Population & Its Attributes - Practice Questions & MCQ

• Individuals live in groups in a well defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population.
• The population that occupies a very small area, is smaller in size, such a population is called local population. 
• A group of such a closely related local population is called meta-population.
• A population has certain attributes that an individual organism does not. 

Population Size or Density:

• It is the number of individuals of a species per unit area or volume.
• It is usually expressed as the number of individuals per unit area or volume, for example, 100 trees per acre.
• It is affected by the following factors:

1. Birth Rate or Natality:

• Populations increase because of natality. 
• Natality is equivalent to birth rate and is an expression of the production of new individuals in the population by birth, hatching, germination (or) fission.
• For example, if in a pond, there are 20 lotus plants last year and through reproduction, 8 new plants are added, taking the current population to 28. Then, birth rate = 8/20 = 0.4 offspring per lotus per year.
   

2. Death Rate or Mortality:

• It is the rate of loss of individuals (death rate) per unit time due to death or due to the different environmental changes, competition, predation, etc. 
• For example, if individuals in a laboratory population of 40 fruit flies died during a specified time interval. Then, the death rate = 4/40 = 0.1 individuals per fruit fly per week.
   

3. Sex Ratio:

• An individual is either a male or a female but a population has a sex ratio like 60% of the population are females and 40% are males.

• Population at any given time is composed of individuals of different ages. 
• When the age distribution (percent individuals of a given age or age group) is plotted for the population, this is called age pyramid.
• The age pyramid of human population generally shows the age distribution of males and females in a combined diagram.
• The growth status of the population is reflected by the shape of the pyramids.

Expansive Pyramid: 

• The population pyramid with broad base and with successive decline in the share of population of higher age groups is known as expansive pyramid.
• This pyramid represents a situation of high fertility, high mortality, low life expectancy, higher population growth rates and low share of old age persons. 

Stationary or Stable Pyramid:

• A pyramid is described stationary when the share of population remains constant in different age groups over the period of time. 
• It represents a situation of low fertility, low mortality and high life expectancy. 
• It indicates slow population growth or stable population. 
• The stationary or near stationary population pyramid displays a somewhat equal share of juvenile and adult age groups.

Constrictive or Declining Pyramid:

• A pyramid with a narrow base is called a constricted pyramid. 
• It represents low fertility, low mortality, high life expectancy and ageing of population. 
• It is typically associated with very advanced countries which have a high level of literacy, easy access to birth control measures and very good health and medical facilities.
   

• The size of a population for any species is not a static parameter, it keeps changing with time.
• The density of a population in a given habitat during a given period, fluctuates due to the four basic processes:
  • Natality refers to the number of births during a given period in the population that are added to initial density.
  • Mortality is the number of deaths in the population during a given period.
  • Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
  • Emigration is the number of individuals of a population who left the habitat and moved elsewhere during a given period of time.
• Out of these four, natality and immigration contribute an increase in population density while mortality and emigration contribute to the decrease in population density.
• So, if N is the population density at time t, then its density at time t +1 is:

Where, 
N = Population density
t = Time,
B = Birth rate,
I = Immigration,
D = Death rate,
E = Emigration

• From the above equations, we can see that population density will increase if, (B + I) is more than (D + E).