Organisms and populations

Organisms and Environment

Population:

• It is a group of similar individuals living in a geographical area, sharing similar resources, and capable of interbreeding.

• Population has certain attributes, which individual organisms do not possess:

  • Birth rate per capita births

  • Death rate per capita deaths

  • Sex ratio − Ratio of number of males to females in a population

• Demography: It refers to the statistical study of human population considering the following factors:
  • Distribution of population
  • Size and Density of population
  • Birth rate
  • Death rate 
  • ​Growth rate of population (​Growth rate = Birth rate - Death rate)

Age distribution

• A population can be composed of individuals of different age groups.

• Age distribution plot for a given population is given by the age pyramid.

• The structure of the age pyramid determines the growth status of the population, whether it is growing, stable, or declining.

Population size, more technically, is referred to as population density (N), which indicates the number of individuals inhabiting a particular niche.

If the population is huge, then relative density is measured instead of absolute density whose measurement is time-consuming.

Population:

• It is a group of similar individuals living in a geographical area, sharing similar resources, and capable of interbreeding.

• Population has certain attributes, which individual organisms do not possess:

  • Birth rate per capita births

  • Death rate per capita deaths

  • Sex ratio − Ratio of number of males to females in a population

• Demography: It refers to the statistical study of human population considering the following factors:
  • Distribution of population
  • Size and Density of population
  • Birth rate
  • Death rate 
  • ​Growth rate of population (​Growth rate = Birth rate - Death rate)

Age distribution

• A population can be composed of individuals of different age groups.

• Age distribution plot for a given population is given by the age pyramid.

• The structure of the age pyramid determines the growth status of the population, whether it is growing, stable, or declining.

Population size, more technically, is referred to as population density (N), which indicates the number of individuals inhabiting a particular niche.

If the population is huge, then relative density is measured instead of absolute density whose measurement is time-consuming.

• The size of a population is an ever-changing aspect since it depends upon availability of food, predation, weather conditions, etc.

• This gives us an idea whether a certain population is growing or declining.

• Some of the reasons for the increase or decrease in population:

  • Natality (B) − Number of births during a given period in the given population

  • Mortality (D) − Number of deaths during a given period in the given population

  • Immigration (I) − Number of individuals of the same species who have come into the habitat from elsewhere during a given period

  • Emigration (E) − Number of individuals of the same species who have left the habitat and gone elsewhere during a given period

• If N is the population at time t, then its density at t + 1 is

Nt + 1 = Nt + [(B + I) − (D + E)]

Growth Models

• Exponential Growth − When the resources are unlimited, population tends to grow in an exponential pattern.

  • If the population size is N and the birth and death rates (not per capita) are b and d respectively, then increase or decrease in N at t (time period) is given by,

dN /dt = (b − d) × N

If (b − d) = r, then

dN/ dt = rN

r is the “intrinsic rate of natural increase”.

Or,

N_t = N₀ e^rt

Where,

N_t − Population density at time t

N₀ − Population density at time 0

r − Intrinsic rate of natural increase

e − Base of natural logarithms (2.71828)

• Logistic growth − When the resources are limited leading to competition between individuals and survival of the fittest, the population tends to grow in a logistic manner.

  • In this kind of growth, there is an initial lag phase followed by acceleration or deceleration phases and finally asymptote, when it reaches its carrying capacity (K).

  • When N in relation t…