The size of population is not static. It keeps changing with time, depending upon food availability, predation pressure and reduces weather. The main factors that determine the population growth are :

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1 Natality (number of birth during a given period in the population)

– Mortality (number of death during a given period in the population)

$\Rightarrow \quad$ Immigration (individual of same species that have come into the habitat)

– Emigration (individual of population that have left the habitat)

If ' $\mathrm{N}$ ' is the population density at a time't', then its density at time $\mathrm{t}+1$ is

$\mathrm{N}_{1+1}=\mathrm{N}_{1}+[(\mathrm{B}+\mathrm{I})-(\mathrm{D}+\mathrm{E})]$

Population density will increase if the number of births plus the number of immigrants $(B + I)$ is more than the number of deaths plus the number of emigrants $(D + E)$, otherwise it will decrease.