Population Ecology
Presenter
Dr. Md. Jasim Uddin
Professor
Department of Fisheries Management
Definition of population
Collective group of organisms
Same species
Occupying in a particular area
Perpetuating year after year through
reproduction and recruitment
E.g., Labeo rohita population in the Halda
Characteristics of population
1. Density
2. Natality or birth rate
3. Mortality or death rate
4. Biotic potential
5. Age distribution
6. Growth form
7. Dispersion or internal distribution pattern
8. Dispersal
9. Isolation and territoriality
1. Density
Population size in relation to per unit space
No. of individuals, or the population biomass in per
unit area or volume
Two types: a) crude density b) specific or
ecological density
Crude density: No. of individuals or biomass in
per unit total space
Specific or ecological density: No. of organisms or
biomass in per unit of habitable space
Crude density & Ecological density
2. Natality or Birth rate
Inherent ability of a population to increase in no.
New individuals- by born, hatch, germination or by
division
Rate- divide the change in number of individuals by
the period of time elapsed during change
Two types: a) maximum natality b) ecological or
realized natality
Maximum natality
Theoretical maximum production of new
individuals under ideal conditions
No ecological limiting factors
Can be limited by physiological factors
Constant
Ecological or realized natality
Increase of new individuals under actual
environmental conditions
Not constant
Vary with population and environment
3. Mortality or Death rate
Death of individuals in the population
Number of individuals dying in a given period, or
Specific rate in terms of the percentage of the total
population dying at a given time interval
Two types: a) ecological or realized mortality b)
minimum mortality
Ecological or realized mortality
Loss of individuals under actual environmental
condition
Not constant
Vary with population and environment
Minimum mortality
loss of individuals under ideal or nonlimiting
conditions
Constant
4. Biotic Potential
Inherent power of a population to increase in
numbers
No limiting effect on organisms
Unlimited environment
Maximum growth
5. Age distribution
Influences both natality and mortality
Mortality varies with age
Reproduction is limited to certain age groups
Ratio of various age groups determines the current
reproductive status
Indicates what may be expected in the future
Three types of age pyramids:
a) expanding
b) stationary or stable
c) declining age pyramids
Expanding: Large proportion of young individuals
Stable: Even distribution of age classes
Declining: Large proportion of old individuals
Expanding Stable
Age
Declining
Percent in age class
6. Growth Form
Populations have characteristic patterns of increase
which are called population growth forms.
Two types:
a) J-shaped growth form
b) S-shaped growth form
J-shaped growth form
Population density increases rapidly in exponential
fashion
Then stops abruptly due to environmental resistance
Generally found in microorganisms
N
Has no equilibrium level
No. of individuals
Has no carrying capacity
Represented by the model
∆N
= rN with definite limit on N
∆t
Time
S-shaped growth form (Verhulst, 1845)
Population growth slow at first
Then rapid at exponentially
Slows down gradually due to environmental
resistance
Generally found in higher animals
No. of individuals
Has equilibrium level K
Has carrying capacity
Represented by the model
∆N (K-N)
= rN
∆t K Time
Carrying capacity (K)
Maximum biomass that can be obtained from a
given area at a particular time interval
500 kg/ha/yr
Standing crop
Total biomass of a given area at a particular time
500 kg/ha
Biomass: Weight of living organisms
7. Dispersion or population internal
distribution pattern or population structure
Spatial distribution of the individuals of a
population at any particular moment
Three broad patterns
a) random
b) uniform
c) clumped
Random distribution
Relatively rare in nature
Occur where the environment is very uniform
There is no tendency to aggregate
Uniform distribution
Occur where competition between individuals is
severe
Where there is a positive antagonism which
promotes even spacing
Clumped
Commonest pattern of dispersion
Groups could be the same or of varying size
They could be randomly distributed, or uniformly
distributed, or themselves aggregated
Overall 5 types of population distribution
1. Random
2. Uniform
3. Random clumped
4. Uniform clumped
5. Aggregated clumped
Aggregation may occur
In response to local habitat differences
In response to daily and seasonal weather
changes
As the result of reproductive processes
As the result of social attractions
Allee’s Principle
The degree of aggregation and overall-
density result in optimum population growth
and survival
It varies with species and conditions
Undercrowding (lack of aggregation), as well
as overcrowding, may be limiting
8. Dispersal
Movement of individuals or their
disseminules (seeds, spores, larvae etc) into
or out of the population or population area
Three forms-
a) Emigration
b) Immigration
c) Migration
Emigration
One-way outward movement
Leave the population area and never come back
Immigration
One-way inward movement
individuals enter into the population area but
never go back
Migration
Periodic departure and return again
Individuals leave the population area, again
come back to the population area
Purpose of migration
Feeding
Breeding
Overwintering
9. Isolation and Territoriality
Vertebrates and the higher invertebrates commonly
restrict their activities to a definite area is called home
range
If this area is actively defended, it is called a territory
Isolation is the result of– inter-individual competition and
actual directed antagonism
Most pronounced in vertebrates and certain arthropods
having complicated reproductive behavior
Reduce competition, prevents overcrowding and
exhaustion of food supply
