Population Ecology: Topic- Population Characteristics

1. Population Ecology
Topic- Population Characteristics
Dr. Emasushan Minj
Assistant Professor
Department of Botany

2. • 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.

3. 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.

4. 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.
• 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.

5. • We can use a simple formula for estimating total population size
(N):

6. 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.

7. • 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'.

8. 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.
• Ecological or realized mortality is the loss of individual under a
given environmental condition.

9. • 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)

10. • 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.

11. 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
• 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.

12. 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

13. 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

14. 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.

15. 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.

16. 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.

17. 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.

18. Thank you