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Population Ecology
PACKET #80
CHAPTER #52
Introduction & Review
 Population
 Group consisting of
members of the same species
that live together in a
prescribed area at the same
time.
 Population Ecology
 The study of numbers of
individuals of a particular
species, at a given time and
location, and the interactions
of that population with other
populations and the
environment
 Population dynamics
 Study of changes in
populations
Population Ecology
IMPORTANT FEATURES OF POPULATIONS
Population Density
 Population Density
 This is how population size is expressed
 The number of individuals of a species per unit space
Dispersion
 Dispersion

Describes the spacing of
individuals and there are
three types.
Types of Dispersion
 Uniform Dispersion

Individuals are evenly distributed
 Clumped (Aggregated) Dispersion


Most common type
Individuals are concentrated in specific parts of the habitat

Patchiness
 Occurs as a result of distribution of resources or by asexual
reproduction
 Random dispersion



Occurs when individuals of a population are spaced throughout an
area in a manner that is unrelated to the presence of others
Least common; hardest to observe
Results from a lack of interaction between individuals or a
homogenous environment
Population Ecology
MATHEMATICAL MODELS
Equation

∆N/




∆t =
b–d
∆N is the change in numbers in the population
∆t is the change in time
b is the natality (birth) rate
d is the mortality rate
 r=b–d

r is the growth rate of the population
If r is 0, the population is stable
 If r > 1, the population is increasing in size
 If r < 1, the population is declining

 These equations can be expressed as dN/dt = rN


N is the population size
Represents arithmetic growth

Population increases by the same amount over each interval of time
Population Ecology
TYPES OF GROWTH RATES
Exponential Growth
 Exponential Growth


Population growth is at a rapid
pace
Reflects the maximum
intrinsic rate of growth

Maximum rate of growth under
ideal conditions for that
population
 dN/dt = rmaxN
 rmax represents
growth rate

the maximum
This type of growth may exist
for a period of time until
limiting factors become
important

Humans have exhibited this
growth pattern for centuries—
but can it last?
Exponential Growth & The Human Population I
 Thomas Malthus
recognized that the
human population
cannot continue in
exponential growth

There has been a large
decrease in death rate and
the population continues
to increase to unknown
proportions

No one knows the carrying
capacity of humans.
Exponential Growth & The Human Population II
 Not all countries have the
same growth rate

Higher in developing countries



India
South Asia
Africa
 Age structure of a country can
be used to predict future
population growth


Shows the percentages of
population at different ages
A broader base to the age
structure indicates a growing
population while a narrower
base indicates a stable or
shrinking population.
Logistic Growth
 Logistic Growth
 Exponential growth with
environmental resistance
(carry capacity of the
environment = K)
incorporated into the
equation
 dN/dt = rmaxN((K – N) /K)
 N = population size
 K = carrying capacity
 Maximum number of
organisms that the
environment can sustain
indefinitely.
 Exhibited by most natural
populations.
Potential Examination Question
Logistic Growth
 Populations do not always approach and stabilize at
K, but may overshoot K, followed by a population
crash.




Why does this occur?
Provide an example.
What reproductive strategies would be advantageous at high
population densities (At or close to K)?
What reproductive strategies would be advantageous at low
population densities?
Population Ecology
FACTORS INFLUENCING POPULATION SIZE
Dispersal
 Dispersal
 Movement of individuals among populations
 Immigration


Emigration


Migration of individuals into the population
Migration of individuals out of the population
If incorporated into the growth rate mathematical equation

r = (b – d) + (i – e)
Density Dependent Factors
 Regulate population size and is difficult to access in
nature and are limiting factors that increase in
intensity as population size increases
Density Dependent Factors II
 Predation
 Disease
 Increases when contact with toxic waste, produced by
population itself, increases with population size.
 Competition
 Intraspecific


Competition between members of the same species
Interspecific

Competition between members of different species
Density Independent Factors
 Limit population size and are factors that do not
increase in intensity as the population size increases.
 Most density-independent factors have some
relationship to population density.
 Many climatic factors are density-independent.
Density Independent Factors
 Climatic factors
 Sunlight
 Rainfall
 Temperature
All have impacts no matter what the population size
 Climatic and seasonal factors, monsoon season for example, can
severely decrease populations

Population Ecology
LIFE “HISTORY” TRAITS OF POPULATIONS
Introduction
 Life history traits, products of natural selection, are
traits that affect an organism’s schedule of
reproduction and survival.

Birth  Reproduction  Death
Semelparous vs. Iteroparous Species
 Semelparous Species

Expend their energy in a
single, immense
reproductive effort
Pacific salmon
 Agave

 Iteroparous Species

Exhibit repeated
reproductive cycles
Most vertebrates
 Most shrubs
 Most trees

r Strategists
 r strategists

Have traits that contribute
to a high population
growth rate.
High r
 Small size
 Large numbers of
offspring
 May live in unpredictable
habitats
 Weeds
 Pests

• Roaches
K Strategists
 K strategists
 Maximize the chance of
surviving in an environment
where the number of
individuals (N) is near the
carrying capacity (K) of the
environment.
 Population size near K
 Larger body size
 Long life span
 Late reproduction and slow
development
 Have fewer offspring
 May care for their young
 African elephants
 Humans
Survivorship
 Survivorship is related to r and k selection
 Type I Survivorship
Typical of K-selected organisms
 High mortality rate later in life


Type II Survivorship


Characterized by constant mortality rate over time
 Birds and lizards
Type III Survivorship
r selected organisms.
 Characterized by high mortality rate early in life

Survivorship Curves
Review
Review
 Students are encouraged to place their own
questions and charts on following slides.