Population and Population Attributes

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources and capable of interbreeding.

• A group formed by asexual reproduction is also treated as a population for ecological study.
• Examples include cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, and lotus plants in a pond.
• Population ecology is important because it links ecology with population dynamics, genetics, and evolution, and natural selection operates at the population level.

A population has attributes of its own, while an individual organism does not. An individual has birth and death, but a population has birth rate and death rate.

Main attributes included here are:

• size
• density
• natality
• mortality
• sex ratio
• age distribution or age pyramid
• immigration
• emigration
• dispersion
• biotic potential
• population growth forms

Population density is the total number of individuals present in a unit area or volume at a specific time. It may also be described as population size in relation to unit space and time.

Formula:

Natality is the birth rate of a population. It is the production of new individuals in a population by birth, hatching, germination, or fission. It has a major influence on population growth.

Natality may be expressed by the formula:

Types of natality:

• Crude birth rate: number of births per 1000 population per year.
• Specific birth rate: birth rate related to a specific criterion, such as age.
• Absolute natality: number of births under ideal conditions with no competition and abundant resources.
• Realised natality: number of births when environmental pressures operate.

Relation:

Example:

Mortality is the death rate of a population. It is the number of deaths in a population in proportion to its size per unit time.

Mortality may be expressed by the formula:

It may also be expressed as:

• deaths per 1000 individuals per year
• loss of individuals in unit time
• Specific mortality, that is, the number of members of an original population dying after a given time

Types of mortality:

• Absolute mortality: number of deaths under ideal conditions with no competition and abundant resources.
• Realised mortality: number of deaths when environmental pressures operate.

Relation:

Example:

Mortality is high at high density because of overcrowding, increased predation, and the spread of disease. It may also increase due to storms, floods, accidents, predators, and parental desertion.

Sex ratio is the ratio of the number of individuals of one sex to that of the other sex in a population. It may also be expressed as the number of females and males per 1,000 individuals or as a percentage.

Example:

• In a surveyed deer population, 60% of the individuals are females and 40% are males.

A male:female ratio of 1:1 is generally considered the most common evolutionary stable strategy.

Sex ratio is affected by:

• birth rate
• death rate
• immigration
• emigration

A population contains individuals of different ages. The percentage of individuals of a given age or age group in the population is called age distribution.

When age distribution is plotted graphically, the structure formed is called an age pyramid.

The shape of the age pyramid shows whether the population is:

• growing
• stable
• declining

General age groups:

• Pre-reproductive group
• Reproductive group
• Post-reproductive group

Age distribution pyramids

• Dispersion is the tendency of a population to spread out in all directions until it reaches barriers.
• Immigration is the movement of individuals into a population area and causes a rise in the population level.
• Emigration is the movement of individuals out of a population area. It generally occurs in overcrowded populations and helps regulate population size and prevent overexploitation of habitats.
• Migration is a mass movement of population from one place to another and back. Examples include Siberian cranes migrating from Siberia to Vedanthangal, salmon moving from sea to freshwater, and eels moving from freshwater to the sea.

• Population is the basic ecological unit for studying genetics, evolution, and ecological change.
• Population density tells the number of individuals per unit area or volume.
• Natality increases population size, while mortality decreases it.
• Sex ratio describes the proportion of males and females in a population.
• The age pyramid helps in identifying whether a population is growing, stable, or declining.