2.6.1-.2 Populations
A look at the factors that tend to increase or
decrease the size of a population.
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.
POPULATION SIZE
Growth factors
(biotic potential)
Abiotic
Favorable light
Favorable temperature
Favorable chemical environment
(optimal level of critical nutrients)
Biotic
High reproductive rate
Generalized niche
Adequate food supply
Suitable habitat
Ability to compete for resources
Ability to hide from or defend
against predators
Ability to resist diseases and parasites
Ability to migrate and live in other
habitats
Ability to adapt to environmental
change
Decrease factors
(environmental resistance)
Abiotic
Too much or too little light
Temperature too high or too low
Unfavorable chemical environment
(too much or too little of critical
nutrients)
Biotic
Low reproductive rate
Specialized niche
Inadequate food supply
Unsuitable or destroyed habitat
Too many competitors
Insufficient ability to hide from or defend
against predators
Inability to resist diseases and parasites
Inability to migrate and live in other
habitats
Inability to adapt to environmental
change
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
REPRODUCTIVE STRATEGIES
Carrying capacity
K
Number of individuals
K species;
experience
K selection
r species;
experience
r selection
Time
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
SURVIVORSHIP CURVES
Population density affects population growth.
DENSITY INDEPENDENT FACTORS = affect a populations’ size regardless
of its population density.
1. Weather
2. Earthquakes
3. Floods
4. Fires
. . . Natural disasters
R-strategists populations are most affected by these.
DENSITY DEPENDENT FACTORS = affect a populations’ size depending
on its population density.
1. Predation
2. Disease
3. Availability of food and water
4. Space
Negative Feedback!!
INTERNAL FACTORS = might include density-dependent fertility or size of breeding
territory.
EXTERNAL FACTORS = might include predation and disease.
Species interactions influence population growth and carrying capacity = SYMBIOSIS
Competition for resources.
High
Low
0
Relative population density
Relative population density
High
Paramecium
aurelia
Paramecium
caudatum
2
4
6
8
10
12
14
Days
Each species grown alone
16
18
Low
0
Paramecium
aurelia
Paramecium
caudatum
2
4
6
8
10
Days
12
14
Both species grown together
16
18
Resource Portioning
PREDATION
PREY
POPULATION
PREDATOR
POPULATION
Avoiding predators
Span worm
Wandering leaf insect
Poison dart frog
Viceroy butterfly mimics
monarch butterfly
Bombardier beetle
Hind wings of io moth
resemble eyes of a
much larger animal
Foul-tasting monarch
butterfly
When touched, the
snake caterpillar
changes shape to look
like the head of a snake
Oxpeckers and black rhinoceros
Clown fish and sea anemone
Shark and ramora
Cleaning station
Cleaner blenny
Sabertooth blenny
change in the size of a population over time.


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 
pop. size
• resources 
pop. size
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)
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
a given space.
2.0
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.
A population that has few if any resource
limitations grows exponentially.
“J” population
growth curve
Population size (N)
EXPONENTIAL GROWTH starts out slowly and then
proceeds faster and faster as the population
increases.
Time (t)
24 hours
later
8
1024
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)
Number of reindeer
2,000
1,500
1,000
500
1910
1920
1930
Year
1940
1950