The size of population of any species in not a state parameter .If keeps on changing with time , depending on various factors such as availability of food ,predation pressure and prevaling weather .Such changes give some idea of what is happening to the population , whether it is increasingor decreasing .Through a number of factors affect population size, the density of population in a a given habitat during a given period , fluctuates due to four basic processes - natality , mortality , immgigration and emigration.
The population density is the number of individuals of a species per unit area /space at a given time Methamatically population density is expressed as :
`D=N/S`
Where , D stands for the population density ,N denotes the number of individuals of a species at a specific time and S represents the number of units of the space.
(i) Natality it refers to the number of birth during a given a period in the popultaion that are added to the initial density.
(ii) Mortality : it is the number of deaths in the poplutaion during a given period .
Immigration : it is the number the of indiivduals to the same species that have come into the habitat from elsewhere during the time period under consideration.
(iv) Emigration : It is number of individuals of the same species who have left the habitat and gone elsewhere during the time period under consideration Natality and immigration contribute to an increase growth or change in the size of population density, while mortality and emigration to a decrease . Therefore , population growth or change in the size of population in a given time is determined by the above factors.
Change in population size= (Births + Immigration) - (Deaths + Emigration)
If N the population density at time t, then its density time t+1 is `N_(t+1)+N(t)+[(B+I)-(D+E)]`
It is clear from the above equation,that population density increases if the number of birth plus the nummber of immigrants `(B+I)` is more than the number of death plus the number of emigration `(D+E)`, otherwise it will decrease. Under normal conditions ,birth and death are the most importan factors influencing population density . If a new habitat is just being colonized , than immigration may contribute more signficantly to the population drowth than birth rates.
