Population growth: what changes population density
Population density rises with natality and immigration and falls with mortality and emigration: Nₜ₊₁ = Nₜ + [(B + I) − (D + E)].
Why population density changes
The density of a population is never fixed. It rises and falls with food, weather, predators and other conditions. Ecologists track the change through four basic processes. Two add individuals: natality (births) and immigration (arrivals). Two remove individuals: mortality (deaths) and emigration (departures). Immigrants and emigrants must be of the same species as the population.
The four processes
| Process | Meaning | Effect on N |
|---|---|---|
| Natality (B) | Births in the population during a given period | Increases |
| Immigration (I) | Individuals of the same species coming into the habitat from elsewhere | Increases |
| Mortality (D) | Deaths in the population during the period | Decreases |
| Emigration (E) | Individuals of the population leaving the habitat for elsewhere | Decreases |
The growth equation
If Nₜ is the density at time t, the density one period later is Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]. Density increases when births plus immigrants (B + I) are more than deaths plus emigrants (D + E). It decreases when (D + E) is larger. Under normal conditions, births and deaths matter most. Immigration and emigration matter only in special conditions. For example, when a new habitat is being colonised, immigration may add more individuals than births do.
Worked example 1: using the growth equation
A lake has 500 egrets at the start of a season. During the season, 120 chicks hatch and survive, 30 egrets arrive from a nearby wetland, 80 die and 50 fly away. Find the number of egrets at the end of the season.
- Write the equation: Nₜ₊₁ = Nₜ + [(B + I) − (D + E)].
- Additions: B + I = 120 + 30 = 150.
- Losses: D + E = 80 + 50 = 130.
- Nₜ₊₁ = 500 + (150 − 130) = 500 + 20 = 520.
520 egrets. The population grew by 20 because additions (150) were more than losses (130).
Worked example 2: when immigration matters most
A new reservoir has no fish at first. In its first year, 400 fish swim in through a canal, 60 are born, 40 die and none leave. Find the population at the end of the year and the process that added most fish.
- Nₜ = 0 at the start.
- B + I = 60 + 400 = 460.
- D + E = 40 + 0 = 40.
- Nₜ₊₁ = 0 + (460 − 40) = 420.
420 fish. Immigration (400) added far more than births (60), as expected in a newly colonised habitat.
Indian cities such as Bengaluru grow partly by immigration, as people move in from villages and towns for work. Census planners count births, deaths and migration to forecast how big a city will become.
2 marks: “Name the four basic processes that change population density. Which two increase it?” Natality, mortality, immigration and emigration. Natality and immigration increase density; mortality and emigration decrease it.
3 marks: “Write the equation for population density at time t + 1. When will density increase, and when does immigration matter more than births?” Nₜ₊₁ = Nₜ + [(B + I) − (D + E)], with each symbol defined. Density increases when B + I is more than D + E. Births and deaths dominate normally; immigration dominates when a new habitat is being colonised.
BIDE: Births and Immigrants add; Deaths and Emigrants subtract.