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Population Ecology
Chapter 5, Section 3
Population Dynamics
Population: all the individuals of a
species that live together in an area
Demography: the statistical study
of populations, make predictions
about how a population will change
Population Dynamics
Three Key Features of Populations
•Size
•Density
•Dispersion
Three Key Features of Populations
1. Size: number of individuals in an
area
Three Key Features of Populations
Four Variables:
(Births+ Immigrants) − (Deaths + Emigrants) =
Population Change
Factors That Affect Future
Population Growth
Immigration
Natality
+
+
Population
Emigration
-
Mortality
Three Key Features of Populations
2. Dispersion: describes their
spacing relative to each other
• clumped
• even or uniform
• random
clumped
even
(uniform)
random
*Species tend to cluster where resources are
available
*Groups have a better chance of finding
clumped resources
*Protects some animals from predators
*Packs allow some to get prey
*Temporary groups for mating and caring for
young
Three Key Features of Populations
2. Density: measurement of
population per unit area or unit
volume
Formula: Dp= N
S
Pop. Density = # of individuals ÷ unit of space
Populations CANNOT grow
indefinitely…
Biotic potential – capacity for population
growth under ideal conditions
Intrinsic rate of increase (r) – the rate at
which a population would grow with
UNLIMITED resources
Factors that affect population growth
Limiting factors =
ENVIRONMENTAL RESISTANCEthe combination of all factors that
act to limit the growth of a
population
 Ex:
Amount of water
Amount of food
Temperature
Together, biotic potential and environmental
resistance determine the CARRYING
CAPACITY!
Environmental Resistance
Density-dependent factors- Biotic
factors in the environment that have an
increasing effect as population size
increases
Predation
 Parasitism
 Infectious disease
 Competition for resources

Environmental Resistance
Density-independent factors- Abiotic
factors in the environment that affect
populations regardless of their density
Ex. temperature
storms
habitat destruction
drought
Other factors that affect population
growth
Carrying Capacity = K = the maximum
population size that can be supported
by the available resources
There can only be as many
organisms as the environmental
resources can support
Logistic growth = “S”
A sheep population on the
island of Tasmania between
1800 and 1925 - After sheep
were introduced in 1800,
their population grew
exponentially, thanks to an
ample food supply. By
1855, they had overshot the
land’s carrying capacity.
Their numbers then
stabilized and fluctuated
around a carrying capacity.
Carrying Capacity
N
u
m
J-shaped curve
(exponential growth)
Carrying Capacity (k)
b
S-shaped curve
(logistic growth)
e
r
Time
r-selected versus k-selected species
2 Life History Patterns
1. R Strategists






short life span
small body size
reproduce quickly
have many young
little parental care
Ex: cockroaches,
weeds, bacteria
2 Life History Patterns
2. K Strategists
 long life span
 large body size
 reproduce slowly
 have few young
 provides parental
care
 Ex: humans,
elephants
4 General Patterns of Variation
1. Stable – pop fluctuates slightly around
carrying capacity
2. Irruptive - many short-lived, rapidly
reproducing organisms – high peak, then
crash
3. Cyclic - predator-prey
4. Irregular – no recurring patterns
Population Cycles for the Snowshoe Hare and
Canada Lynx