This document provides an overview of key concepts in population ecology. It discusses population characteristics like density, natality, mortality, dispersal, growth curves, fluctuation, distribution, age pyramids, and equilibrium. It describes population density, natality rates, types of mortality, dispersal through emigration, immigration and migration. Growth curves can follow a J-curve or S-curve pattern. Populations can fluctuate cyclically. Distribution can be random, uniform, or clumped. Age pyramids reflect the age structure of a population.

1. By: Nandita Purohit
(M.Sc.
Zoology)
Population Ecology

2. Synopsis
• Introduction
• Population Characteristics
– Density
– Natality
– Mortality
– Dispersal
– Growth curve
– Population Fluctuation
– Distribution
– Age pyramid
– Equilibrium
• Importance

3. introduction
A population is generally a group of particular species occupying a
particular area at a specific time. Some of the ecologists however
recognize two types of population;
• Monospecific population- a population with individuals only one
species
• Mixed or Polyspecific population- a population of more than one
species
However in Ecology, a polyspecific population is considered as a
COMMUNITY, and the term POPULATION is used for a group
of individuals of any kind of organism.
POPULATION ECOLOGY is the study of individuals of the same
species where the processes are aggregation, interdependencies
between individuals etc, and the various factors governing such
processes are emphasized.

4. • Demography is the study of human
populations – their size, composition and
distribution across space and the process
through which populations change.
• Births, deaths and migration are the ‘big three’
of demography, jointly producing population
stability or change.
• It is the study of human population dynamics.
• Population dynamics is the branch of life
sciences that studies the size and age
composition of populations as dynamical
systems, and the biological

5. density
• Population density is the
number of individuals per
unit geographic area, for
example, number per
square meter, per hectare,
or per square kilometer.
• Density may be numerical
density (number of
individuals per unit area or
volume) or biomass
density (biomass per unit
area or volume)

6. • Further, since the pattern of distribution of
organisms in nature is different it becomes
important to distinguish between crude density
and ecological density.
• Crude density is the density (number or
biomass) per unit total space.
• Ecological density (specific or economic
density) is the density (number or biomass) per
unit of habital space, i.e., available area or
volume that can actually be colonized by the
population.

7. Density is affected by four primary population
parameters such as natality (births),
mortality (deaths), immigration and emigration

8. Natality
• Natality is a broader term covering the production of new
individuals by birth, hatching, germination, or fission.
• The natality rate may be expressed as the number of
organisms born per female per unit time.
• In human population, the natality rate is equivalent to the
‘birth-rate’.
• The two essential aspects of the reproduction which forms
main component of natality :
– Fertility is the physiological notion which indicates that an
organism is capable of breeding.
– Fecundity is an ecological concept that is based on the numbers
of offspring produced during a period of time

9. • Natality is of following two types :
– Maximum natality. (absolute, potential or physiological
natality). It is theoretical maximum production of new
individuals under ideal conditions which simply means that
there are no ecological limiting factors and that
reproduction is limited only by physiological factors.
– Ecological natality. (realized natality or simply natality). It
refers to population increase under an actual, existing
specific condition. Thus, ecological natality takes into
account all possible existing environmental conditions.
I’m here to contribute
for density increase

10. • ∆ Nn/ Dt = the absolute natality rate
• ∆ Nn/NDt = the specific natality rate
– where
N = the reproductive part (mature female) of the
population or initial number of organisms or total
population
n = new individuals in the population
t = time
∆ = delta : a change in value

11. mortality
• Mortality means the rate of death of individuals in the
population.
• It is a negative factor for population growth
• It is of following types
– Minimum mortality (specific or potential mortality)
represents the theoretical minimum loss under ideal or non-
limiting conditions. It may be constant for a population.
– Ecological mortality (realised mortality) is the actual loss
of individuals under a given environmental condition.
• Mortality can be expressed as-
– number if individuals dying/ unit time.

12. • A survivorship curve is a graph that measures
the proportion of individuals in a given species
that are alive at different ages.
• Typically, the number of individuals of the
population is plotted on the y-axis of the graph
and the age of survivorship is plotted on the x-
axis of the graph.
• There are three types of survivorship curves,
and they are simply referred to as type I, type
II, and type III.

13. •Type I (concave curve) survivorship curve shows individuals that have a high
probability of surviving through early and middle life but have a rapid decline in
the number of individuals surviving into late life. E.g. human, flies.
•Type II (diagonal curve) survivorship curve shows a roughly constant
mortality rate for the species through its entire life. E.g. hydra, mice, birds.
•Type III (convex curve) survivorship curve depicts species where few
individuals will live to adulthood and die as they get older because the greatest
mortality for these individuals is experienced early in life. E.g. oyster, fishes

14. Dispersal
• Populations have a tendency to disperse, or spread
out in all directions, until some barrier is reached.
• Accordingly, population dispersion is the
movement of individuals into or out of the
population area.
• It takes three forms-
– Emigration
– Immigration
– Migration

15. • Emigration (one-way outward movement) under natural conditions occurs
when there is overcrowding in the migratory locust, lemming, grouse, snowy
owl, snowshoe rabbit.
• This is generally regarded as an adaptive behavior that regulates the
population on a particular site and prevents over-exploitation of the habitat.
• Further, it leads to occupation of new areas elsewhere
• By dispersing into new localities, there is opportunity gained for
interbreeding with other populations leading to more genetic heterozygosity
and adaptability.
• Immigration (one-way inward movement) leads to a rise in population
level, causing an overpopulation which may lead to an increase beyond the
carrying capacity.
• These immigrations result in increased mortality among the immigrants or
decreased reproductive capacity of the individuals

16. • Migration (periodic departure and return) is a
peculiar kind of population dispersion which
involves the mass movement of entire population.
• This can occur only in mobile organisms and best
developed in insects such as desert
locust(Schistocerca gregaria), migratory
locust(Locusta migratoria), butterfly (Danaus
plexippus)
• In most cases, it may occur for food, shelter, or
reproduction.
• Better utilization of uninhabited untouched
habitats and their resources are the greatest
benefits of migratory movements.

18. Growth Curves
• Population growth is the increase in the number of
individuals in a population.
• Two types of population growth patterns may occur
depending on specific environmental conditions:
– An exponential growth pattern (J curve) occurs in an ideal,
unlimited environment.
– A logistic growth pattern (S curve) occurs when environmental
pressures slow the rate of growth.

19. J- Curve
• In J-shaped curve there is a rapid increase in density
with the passage of time (called exponential growth).
• The density values when plotted against time give a J-
shaped growth curve.
• If 1000 bacteria are placed in a large flask with an
unlimited supply of nutrients, after an hour there will be
one round of division, resulting in 2000 organisms. In
another hour, each of the 2000 organisms will double,
producing 4000, and so on.
• The bacteria example is not representative of the real
world where resources are limited, and ultimately
competition for food would result in deaths and
population decline.

20. S- curve
• The population growth curve in human
populations and growth of yeast, Drosophila and
rabbit under laboratory conditions show an initial
slow rate and then it accelerates and finally slows
giving the growth curve which is sigmoid or S-
shape.
• The peak constant level represented by K or upper
level (called asymptote) of the sigmoid curve is
called the maximum carrying capacity. It marks
the limit to which the environment can support
the population

21. Population fluctuation
• Populations are said to be cyclic when they alternatively
errupt and subside in a more or less uniform manner
between high and low levels of density.
• Different animals exhibit population cycles different times.
• Population ecologists commonly divide the factors that
affect the size of populations into
– Density-dependent factors (regulating factors), intensify as
the population increases in size, and has a potential for
maintaining population density within a narrow range of values.
– Density-independent factors (limiting factors), such
as weather and climate, exert their influences on population size
regardless of the population’s density.

22. Cyclical fluctuations in the population density of the snowshoe hare and
its effect on the population of its predator, the lynx.

23. Distribution
• Individuals of a population arrange themselves in
a manner that is specific for each population.
• In nature, due to various biotic interactions and
influence of abiotic factors, the following three
basic population distributions can be observed :
– Random distribution
– Uniform or regular distribution
– Clumped distribution

24. I. Regular Dispersion: here the
individuals are spaced at equal
distance from one another. This is
rare in nature but common in
managed systems (croplands).
Animals with territorial behavior
tends towards this dispersion
II. Random Dispersion: the position of
one individual is unrelated to the
position of its neighbor. This us also
relatively rare in nature.
III. Clumped Dispersion: most population
exhibit this dispersion to some extent,
with individuals aggregated into
patches interspersed with no or few
individuals.

25. Age pyramid
• In most types of populations, individuals are of
different age.
• The proportion of individuals in each age group is
called age structure or age distribution of the
population.
• Age distribution of the population influences
natality and mortality.
• A model representing geometrically the
proportion of different age groups in the
population of any organism is called age pyramid
(or age-sex pyramid)

27. • According to Bodenheimer (1938), there are following three basic
types of age-sex pyramids:
1. Pyramid with broad base (or Triangular structure)
indicates a rapidly expanding population with a high
percentage of young individuals and only few old
individuals.
 Thus, in rapidly growing young population, birth rate is high
and population growth may be exponential as in yeast,
housefly, Paramecium, etc.
2. Bell-shaped polygon indicates a stationary population
having an equal number of young and middle-aged
individuals.
 As the rate of growth becomes slow and stable, i.e., the pre-
reproductive and reproductive age groups become more or
less equal in size, post-reproductive group remaining as the
smallest.

28. 3. Urn-shaped structure indicates a low
percentage of young individuals.
 It shows a declining population.
 Such an urn-shaped figure is obtained when
the birth rate is drastically reduced, the pre-
reproductive group dwindles in proportion to
the other two age groups of the population.

29. importance
• The study of human population is of immense
importance to an economy.
• Population studies help us to know how far the
growth rate of the economy is keeping pace
with the growth rate of population.
• Data relating to the present trend in population
growth help the planners in formulating
policies for the economic plan of the country.