Population density (how many individuals live in a given area or volume) can change whenever the number of individuals or the space they occupy changes. In biology questions, this is usually framed in terms of births, deaths, immigration, and emigration.

Core conditions that change population density

For any population (animals, plants, microbes, even humans), density can go up or down when:

  1. Birth rate changes (natality)
    • More births than before → population density increases because more individuals are added to the same area.
 * Example: A mild winter and plenty of food cause more rabbits to survive and reproduce, raising their density in a field.
  1. Death rate changes (mortality)
    • Higher death rate → density decreases as individuals are removed from the population.
 * Example: A disease outbreak in a deer herd kills many individuals, lowering their density in the forest.
  1. Immigration changes (individuals moving in)
    • More individuals move into the area → density increases.
 * Example: Fish from surrounding reefs move into a protected marine reserve, increasing the density there.
  1. Emigration changes (individuals moving out)
    • More individuals move out of the area → density decreases.
 * Example: Birds leaving a region because of habitat loss reduce the local population density.

In many exam-style biology questions, you’ll see this summarized as:

Population density in a habitat is influenced by four basic processes: natality, mortality, immigration, and emigration.

  • Births and immigration → increase density.
  • Deaths and emigration → decrease density.

Environmental conditions that drive those changes

Behind those four processes are environmental and ecological conditions that make them rise or fall:

  • Food and nutrient availability
    • Plenty of food → higher birth rates, lower deaths → density tends to increase.
* Food shortage → higher deaths, lower births → density decreases.
  • Climate and weather extremes
    • Heat waves, extreme cold, droughts, storms, or floods can kill individuals directly or reduce reproduction → lower density.
* Favorable climate (mild temperatures, reliable rainfall) supports survival and reproduction → higher density.
  • Disease and predation (density-dependent factors)
    • At high density, disease spreads more easily and predators find prey more quickly → can raise death rates and reduce density.
* At low density, disease spread may slow, and predation pressure per individual can drop.
  • Habitat quality and space
    • Good habitat (shelter, nesting sites, fertile soil, water) supports more individuals → higher density.
* Habitat loss or degradation (pollution, deforestation, urbanization) reduces carrying capacity → density falls as deaths/emigration rise.
  • Catastrophic events (density-independent factors)
    • Fires, hurricanes, floods, or human disasters can sharply reduce population size regardless of initial density → density decreases suddenly.
  • Human and social factors (especially for human populations)
    • Jobs, infrastructure, low crime, good health care and education attract people (immigration) → local human population density rises.
* War, persecution, poor services, or high crime drive people away (emigration) or increase mortality → density falls.

Simple way to remember it

Think of population density as:

Density=Number of individualsArea (or volume)\text{Density}=\frac{\text{Number of individuals}}{\text{Area (or volume)}}Density=Area (or volume)Number of individuals​

Any condition that:

  • Adds individuals (births, immigration),
  • Removes individuals (deaths, emigration), or
  • Effectively changes usable area (habitat loss or expansion)

will change the density of the population.

Information gathered from public forums or data available on the internet and portrayed here.