• POPULATION = a group of interbreeding
organisms (same species) that live in the
same place at the same time and compete for
the same resources.
• Resources = food, water, shelter, mates, and
so on . . .
• resources 
• resources 
pop. size
pop. size
The population size of a species in a given
space at a given time is determined by the
interplay between BIOTIC POTENTIAL and
ENVIRONMENTAL RESISTANCE.
Biotic potential = growth rate with unlimited
resources.
Environmental resistance = all the factors
acting jointly to limit population growth.
Four variables change population size:
1. NATALITY = birth rate
2. MORTALITY = death rate
3. IMMIGRATION = rate of organisms moving
in
4. EMIGRATION = rate of organisms moving
out
DENSITY DEPENDENT FACTORS = affect a
populations’ size depending on its
population density.
1. Predation
2. Disease
Feedback!!
3. Availability of food Negative
and water
4. Space
Populations change in response to
environmental stress or changes in
environmental conditions.
1. In size = # of individuals
2. Density = # of individual / specific space
3. Age distribution = proportions / age group
4. Dispersion =
Clumped
(elephants)
Uniform
(creosote bush)
Random
(dandelions)
DENSITY INDEPENDENT FACTORS = affect a
populations’ size regardless of its
population density.
1. Weather
2. Earthquakes
3. Floods
4. Fires
R-strategists populations are most affected by
these.
. . . Natural disasters
No population can grow indefinitely!
Number of sheep (millions)
Every environment has a CARRYING
CAPACITY = the maximum number of
individuals of a given species that
can be sustained indefinitely in
2.0
a given space.
1.5
1.0
.5
1800
1825
1850
1875
Year
1900
1925
Factors that affect carrying capacity:
1. Competition with/in and between
species.
2. Natural and human caused catastrophes.
3. Immigration and emigration.
4. Seasonal fluctuations in food, water,
shelter, and nesting sites.
“J” population
growth curve
Population size (N)
A population that has few if any resource
limitations grows exponentially.
EXPONENTIAL GROWTH starts out slowly
and then proceeds faster and faster as the
population increases.
Time (t)
LOGISTIC GROWTH involves initial exponential
growth and then there is a steady decrease in
growth as the population encounters
environmental resistance and approaches
carrying capacity and
levels off.
“S or sigmoid”
population growth
curve
Population size (N)
K
Time (t)
Plateau phase
K
Population size (N)
Transitional phase
Exponential phase
Time (t)
Opportunistic or r-Selected Species
cockroach
dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
Competitor or K-Selected Species
elephant
saguaro
Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close
to carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
REPRODUCTIVE STRATEGIES
Carrying capacity
K
Number of individuals
K species;
experience
K selection
r species;
experience
r selection
Time
SURVIVORSHIP CURVES
Reindeer on St. Paul Island
• 41 mi2
• Started with 25
Reindeer (4 males
and 21females)
• No predators
• 200 mi from
mainland so no on
or off
• No hunting
End Result
Start with 25
1938= 2046 deer
1950= 8 deer
Growth Rate of 16%
Did bounce back to a
steady 250 deer
Birth and Death Calculations
Crude Birth Rate (CBR)= # of births per
1000 individuals/yr
CBR=(Births/Pop. Size)x1000
The world CBR is 20.3
Crude Death Rate (CDR) = # of deaths per
1000 individuals/yr
CDR= (Deaths/Pop. Size)x1000
The world CDR is 9.6
More Calculations
Natural Increase Rate in % (NIR)
NIR = (CBR-CDR)/10
Doubling Rate = how many years it will take
to double the population
DR= 70/NIR
Density= # of individuals in a given area
D= Pop./ Area
IB Acronyms for Countries
• MEDC= More Economically Developed
Country
• LEDC= Less Economically Developed
Country
• NIC= Newly Industrialized Country