Explain the factors that change population size and describe methods used to estimate populations.

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.

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-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.

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:

1. Capture a sample of animals, count and mark them harmlessly, then release them.
2. Allow time for them to mix back into the population.
3. 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.