A population is a group of individuals of the same species that live together in a region. Population ecology studies populations and their interactions with the environment. Populations have characteristics like density, birth rate, death rate, and distribution that can be measured and compared. [END SUMMARY]

1. Population Ecology
Topic- Population Characteristics
A population is a collection of individuals of the same species that live together in a
region.
Population ecology is the study of populations and their interaction with existing
environment.
A population has several characteristics which is the function of the whole group and not
of the individual. Different populations can be compared by measuring these attributes or
characteristics, such as density, natality, mortality, growth forms, distribution, etc.
The study of the group characteristics or parameters of the human population, their
changes over time and prediction of future changes is known as demography.
Population density
The size of the population is represented by its fundamental properties called density. It
can be defined as the total number of individuals per unit area or volume at the given
time.
There are two types of density-
• Crude density- It is the density per unit total space.
• Specific (ecological) density- It is the density per unit of habitat space.
Determining population size-
• Usually population size is estimated by counting all the individuals from a small sample
area then calculated for large area.
• Another very commonly known method is Mark recapture technique.
• Where a small random sample of the population is captured, marked, then release to
disperse within the general population.
• The marked individuals mix with the unmarked individuals within a period of time are
randomly mix freely.
• The population was resampled and the number of marked and unmarked individuals is
recorded.
• And it was considered as second sample taken in the same ratio as first one.

2. We can use a simple formula for estimating total population size (N):
N = Total individual mark in first sample × size of second sample
Number of marked individual re captures in second sample
Natality
It refers to the birth of individuals in a population.
Natality rate or birth rate is expressed as the number of individuals produced per female
per unit time.
Natality may be maximum natality or eclogical natality.
Maximum natality is also known as absolute of physiological natality.
Ecological or realised natality refers to number of individuals produced under an actual
or specific environmental condition. It is not a constant for a population but may vary
with the size and age composition of the population and the physical environmental
condition.
In ecology, fecundity and fertility is not same.
The 'fecundity' describes as the maximum reproductive outcome potential of an
individual over its lifetime under favourable environmental condition.
But the term 'fertility' differs from fecundity which describes the actual reproductive
performance of an individual under favourable condition and it is a generalization of the
terms 'birth rate' and 'natality rate'.
Mortality
Mortality refers to the death of individual in a population.
Like natality rate mortality rate or death rate may be expressed as the number of
individuals dying in a given period.
Mortality may be minimum mortality or ecological mortality.
• Minimum mortality is the constant for the population represents the laws under
favorable or non-limiting condition.

3. • Ecological or realized mortality is the loss of individual under a given
environmental condition.
Mortality rate of the population can be expressed by survivorship curve.
Survivorship curve plot the number of surviving individual to the particular age.
Generally there three types:
• Highly convex curve (Type I curve)
• Type II curve
• Highly concave curve (Type III curve)
• A highly convex curve is characteristic of the species in which the population mortality
rate is low until near the end of the life span. Many species such as deer mountain sheep
and man show such curve.
• A highly concave curve is characteristic of those species where the mortality rate is high
during the young stages. Examples are oysters or shell fish. In oysters mortality is
extremely high during free swimming larval stages.
• In type III curve which falls between type I and III the rate of mortality is constant at all
age groups, so that an individual chance of living another year is just as good at one age
as another this curve is typically of several birds and of human beings exposed to pure
poor nutrition and hygiene.
Dispersion
Dispersion refers to the spatial and temporal distribution pattern of individuals of a
population.
It shows three broad patterns of distribution-
▪ Regular
▪ Random and
▪ Clumped

4. • In regular dispersion, the individuals are more or less spaced at equal distance from one
another. This is rare in nature but is common in managed systems like croplands.
• In random dispersion the position of one individual is unrelated to the position of its
neighbours. Here the environment is very uniform and there is no tendency to aggregate.
This is also rare in nature.
• In clumped dispersion individuals are aggregated into groups of varying size.
Age structure
Individuals are present in the different age groups.
Relative numbers of young and old individuals in a population will significantly
influence the behavior of a population such as natality and mortality.
The population of individuals in each age group is called the age structure of the
population.
A group of individuals are all roughly of same age known as cohert.
Age distribution affects both natality and mortality rate.
Reproductive status of the population also affects the ratio of various age groups.
Age structure is classified into three categories-
• Pre-reproductive
• Reproductive
• Post-reproductive ages
Age pyramids
The most convenient way to represent the age distribution is in the form of age pyramid.
An age pyramid is a vertical bar graph which represents the number or proportion of
individuals in various age groups at any given time i.e. age pyramid is the model
representing geometrically the proportions of different age groups in the populations of
any organisms.
There are three types of hypothetical age pyramids:
• Expanding population
• Stable population
• Diminishing population

5. • Expanding population
It shows high birth rate and population growth may be exponential, as in the case of the housefly,
yeast and alga. Each successive generation will be higher than the previous one, as a result
pyramid-shape age structure results.
• Stable population
As the rate of growth decreases and stabilities, the pre-reproductive and reproductive age groups
become more or less equal in size, the post-reproductive groups remain as the smallest. The
graphical representation of this stabilized population is bell-shaped.
• Diminishing population
It is an urn-shaped age structure, where the birth rate is drastically reduced. This is representative
of a population that is dying off.

6. Population dispersal
It is the movement of individuals into or out of the population or the population area. But
these changes do not affect the size of the population.
It occurs in three following ways-
• Emigration- One way outwards movement of individuals from an area.
• Immigration- One way inward movement of individuals into an area.
• Migration- Periodic departure and return of individuals to the same area.