POPULATION DYNAMICS
Biology ATAR Year 11
Biology 1AB
Biology 3AB
KEYWORDS
Population change
 Births
 Deaths
 Immigration
 Emigration
 Measuring techniques
 Capture/re-capture
 Transects
 Quadrats
 Net trawling
 Radio trapping

Abundance
 Distribution
 Carrying capacity
 Limiting factors
 Ecosystem stability
 Density dependent
factors
 Density
independent factors

POPULATION DYNAMICS
Carrying capacity
 The maximum
population size of the
species that the
environment can
sustain indefinitely
 Population
abundance and
distribution is an
indication of an
ecosystem’s health
and stability
ABUNDANCE AND DISTRIBUTION
Distribution
 The area in which one
or more members of a
species may be found
Abundance
 The numbers of each
species in that area at
that time
 Abundance can be
referred to as
population density
DISTRIBUTION PATTERNS
Random distribution
 Rare
 Usually only occur for a
short period of time
while resources are
greater than demand
Uniform distribution
 Rare
 In some forests, large
trees may have an even
spacing because of
competition for light and
soil nutrients
 For animals, territorial
behaviour may
contribute to a uniform
distribution
Clumping
 Most common pattern
 Environmental conditions
are seldom uniform
 Animal reproductive
patterns and rearing
habits favour clumping
 Animals often congregate
in loose groups, schools,
flocks and herds
ABUNDANCE & DISTRIBUTION
FACTORS AFFECTING ABUNDANCE AND
DISTRIBUTION
ENVIRONMENTAL


Climate
 weather
patterns
 temperature
 light intensity
 humidity
Soil
characterisitics
 Depth soil
 temperature
 pH
 minerals &
nutrients
AVAILABILITY
OF RESOURCES







Food
Living space
Shelter
Nesting sites &
materials
Oxygen
Sunlight
Water
BIOTIC
COMMUNITY






Predator-prey
ratio
Biotic
relationships
mutualism,
commensalism
competition
Immigration &
emigration
ABUNDANCE AND DISTRIBUTION




Availability of a resource may directly influence
the maximum population or carrying capacity
of an area.
These factors are called limiting factors
Factors affecting a population’s abundance and
distribution affect the biodiversity of an
ecosystem.
The larger the biodiversity, the larger the food
web and the more stable the ecosystem.
ECOSYSTEM STABILITY
Density dependent
factors
These factors do not affect
the survival of individual
organisms within that
species until their density
reaches a particular
threshold
 Once that density is
reached, an individual’s
chances of survival
decrease.
Factors
 Availability of resources
 Biotic community

Density independent
factors

These factors are not
influenced by the number
of organisms in an area.
Factors
 Environmental
ECOLOGICAL NICHE



In any ecosystem,
specific species will
occupy a niche
A niche is an
organism’s place in the
ecosystem
No two species can
occupy the same niche
ie. have exactly the
same requirements
and the same role in a
community
Describing a niche
includes a species’:
The physical limitations
temperature range
water requirements
Food & other resource
requirements
nest sites
shelter
Behavioural patterns
movement
daily/seasonal
rhythms
ECOLOGICAL NICHE
PREDATOR/PREY RELATIONSHIPS
Competition
 Competition is the main
density-dependent
limiting factor on
population size
 As population size
increases, the
competition for food,
water, light & shelter
also increases
 This limits to increase
in population size
Predator/prey
relationships
 Can be a significant
limiting factor for each of
the species involved
 It is a complex relationship
because mostly predators
have multiple food sources
 Prey numbers also don’t
necessarily drop below the
carrying capacity, though
the age structure of the
population may change
PREDATOR/PREY RELATIONSHIP
PREDATOR/PREY RELATIONSHIPS

A food web, and therefore ecosystem, is more
stable when many diverse predator-prey links
connect high and intermediate trophic levels.
RATES OF POPULATION CHANGE
Populations are
constantly changing.
 To understand what
is happening in an
ecosystem with a
population, we need
to know not only if it
is increasing or
decreasing, but how
fast it is changing.

Changes in populations
 Rate of change
 How many individuals
are being born
 How many are dying
 How many new
individuals are moving
into the population
 How many are leaving
the population
RATES OF POPULATION CHANGE
B = birth
 D = death
 I = immigration
 E = emigration

R = (b + i) – (d + e)
It is not the actual
number of individuals
a population has
changed by that is of
concern, but the
number it has
changed by compared
to the initial
population
 The amount of change
per unit of population,
usually per thousand

RATE OF POPULATION CHANGE
Overall rate of population change
 Positive means an increase, negative means a
decrease
Birth rate
 The number of individuals born per unit of population
(usually per 100)
Death rate
 The number of individuals dying per unit of
population during the same time (usually 1 year)
Immigration
 The number of individuals per unit of population
entering the population during that time
Emigration
 The number of individuals per unit of population
leaving the population for other areas during that
same period of time.
RATES OF POPULATION CHANGE
MEASURING POPULATION
Capture/recapture
 Capture a number of the animal, tag these animals,
release them back in the wild.
 Sometime later, capture another sampe and count the
number tagged in this second sample.
Transects
 A ling of strip along which the distribution of
organisms is plotted
Quadrats
 An area of study
 Usually involved defined areas (1m2) selected
randomly
MEASURING POPULATION
Net trawling
 Drawing an open net through water in order to
catch aquatic animals
Radio trapping
 Attaching a transmitter to an animal
 This device sends out a signal that can only be
detected using a radio recorder.
 A way of tracking animals in the wild.
REVIEW QUESTIONS
1.
Many organisms undergo a periodic population
explosion followed by a dramatic decline in the
numbers. List factors that contribute to both the
increase and the decrease in numbers.
2.
Why are predators not considered the main
contributors to the maintenance of a stable
population size of the prey?