# Three Key Features of Populations 3. Density

```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
Three Key Features of Populations
2. Dispersion: describes their
spacing relative to each other
• clumped
• even or 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
3. Density: measurement of
population per unit area or unit
volume
Formula: Dp= N
S
Pop. Density = # of individuals &divide; 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
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