Natality and Mortality
பிறப்பு மற்றும் இறப்பு விகிதம்
Population density is not static; it is a dynamic equilibrium. If you ignore Natality and Mortality, you cannot calculate the intrinsic rate of natural increase. NEET traps you by confusing these with migration factors. Focus on the basics first.
The idea
Natality refers to the number of births during a given period in the population that are added to the initial density. Mortality is the number of deaths in the population during the same period. These two are the most important factors influencing population density variation. They act in opposite directions to regulate the total count.
Why it matters
These processes regulate the size of a population based on resource availability and environmental conditions.
NEET lens
NEET asks this 9 times in a decade. Memorize the formula: Nt+1 = Nt + [(B + I) - (D + E)]. Do not confuse Natality (births) with Fecundity (potential to reproduce). Always check if the question specifies 'per capita' or 'absolute' numbers.
Clinical significance
In human epidemiology, these metrics determine the R0 (basic reproduction number) of a pathogen. When mortality exceeds natality, a population faces extinction risk. Conversely, uncontrolled natality in a closed ecosystem leads to resource depletion and subsequent population collapse.
Introduction
Natality is the number of births during a given period in the population that are added to the initial density; mortality is the number of deaths in the population during a given period.
Core concept
Birth rate and death rate are expressed per capita. Example (NCERT): if in a pond there were 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, the birth rate is 8/20 = 0.4 offspring per lotus per year. If 4 individuals in a laboratory population of 40 fruit flies died during a specified time interval, say a week, the death rate in the population during that period is 4/40 = 0.1 individuals per fruit fly per week. Population density changes by four basic processes: natality and immigration increase density; mortality and emigration decrease it. Nt+1 = Nt + [(B + I) − (D + E)].
Go deeper
These rates are characteristic of a population, not of an individual — an individual is born or dies, but a population has a birth rate and death rate.
From Class 10
If 10 babies are born in a village of 100 people in a year, the birth rate is 0.1 per person per year.