Population Density Is Not Static

The size of a population is never a fixed, static parameter. It keeps changing with time, depending on factors such as food availability, predation pressure and adverse weather. In fact it is precisely these changes in density that tell us what is happening to a population — whether it is flourishing or declining.

Whatever the ultimate reasons behind them, the density of a population in a given habitat over a given period fluctuates because of four basic processes. Two of them push the density up and two pull it down.

The Two Processes That Raise Density

Natality is the number of births during a given period that are added to the initial density of the population. It is the main way a population replenishes and grows from within.

Immigration is the number of individuals of the same species that come into the habitat from elsewhere during the period under consideration. These new arrivals add to the local density even though they were not born there.

The Two Processes That Lower Density

Mortality is the number of deaths in the population during a given period. Every death removes an individual and lowers the density.

Emigration is the number of individuals of the population that leave the habitat and go elsewhere during the period under consideration. Like death, emigration removes individuals from the local population — though here they are lost to movement rather than to death.

Putting It Together: The Balance Equation

If NN is the population density at time tt, then its density at time t+1t+1 is given by

Nt+1=Nt+[(B+I)−(D+E)]N_{t+1} = N_t + [(B + I) - (D + E)]

Here BB = number of births, II = number of immigrants, DD = number of deaths and EE = number of emigrants. The population density will increase whenever the number of births plus immigrants (B+I)(B + I) is greater than the number of deaths plus emigrants (D+E)(D + E).

Under normal conditions, births and deaths are the most important factors influencing density, with the other two mattering only in special circumstances. When a new habitat is just being colonised, for example, immigration may contribute more to growth than the birth rate does.

Quick Recap

  • Population density is not static; it changes with food, predation and weather.
  • Density fluctuates due to four basic processes.
  • Two increase density: Natality (births added) and Immigration (individuals of the same species entering the habitat).
  • Two decrease density: Mortality (deaths) and Emigration (individuals leaving the habitat).
  • The balance is captured by Nt+1=Nt+[(B+I)−(D+E)]N_{t+1} = N_t + [(B + I) - (D + E)], where BB, II, DD and EE are births, immigrants, deaths and emigrants.
  • Density increases when (B+I)>(D+E)(B + I) > (D + E).
  • Normally births and deaths matter most; immigration and emigration count only in special conditions, such as a habitat being newly colonised.

Solved Examples — Section 4

Q1. Name the four basic processes that change population density.

Answer: Natality, mortality, immigration and emigration.

Q2. Which two processes increase population density?

Answer: Natality (births added) and immigration (individuals of the same species entering the habitat).

Q3. Which two processes decrease population density?

Answer: Mortality (deaths) and emigration (individuals leaving the habitat).

Q4. Write the equation relating population density at time t+1 to the density at time t.

Answer: Nt+1=Nt+[(B+I)−(D+E)]N_{t+1} = N_t + [(B + I) - (D + E)], where B, I, D and E are births, immigrants, deaths and emigrants.

Q5. According to the equation, when will population density increase?

Answer: When the births plus immigrants exceed the deaths plus emigrants, that is when (B+I)>(D+E)(B + I) > (D + E).

Q6. Under what special condition can immigration contribute more to growth than the birth rate?

Answer: When a new habitat is just being colonised.