POPULATIONS
& CARRYING CAPACITY
02 June 2010 Populations.ppt 1
• 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
02 June 2010 Populations.ppt 2
Three Key Characteristics
of Populations
• Size, number of
individuals (N)
• Density (N/ area)
• Dispersion,
– Random, uniform,
clumped,
• (appropriate scale)
02 June 2010 Populations.ppt 3
Three Key Characteristics of
Populations
• 1. Size: number of individuals in an area
– Characterized by (N)
Growth Rate:
Birth Rate (natality) - Death Rate (mortality)
(How many individuals are born vs. how many die)
02 June 2010 Populations.ppt 4
Three Key Characteristics of
Populations
• 2. Density: measurement of population per
unit area or unit volume
Pop. Density =
# of individuals ÷ unit of space
02 June 2010 Populations.ppt 5
4 Factors that affect density
1. Immigration- movement of
individuals into a population
2. Emigration- movement of
individuals out of a population
02 June 2010 Populations.ppt 6
4 Factors that affect density
3. Density- 4. Density-
dependent factors- independent factors-
Biotic factors in the Abiotic factors in the
environment that environment that
have an increasing affect populations
effect as population regardless of their
size increases density
Ex. disease Ex. temperature
competition storms
parasites habitat destruction
drought
02 June 2010 Populations.ppt 7
Three Key Characteristics of
Populations
• Dispersion:
describes their
spacing relative to
each other
02 June 2010 Populations.ppt 8
Dynamic characteristics
of populations
• Age distribution,
– proportions of young, middle-aged, old
– Differs in growing, stable, decreasing populations
02 June 2010 Populations.ppt 9
Factors That Affect Future
Population Growth
Immigration
+
+ -
Natality Population Mortality
-
Emigration
Changes in populations
• Growth
– Expansion of species’ populations may lead to
evolution of new species
• Decline
– Shrinking species’ populations may lead to
extinction
• Small populations
• Narrowly specialized species
02 June 2010 Populations.ppt 11
Changes in populations
• ΔN = +B +I –D –E
– +B = births (birth rate)
– +I = immigrants (immigration rate)
– – D = deaths (death rate)
– – E = emigrants (emigration rate)
– (For many [most] natural populations I and E
are minimal.)
02 June 2010 Populations.ppt 12
Other factors that affect
population growth
Limiting factor- any biotic or abiotic
factor that restricts the existence of
organisms in a specific environment.
– EX.- Amount of water
Amount of food
Temperature
02 June 2010 Populations.ppt 13
Other factors that affect
population growth
• Carrying Capacity- 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
02 June 2010 Populations.ppt 14
Carrying Capacity
J-shaped curve
N (exponential growth)
u
Carrying Capacity (k)
m
b S-shaped curve
e (logistic growth)
r
Time
02 June 2010 Populations.ppt 16
Logistic model
• Logistic model works, to a point.
– Real organisms have time lags for growth, time to
develop eggs, flowers, etc.
• seasonality, longevity
– Real populations may exceed carrying capacity.
• Easter Island
• Pribloff Reindeer
• Kaibab Deer
02 June 2010 Populations.ppt 17
Easter Island
• Discovered by Polynesians ~ A.D. 1000
• Population grew to several thousand
– Used trees for canoes to hunt dolphins
– Used wood for cooking
– Also ate birds, eggs, vegetables
• Resources (trees) depleted
– No canoes, no dolphins
– Warfare over land, food resources
• Population fell to ~ 100 when discovered by
Dutch, Easter Sunday 1722.
02 June 2010 Populations.ppt 18
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
Two general types of
Life History Strategies
Life History trait r-adapted, K-adapted,
Opportunistic Equilibrium
Offspring Many, small (high r) Fewer, large (low r)
Offspring survival Low High
Parental care Rare Common
Reproductive age Early Later
Reprod. “seasons” 1-few Many
Habitat Unstable, temporary Stable, permanent
Competitiveness Low High
Population regulation Density independent Density dependent
Population fluctuation Irruptive Stable near K
Life History Strategies
• Survivorship curves of Opportunistic and
Equilibrium species
– Opportunistic have Type III
– Equilibrium have Type I
02 June 2010 Populations.ppt 22
Life History Strategies
• Fluctuating populations of two interacting populations
– Based on pelts sold by Canadian trappers to the Hudson Bay
Company, ~1840-1940.
02 June 2010 Populations.ppt 23
