(i) Population size and population density

(ii) Birth or natality rate

(iii) Death or mortality rate.

(i) Population size. The size of population keeps changing with time depending on the factors like :

(i) Food availability (ii) Weather

(iii) Predation pressure and (iv) Competition.

The population density in a given habitat during a given period, changes due to four basic processes namely (1) Natality, (2) Mortality, (3) Immigration and (4) Emigration.

While natality and immigration contribute to an increase in the size and density of a population, mortality and emigration contribute to a decrease in them.

So the equation for population growth is :

N_{t +1} = N_t + [(B + I) - (D + E)],

where N_t = population density at time t.

B = birth rate.

I = Immigration.

D = death rate.

E = emigration.

If B + I is more than D - E, the population density increases.

If B + I is less than D - E, the population density decreases.

(ii) Birth or Natality rate. It is generally expressed as number or births per 1,000 individuals of a population per year. It increases the population size (total number of individuals of a population) and population density.

(iii) Death or Mortality rate. It is opposite of natality rate. It is commonly expressed as the number of deaths per 1,000 individuals of a population per year.