Population Ecology
• Population ecology is the study of populations
in relation to environment
– Including environmental influences on population density
and distribution, age structure, and variations in
population size
Levels of
Organization
•Organism
•Population
•Community
•Ecosystem
•Biome
Describing
populations
Describing populations
• Size: total # of individuals in a pop (N)
• Density: # / unit area
Describing populations
Estimation of Population Size
Sampling technique
• Mark and recapture
Describing populations
Mark and Recapture
• Orgs are captured, tagged, and then
released.
• Some time later, the same process is
repeated and the following is used:
Describing populations
• Ex:
– Suppose that 50 zebra mussels are captured,
marked, and released. One week later, 100 zebra
mussels are captured and 10 are found to have
markings already. The estimated population
would then be
• N= (50·100) / (10) = 5000/10 = 500
• N=500
Dispersion
•
Describes how
individuals in a
population are
distributed
1. Clumped
2. Uniform
3. Random
Describing populations
Clumped dispersion
–
–
(most common)
Describing populations
Individuals aggregate in patches
May be influenced by resource availability & behavior
Example:
For many animals, such as these wolves,
living in groups
1.increases the
effectiveness of hunting.
2.spreads the work of
protecting and caring for
young.
3.helps exclude other
individuals from their
territory.
Fish travel in
schools –
safety in
numbers
Uniform dispersion
Describing populations
-Individuals are evenly distributed
-May be influenced by social interactions such as territoriality
Birds nesting on small islands,
such as these king penguins on
South Georgia Island in the
South Atlantic Ocean, often
exhibit uniform spacing,
maintained by aggressive
interactions between
neighbors.
Some plants
secrete
toxins that
keep others
away
Random dispersion
Describing populations
– The position of each individual is
independent of other individuals
Dandelions grow from
windblown seeds that
land at random and
later germinate.
Changes in Population Size
Describing populations
• Biotic potential of a population=
– max rate at which a population could increase under ideal
conditions
• Influenced by several factors:
– Age at which reproduction begins
– Life span during which the organisms are capable of
reproducing
– Number of reproductive periods in the lifetime
– Number of offspring the organism is capable of having
Age Structure
Describing populations
• Description of the abundance
of individuals of each age in a
population.
Age Structure Diagrams
Describing populations
2004
China
India
United States
Zero
population
growth
Describing populations
1954
Animation
Describing populations
Survivorship Curves
Describing populations
• Describes how the mortality of individuals in
a species varies during their lifetime
The survivorship
curve for
Belding’s
ground
squirrels
–
Shows that
the death
rate is
relatively
constant
Number of survivors (log scale)
1000
100
Females
10
Males
1
0
2
4
6
Age (years)
8
10
•Type I
•A species in which
most survive to
middle age
•Parenting
•Type II
•Length of
survivorship is
random. Death
rate is constant.
Number of survivors (log scale)
• Survivorship curves can be classified into three general
types: Type I, Type II, and Type III
1,000
I
100
II
10
III
1
0
100
50
Percentage of maximum life span
•Most individuals
die young.
•Few reach
reproductive
maturity.
•Fish release
1000s of eggs.
•No parenting
Number of survivors (log scale)
•Type III
1,000
I
100
II
10
III
1
0
100
50
Percentage of maximum life span
Animation
Factors Affecting
Population
Limiting Factors
Factors Affecting
Population
 Things that prevent a population from
attaining its biotic potential
–
2 categories
1. Density-dependent
2. Density-independent
Density
Dependent
Density
Independent
Density Dependent
Factors Affecting
Population
Populations regulated by density-dependent factors
affected by the number of
organisms present.
are
a)
b)
c)
d)
Predation
Parasitism
Disease
Competition
Intraspecific Competition = the
struggle between members of a
population for scarce resources
-the more organisms crowd together, the more damaging are
food shortages, parasites, and predators.
Density Independent
Factors Affecting
Population
• The number of organisms present does not affect
the influence of the factor.
– Fire, earthquakes, storms, floods
• Density-independent factors show no correlation
with the size of the population.
Describing Population Growth
Population ecologists describe two general patterns of populations growth:
1. Exponential
2. Logistic
Types of
The growth of a population can be
Population
described by the following equation: Growth
• r= reproductive rate (or growth rate)
• N= population size at the beginning of the
interval for which the births and deaths are
counted.
.
r=
births-deaths
N
Can r be negative? Zero?
Net increase
of individuals
r=
Ex:
births-deaths
N
Types of
Population
Growth
– A population of 1000 had 60 births and 10
deaths over a one year period. What is the
growth rate?
– 0.05 per year
Exponential growth
r>0
Types of
Population
Growth
In a population showing exponential growth the
individuals are not limited by food, disease,
predation or competition.
If the rate of reproduction per individual remains
constant through
time, then the
rate at which
the population
increases is a
multiple of the
number of
individuals in
the population.
Exponential population growth
r>0
Types of
Population
Growth
– Results in a J-shaped curve
– Usually short lived in nature- not sustainable
size
Population
(N) (N)
Population size
2,000
1,500
dN 1.0N
dt
dN 0.5N
dt
1,000
500
0
0
10
15
5
Number of generations
the real
world
Carrying Capacity (K)
• Populations usually
reach a carrying
capacity.
• Upper limit to the
number of individuals
the environment can
support.
Types of
Population
Growth
Logistic growth
dN 
dt rmaxN
Types of
Population
Growth
In most real populations both food and disease become
important as conditions become crowded.
•A population can grow
exponentially only for
short periods of time,
because it would very
quickly deplete all the
resources necessary for its
survival.
•Over long periods of time,
populations tend to attain an
equilibrium population size
which is determined by the
available resources.
Animation
Types of
Population
Growth
• Zero population growth
– Occurs when the birth rate equals the death rate
• The population growth equation can be expressed as
dN
dt
 rN
Life-history
strategies
Logistic and exponential growth are
associated with 2 kinds of life-history
strategies
1.R-selected species (strategists)
• organisms are opportunistic; they reproduce
rapidly when the environment is uncrowned and
resources are vast
2.K-selected species (strategists)
•
operate at a density near K
R-selected species
Life-history
strategies
a. Opportunistic species, which tend to be colonizers.
b. Grasses, many insects
-Usually quickly invade, reproduce many offspring and die
-Offspring mature quickly and require very little parental
care
c. Strategies for continued existence is based on individuals
having the following traits:
1) small size
2) short life span
3) mature fast
4) produce many offspring
5) engage in little care of offspring
K-selected
species
–sensitive to population density
Life-history
strategies
Such populations are equilibrium species, tend to be specialists
rather than colonizers, and may become extinct when their evolved way
of life is disrupted (e.g., the grizzly bear, Florida panther, etc.).
Overall strategy for continued existence is based on having the following
traits:
1) large size
2) long life span
3) slow to mature
4) produce few offspring
5) expend considerable energy in care
Human Population Growth
The following made
exponential growth
possible
1. Increases food supply
2. Reduction in disease
3. Reduction in human
waste
4. Expansion of habitat
As of 28 April 2010, the human population of the world is
estimated by the United States Census Bureau to be
6,817,500,000.
Countries Ranked by Population: 2010
Source: U.S. Census Bureau, International Data Base.
6,817,500,000
Map of countries by population for the year 2007