The study of the vital statistics that affect population size is called demography. Birth rate and death rate vary among subgroups within a population. This birth rate and death rate control the population size. Addition occurs due to birth and immigration. Deaths (Mortality) and emigration decreases the population. Certain vial factors like age structure and sex ratio affect the population size. These are studied in demography.
- Age structure
The relative number of individuals of each age in population is called age structure. Mostly average lifespan of individuals in a population is greater than time it takes to ‘nature or reproduce. Therefore, the generation overlaps. It gives different age structure in a population: isw,c of plant can be determined by annual growth rings.
Each age group has characteristic birth and death rate. Young and old individuals have more chance to die than intermediate age group. The intermediate age groups are stronger. They have ability to reproduce. For example the death rate is highest in young plants and old plants. Thus the intermediate populations has e highest birth rate. Such population increases rapidly . The populations with large number of older individuals shows decline.
- Generation time
The average span between the birth of individuals and the birth of their offspring is called generation time. It is an important demographic feature. It directly a fleets the population size. There are two types of generation times:
(a) Short generation time: There is last growth rate in populations With shorter generation time. Such populations have both high birth rate and death rate. Its example is annual plants
(b) Long generation time:They have slow growth rate. Their birth rate is equal to death rate. Its example is annual plants.
- Sex ratio
The proportion of the individual of each sex in a population is called sex ratio. It is also an important demographic statistic. It affects the population growth rate. The birth rate is related to number of females in a population. Single male can provide sperm to many. females. Therefore. the number of males is less significant for birth rate. for example. dates has e separate male and female plants. Only one or two male plants are sufficient to fertilize female plants of all the gardens.
Applications of demography
Demography is used to develop Life tables and survivor-ship curses.
- Life tables
The tables showing mortality summaries of a population is called life table. Cohorts are used to develop these tables. Cohort is a group of individuals of same age from birth till their death. Essential data is taken for the living individuals for sampling time. This sampling time is taken one year for most of the species. This data is put in table with different columns. Life tables ,how following factors of demography:
(a) The variation of mortality with age over a time period. This time period is equal to maximum life span of this population.
(b)Eggs produced by female of particular age. It is called fecundity.
(c) Birth rate in sampling time.
- Survivorship curves
A plot of the numbers of cohort alive at each age group is called survivorship curve. These are classified into three general terms:
(a)Type I: It is a flat curve. It shows low death rates during early and middle age. The death rates increase among older age groups. Therefore, this curve drops steeply. It is shown by human population.
(b)Type II: It is intermediate between type I and type III. Their mortality is constant through out the life span. It is shown by annual plants.
(c)Type III: This curve drops sharply at the left of the graph. It shows very high death rate in the young individuals. But then it becomes flat.