UNIT 3 POPULATION ECOLOGY (1)

UNIT 3:
POPOULATION
ECOLOGY
(Campbell & Reece, 2010. Chapter:53)

POPOULATION
ECOLOGY
Population ecology is the study of
populations in relation to environment,
including environmental influences on
density and distribution, age structure,
and population size.

1. TERMINOLOGY
 A population is a group of individuals
of the same species living in the same
area at the same time.
 Density is the number of individuals
per unit area or volume.
 Dispersion is the pattern of spacing
among individuals within the
boundaries of the population.

TERMINOLOGY
Density is the result of an interplay
between processes that add individuals
to a population and those that remove
individuals.
 Immigration is the influx of new
individuals from other areas
 Emigration is the movement of
individuals out of a population

POPULATION SIZE IS PRIMARILY INFLUENCED
BY:

TERMINOLOGY
 Demography is the study of the vital
statistics of a population and how they
change over time. Death rates and
birth rates are of particular interest to
demographers.

2. PATTERNS OF DISPERSION
Environmental and social factors influence spacing of
individuals in a population:
1. In a clumped dispersion, individuals
aggregate in patches. A clumped dispersion may
be influenced by resource availability and
behaviour.
2. A uniform dispersion is one in which
individuals are evenly distributed. It may be
influenced by social interactions such as
territoriality
3. In a random dispersion, the position of each
individual is independent of other individuals.
It occurs in the absence of strong attractions or
repulsions.

3.Patterns of Population Growth
in an Ecosystem
• Many factors affect population growth, but
one factor is a species' intrinsic growth rate.
• The birth rate minus the death rate with no
environmental restrictions defines a species
intrinsic growth rate.
• Within an ecosystem, however, resource limits
and predation also effect population growth.

There are two main patterns of
population growth:
• Exponential growth/J-pattern: only
one reproductive chance is given to
members of the population during their
entire lifespan. Once mission accomplishes,
they die. Many insects and annual plants
reproduce in this manner.
• Logistic growth/S-pattern: members
experience many reproductive events
throughout their lifetime. Most vertebrates,
and trees have this pattern of reproduction.

• Exponential growth/J-pattern
Lag phase: Growth is slow because population base
is small.
Exponential growth phase: Growth is accelerating,
that is, the rate of growth itself grows.

• Exponential growth/J-pattern
Exponential growth cannot be
sustained for long in any population

• Logistic growth/S-pattern:

• Logistic growth/S-pattern:
Lag phase: Growth is slow because the
population base is small, organisms adapt.
Exponential growth phase: Growth is
accelerating, that is, 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
because births and deaths are about equal.
Area has reached its carrying capacity.

• Logistic growth/S-pattern of Daphnia

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

4. Survival patterns/curves
Survivorship curves can be classified into three
general types:
• Type I: low death rates during early and
middle life, then an increase among older
age groups
• Type II: the death rate is constant over the
organism’s life span
• Type III: high death rates for the young,
then a slower death rate for survivors.

5. Factors that influence and
regulate population growth
There are two general questions about
regulation of population growth:
• What environmental factors stop a
population from growing indefinitely?
• Why do some populations show radical
fluctuations in size over time, while
others remain stable?

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

Density-dependent populations
• The population growth in density-
dependent populations are affected by
many factors, such as :
 competition for resources,
 territoriality,
 disease,
 predation,
 toxic wastes, and
 intrinsic factors

INTRASPECIFIC COMPETITION FOR
RESOURCES
• In crowded populations, increasing
population density intensifies
competition for resources and results in
a lower birth rate.
• Resources like: water, shelter, food,
space, access to mates, ecological
niches.

TERRITORIALITY
• In many vertebrates and some invertebrates,
competition for territory may limit density
• Cheetahs are highly territorial, using
chemical communication to warn other
cheetahs of their boundaries.

DISEASES
• Population density can influence the health
and survival of organisms
• In dense populations, pathogens can spread
more rapidly.

PREDATION
• As a prey population builds up, predators
may feed preferentially on that species.
• This will decrease the prey population size –
and later decrease the predator population
size. – Predator-prey relationship (Graph)

TOXIC WASTE
• Accumulation of toxic wastes can contribute
to density-dependent regulation of
population size.

6. How to determine the size of a
population
 In most cases, it is impractical or impossible
to count all individuals in a population.
 Sampling techniques can be used to
estimate densities and total population
sizes
 Method to determine population size can be
estimated by:
 Census taking – counting each individual
 Mark-recapture method
 Quadrant method (will be done as an
assignment)

MARK-RECAPTURE METHOD
(PETERSON METHOD)
 One way to estimate the size of a population is
to capture and mark individuals from the
population, release them, and then resample to
see what fraction of individuals carry marks.
 Useful when sampling closed populations --
choose populations that do not change in size
at all or very much during the study period
due to births, deaths, immigration, or
emigration.

MARK-RECAPTURE METHOD
(PETERSON METHOD)
 N = estimated population size
 M = the number of individuals marked in the
first sample
 C = total number of individuals captured in 2nd
sample
 R = number of individuals in 2nd sample that are
marked
N = CM
R

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

7. HUMAN POPULATION
 The human population is no longer growing
exponentially but is still increasing rapidly.
 No population can grow indefinitely, and
humans are no exception.
 The human population increased relatively
slowly until about 1650 and then began to grow
exponentially.

HUMAN POPULATION
 Though the global population is still growing, the
rate of growth began to slow during the 1960s

REGIONAL PATTERNS OF
POPULATION CHANGE
• To maintain population stability, a regional
human population can exist in one of two
configurations:
• Zero population growth =
High birth rate – High death rate
• Zero population growth =
Low birth rate – Low death rate
• The demographic transition is the move from
the first state toward the second state

REGIONAL PATTERNS OF
POPULATION CHANGE
• The demographic transition is associated with an
increase in the quality of health care and
improved access to education, especially for
women
• Most of the current global population growth is
concentrated in developing countries.

SWEDEN – DEVELOPED COUNTRY
MEXICO – DEVELOPING COUNTRY

POPULATION GROWTH IN SOUTH
AFRICA

PROJECTIONS OF S.A.POPULATION
SIZE

POPULATION GROWTH IN SOUTH
AFRICA
• The Population; total in South Africa was
last reported at 49991300 in 2010.
49320150 in 2009.
reported at 48793022 in 2008.

AGE AND GENDER STRUCTURES
• One important demographic factor in present
and future growth trends is a country’s age
and GENDER structure
• Age structure is the relative number of
individuals at each age.
• Age structure diagrams can predict a
population’s growth trends
• They can illuminate social conditions and help
us plan for the future.

How many humans can the
biosphere support?
What is the earth’s carrying
capacity?
Our carrying capacity could
potentially be limited by food,
space, non-renewable resources,
or build-up of wastes

8. SOCIAL ORGANISATION
 An organism that is highly interactive with other
members of its species is said to be a social
animal.
 All mammals (and birds) are social to the extent
that mothers and offspring bond.
 A few species, notably insects (ants, bees wasps
and termites) show an extreme form of sociality,
involving highly organized societies, with
individual organisms specialized for distinct roles.
 This form of social behaviour is referred to as
eusociality.

Vertebrate societies may exhibit one
of more of these behaviours:
 cooperative rearing of young by the group
 overlapping generations living in a
permanent, as opposed to seasonal, group
 cooperative foraging or hunting
 cooperative defence from predators and
competitors
 social learning (such as a young chimpanzee
learning by observation to use a twig to fish
for termites)

SOCIAL ORGANISATION
 Many herbivores (e.g. zebra) are always in herds,
to outnumber the predators.
 Wolves, hyena's and wild dogs – hunt in packs –
guarantee a kill.
 Bee’s and ants divide their daily tasks to get
everything done.

9. COMMUNITY STRUCTURES
 A biological community is an
assemblage 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) and
decomposers.

COMMUNITY STRUCTURES
 Ecologists call relationships between species
in a community interspecific interactions
 Examples are competition, predation,
herbivory, and symbiosis (parasitism,
mutualism, and commensalism)
 Interspecific interactions can affect the
survival and reproduction of each species,
and the effects can be summarized as
positive (+), negative (–), or no effect (0).

COMPETITION
 Interspecific competition (–/– interaction)
occurs when different species compete for a
resource in short supply (e.g. water, food,
shelter, space, light)
 Strong competition can lead to competitive
exclusion, local elimination of a competing
species.
 The competitive exclusion principle states
that two species competing for the same
limiting resources cannot coexist in the same
place.

COMPETITION
 The total of a species’ use of biotic and
abiotic resources is called the species’
ecological niche
 An ecological niche can also be thought of as
an organism’s ecological role
 Ecologically similar species can coexist in a
community if there are one or more
significant differences in their niches.
 Resource partitioning is differentiation of
ecological niches, enabling similar species to
coexist in a community

 Resource partitioning example:
Seven species of Anolis lizards live close to each
other, all feed on the same insects, but competition
for food is reduced because they prefer different
perches (niches)

PREDATION
 Predation (+/– interaction) refers to interaction
where one species, the predator, kills and eats the
other, the prey
 Some feeding adaptations of predators are claws,
teeth, fangs, stingers, and poison.
 Prey display various defensive adaptations.
 Behavioral defenses include hiding, fleeing,
forming herds or schools, self-defence, and alarm
calls.
 Animals also have morphological and
physiological defence adaptations
 Cryptic coloration, or camouflage, makes prey
difficult to spot

HERBIVORY
• Herbivory (+/– interaction) refers to an
interaction in which a herbivore eats parts of
a plant or alga.
• It has led to evolution of plant mechanical
and chemical defences and adaptations by
herbivores.

SYMBIOSIS
• Symbiosis is a relationship where two or
more species live in direct and intimate
contact with one another.
• 3 TYPES:
 PARASITISM
 MUTUALISM
 COMMENSIALISM

PARASITISM
• In parasitism (+/– interaction), one
organism, the parasite, derives
nourishment (benefits), from another
organism, its host, which is harmed in
the process.
• Parasites that live within the body of
their host are called endoparasites;
parasites that live on the external
surface of a host are ectoparasites

MUTUALISM
• Mutualistic symbiosis, or mutualism
(+/+ interaction), is an interspecific
interaction that benefits both species.
• A mutualism can be:
 Obligate, where one species
cannot survive without the other.
 Facultative, where both species
can survive alone.

MUTUALISM
Anemones provide the
Clown Fish with
protection from predators
whilst Clown fish defend
the Anemones from
Butterfly fish who like to
eat Anemones.
The flower is pollinated
and the butterfly gets
nectar. – both benefit

COMMENSALISM
• In commensalism (+/0 interaction), one
species benefits and the other is
apparently unaffected
Cattle egrets/tick
birds benefits
(insects) and
water buffalo (not
affected)

10. ECOLOGICAL SUCCESSION
• Ecological succession is the sequence of
community and ecosystem changes
after a disturbance.
• Primary succession occurs where no
soil exists when succession begins.
• Secondary succession begins in an area
where soil remains after a disturbance.

• Example of ecological succession of a
glacial retreat at Glacier Bay, Alaska.

ECOLOGICAL SUCCESSION
• The first inhabitants of a disturbed area are called
the pioneer community.
• As more organisms inhabit the area – it reaches a
climax community – no more new comers can be
accommodated.

UNIT 3 POPULATION ECOLOGY (1)

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