the total population is large; if they make up a large fraction, the population is small. The estimate assumes the marks do not affect survival, no births, deaths, or migration occur during the study, and individuals mix randomly.
Reliability of estimates
Estimates are only as good as the sampling. Reliability improves with larger sample sizes, random placement of quadrats to avoid bias, and meeting the assumptions of the method used. Repeating the sampling and reporting a range rather than a single figure gives a more honest picture of the true population size.
A common error is to label every limiting factor as density-dependent. The test is whether the factor's effect changes with crowding. Competition and disease are density-dependent because they hit harder at high density. A bushfire or drought is density-independent because it affects the same proportion regardless of how crowded the population is.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original6 marksResearchers estimate a possum population using capture-mark-recapture. In the first capture they mark and release possums. In a second capture of possums, are found to be marked. Using the Lincoln index , estimate the population size, and state two assumptions the method relies on.Show worked answer →
A 6 mark answer applies the formula correctly and states two valid assumptions.
Calculation. (marked first), (second sample), (recaptured marked). possums.
Assumptions. Any two of: marked individuals mix randomly back into the population before the second sample; no births, deaths, immigration or emigration occur between samples; marking does not affect an animal's survival or chance of recapture; marks are not lost.
Markers reward the correct estimate of and two valid assumptions of the capture-mark-recapture method.
Original5 marksDistinguish between density-dependent and density-independent factors that limit population size, giving one example of each, and explain what is meant by the carrying capacity of an environment.Show worked answer →
A 5 mark answer distinguishes the two factor types with examples and defines carrying capacity.
- Density-dependent factors
- Their effect increases as population density rises; examples include competition for food, predation, and disease (which spreads faster in dense populations).
- Density-independent factors
- Their effect does not depend on density; examples include floods, fire, drought and extreme temperatures, which affect the same proportion regardless of how crowded the population is.
- Carrying capacity
- The maximum population size an environment can sustain indefinitely given its available resources (food, water, space); population tends to level off around this value.
Markers reward the density-based distinction, one valid example of each, and a correct definition of carrying capacity.