1) Population ecology is the study of populations in relation to their environment, including factors like density, distribution, age structure, and population size. 2) There are two main patterns of population growth - exponential/J-shaped growth seen in insects and annual plants, and logistic/S-shaped growth seen in vertebrates and trees. 3) Population size is influenced by density-dependent factors like competition, disease, and predation as population density increases. 4) Ecological succession is the process of community changes over time, either through primary succession on new habitats, or secondary succession after a disturbance to an existing habitat.

1. UNIT 3:
POPOULATION
ECOLOGY

2. POPOULATION
ECOLOGY
• Population ecology 
study of populations in
relation to environment,
• including
environmental
influences on
density
• distribution
• age structure
• population size.

3. 1.
TERMINOLOGY
• Population  group of
individuals of same species
living in same area at same
time.
• Density number of
individuals per unit area /
volume.
• Dispersion  pattern of
spacing among individuals
within boundaries of
population.

4. TERMINOLOGY
Density  result of processes that add
individuals to a population & those that
remove individuals.
 Immigration influx of new
individuals from other areas
 Emigration movement of
individuals out of a population

5. POPULATION SIZE IS PRIMARILY INFLUENCED
BY:

6. TERMINOLOGY
• Demography  study
of vital statistics of a
population & how they
change over time.
• Death rates & birth
rates are of
particular interest to
demographers.

7. 2. PATTERNS OF DISPERSION
• Environmental & social factors influence spacing of individuals in a population:
• Clumped dispersion  individuals aggregate in patches.
• A clumped dispersion may be influenced by resource availability &
behaviour.

8. 2. PATTERNS OF
DISPERSION
Uniform dispersion  individuals are evenly
distributed.
It may be influenced by social interactions
such as territoriality

9. 2. PATTERNS OF
DISPERSION
• Random dispersion  position of each individual is
independent of other individuals.
• It occurs in absence of strong attractions or repulsions.

10. PATTERNS OF DISPERSION

11. 3.Patterns of Population Growth in
an Ecosystem
• Birth rate minus death rate with no
environmental restrictions defines a
species intrinsic growth rate.
• Within an ecosystem, however,
resource limits & predation also
effect population growth.

12. There are
2 main
patterns of
population
growth:
1) Exponential growth / J-pattern:
only one reproductive chance is
given to members of population
during their entire lifespan.
• Once mission accomplishes 
die.
• Example: Insects & annual
plants
2) Logistic growth / S-pattern: many
reproductive events throughout
their lifetime.
• Examples Vertebrates & trees

13. There are
two main
patterns of
population
growth:
• Exponential growth/J-pattern
(Insects): Exponential growth
assumes that environmental factors
(food, water supply, space & shelter,
disease organisms & weather
conditions & natural disasters) do
not affect the birth or death rate.
• As long as birth rate exceeds death
rate (even slightly) population size will
increase exponentially.
• If death rate exceeds birth rate
population size will decrease
exponentially.

14. Exponential growth/J-
pattern (Insects)
• Lag phase:
Growth is slow
because
population
base is small
(adjustment
period).
Exponential
growth phase:
Growth is
accelerating,
(the rate of
growth itself
grows).

15. Exponential
growth/J-pattern
• Exponential
growth
cannot be
sustained for
long in any
population –
limited by
resources
(food and
space)

16. Logistic growth/S-pattern:

17. Logistic
growth/S-
pattern:
• Lag phase: Growth is slow
because population base is small,
organisms adapt.
• Exponential growth phase:
Growth is accelerating  many
offspring born at a fast rate.
• Deceleration phase: The rate of
population growth slows down –
individuals start to die.
• Stable equilibrium phase: Little
growth  births & deaths are
about equal. Area has reached its
carrying capacity.

18. Logistic growth/S-pattern of Daphnia

19. Carrying
capacity:
In logistic population
growth model  rate of
increase declines as carrying
capacity is reached.
Carrying capacity (K) is the
maximum population size
that environment can
support

20. 4. Survival
patterns/curves
• Survivorship curves can be classified
into 3 general types:
– Type I – Vertebrates (humans)
• low death rates during early &
middle life,
• an increase among older age
groups
– Type II – small animals (rodents)
• death rate is constant over the
organism’s life span
– Type III - insects
• high death rates for the young,
• a slower death rate for
survivors.

21. Survival patterns/curves

22. 5. Factors that
influence & regulate
population growth

23. Factors
that
influence &
regulate
population
growth :
density
In density-independent
populations, birth rate &
death rate do not change
with population density.
In density-dependent
populations, birth rates
fall & death rates rise
with population density.

24. Density-dependent populations
• The population growth in density-dependent populations
are affected by many factors, such as :
1) competition for resources (food & space),
2) Territoriality (space),
3) Disease (spreading of pathogens),
4) predation,
5) toxic wastes, &
6) intrinsic factors

25. 1) INTRASPECIFIC
COMPETITION
FOR RESOURCES
• Crowded populations
(increased population
density)  intensifies
competition for resources
& results in lower birth
rate.
• Less Resources
available (water,
shelter, food, space,
access to mates,
ecological niches)

26. 2) TERRITORIALITY
Many vertebrates & some invertebrates 
competition for territory limit density
Example: Cheetahs are highly territorial,
using chemical communication to warn
other cheetahs of their boundaries.

27. 3) DISEASES
Population density 
influence health &
survival of organisms
In dense populations
 pathogens spread
more rapidly.

28. 4) PREDATION
As prey population builds up, predators may feed
mostly on that species.
Thus decrease prey population size
later decrease predator population size. –
Predator-prey relationship (Graph)

29. 5) TOXIC WASTE
Accumulation of toxic wastes contribute to density-
dependent regulation of population size.
Waste  diseases, deaths, lower birth rates

30. 6) INTRINSIC (BIOTIC) FACTORS
• Biotic (Intrinsic factors) are specific to each species:
• The age of reproductive maturity
• The number of offspring produced per reproductive
event
• The number of reproductive events in an individual's
lifetime

31. 6. How to
determine
the size of
a population
Method to determine population size can be
estimated by:
Census taking –
counting each
individual
Mark-recapture
method
Quadrant method
(will be done during
prac 3)
Sampling techniques used to estimate densities &
total population sizes
In most cases  impractical or impossible to count all
individuals in a population.

32. MARK-RECAPTURE METHOD
(PETERSON METHOD)
 To estimate size of population  capture & mark
individuals from population, release them, and then
resample  what fraction of individuals carry marks???.
 Useful when sampling closed populations --
• choose populations that do not change in size at all
or slightly during study period due to (births, deaths,
immigration, or emigration).

33. MARK-RECAPTURE METHOD
(PETERSON METHOD)
 N = estimated population size
 M = the number of individuals marked in
FIRST sample
 C = total number of individuals captured in
SECOND sample
 R = number of individuals in SECOND
sample that are marked
N = CM
R

34. ASSUMPTIONS
OF THE MARK-
RECAPTURE
METHOD
(PETERSON
METHOD)
The population is
"closed", so N is
constant.
All animals have
same chance of
getting caught in 1st
sample.
Marking individuals
does not affect their
mobility & chance to
be caught again.
Animals do not lose
marks between 2
sampling periods.
All marks are
reported on
discovery in 2nd
sample.

35. 8. SOCIAL
ORGANISATION
Insects (ants, bees wasps & termites) show
extreme form of sociality  individual
organisms specialized for distinct roles.
This form of social behaviour
is referred to as eusociality.
Organism  highly interactive with other
members  social animal.
mothers & offspring bond.

36. VERTEBRATE
SOCIETIES
may exhibit 1
of more of
these
behaviours:
cooperative rearing of young by group
overlapping generations living in permanent, as
opposed to seasonal, group
cooperative foraging / hunting
cooperative defence from predators &
competitors
social learning (such as young chimpanzee
learning by observation to use a twig to fish for
termites)

37. 9.
COMMUNITY
STRUCTURES
 A biological community  GROUP
OF POPULATIONS of various species
living close enough for potential
interaction, in a specific area, at a
specific time.
 Community includes the
producers (plants), consumers
(herbivore, carnivores,
omnivores) & decomposers.

38. COMMUNITY
STRUCTURES
 Relationships between species in
community  interspecific
interactions
 Competition, predation,
herbivory, & symbiosis
(parasitism, mutualism,
commensalism)
 Interspecific interactions can affect
survival / reproduction of each
species,
 effects can be summarized as
positive (+), negative (–), or no
effect (0).

39. COMPETITION
 Interspecific competition
different species compete for
resource in short supply (water,
food, shelter, space, light)
 Strong competition lead to
competitive exclusion  local
elimination of a competing species.
 competitive exclusion principle
states that 2 species competing
for same limiting resources
cannot coexist in same place.

40. COMPETITION
Ecological niche  organism’s
ecological role & how they use
biotic & abiotic factors
Ecologically similar species can
coexist in community if there
are 1 or more significant
differences in their niches.
Resource partitioning  differentiation of ecological
niches  enabling similar species to coexist in community

41. Resource partitioning
example:
Herbivores all
living in same
habitat,
specialized to
feed on
different plants.
Competition
reduced
because they
prefer different
plants.

42. PREDATION
 Predation (+/– interaction)  predator, kills & eats prey
 Feeding adaptations of predators: claws, teeth,
fangs, stingers, poison.
 Prey display various defensive adaptations.
 Behavioural defences  hiding, fleeing, forming
herds or schools, self-defence, & alarm calls.
 Animals also have morphological & physiological defence
adaptations
 Cryptic coloration / camouflage makes prey difficult to
spot

43. HERBIVORY
• Herbivory (+/–
interaction) 
interaction herbivore
eats parts of a plant /
alga.
• Led to evolution of
plant mechanical &
chemical defences &
adaptations by
herbivores.

44. SYMBIOSIS
• Symbiosis  relationship where 2+ species live in direct /
intimate contact with each other.
• 3 TYPES:
 PARASITISM
 MUTUALISM
 COMMENSIALISM

45. PARASITISM
• Parasitism (+/– interaction) 
parasite, get food (benefits), from
host (harmed in process).
• Parasites that live within body of
their host are called endoparasites;
• Parasites that live on external
surface of host are ectoparasites

46. PARASITISM

47. MUTUALISM
• Mutualism (+/+ interaction),  interspecific interaction
that benefits both species.
• A mutualism can be:
 Obligated, where 1 species cannot survive
without other.
 Facultative, where both species can survive
alone.

48. MUTUALISM
• Buffalo provide birds
with protection from
predators whilst birds
like to eat the ticks
from buffalo skin.
• The flower is
pollinated and the
butterfly gets nectar. –
both benefit

49. COMMENSALISM
• Commensalism
(+/0
interaction), 1
species benefits
& other is
unaffected
• Plant grows on
tree trunk,
greater
exposure to
sunlight and
water. Tree
remains
unharmed

50. 10.
ECOLOGICAL
SUCCESSION
Ecological succession  sequence of
community & ecosystem changes after
disturbance.
Primary succession  series of community
changes which occur on an entirely new
habitat which has never been colonized before
– no soil present. For example, a newly
quarried rock face or sand dunes.
Secondary succession series of community
changes which take place on a previously
colonized, but disturbed / damaged habitat.
For example, after felling trees in a woodland,
land clearance or a fire.

51. ECOLOGICAL
SUCCESSION
• Gradual replacement of 1
community by another in
same area over time

52. ECOLOGICAL
SUCCESSION
1ST inhabitants of disturbed area  pioneer
community – developing where only soil was
present.
As more organisms inhabit area – reaches
climax community – no more new comers can
be accommodated.

53. ECOLOGICAL SUCCESSION
• Ecological succession refers to all plant, animal & fungi
communities that exist within ecosystem.
• Succession will not go any further than climax
community:
• Does not imply that no changes will take place:
• If large tree die – new species will fill its place 
secondary succession…
• It may take hundreds of years for climax community
to be established.

54. PRIMARY SUCCESSION

55. PRIMARY SUCCESSION

56. SECONDARY
SUCCESSION