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Population Dynamics
Population Ecology
Population: all the individuals of a
species that live together in the
same area
Branch of ecology that studies the
interaction between members
within the same population
Characteristics of Populations
1. Birth and Death rates
2. Sex Ratios
3. Age Distribution
4. Density
5. Dispersion
6. Reproductive Strategies
Populations have characteristic birth/death rates
1. Natality - number of births
Birthrate - number born in a given time
2. Mortality – number of deaths
Deathrate - number that die in a given time
Sex Ratios
• Comparison in numbers of males and females
• Affects sexual reproductive success – in all
species the number of offspring produced is
tied more closely to number of females than
males
• More mommas…more babies
Age Distribution
• Number of individuals of each age in the
population
• Used to predict population growth rate
POSTREPRODUCTIVE
REPRODUCTIVE
PREREPRODUCTIVE
Population Density
Number of individuals per unit of area
1 grizzly bear/ 10 square miles
24 students / classroom
High Density
Low Density
Dhaka, Bangladesh 112,700 / sqmi
Population Density affects the
availability of resources
Population Dispersion (Spacing)
Dispersion – way in which individuals
are spread out, or spaced, in the area
they live in.
3 Types of Dispersion
Clumped dispersion
• Grouped around resource
Most common pattern in nature
Advantages:
• Offer protection against predators
• Easier to find mates
Uniform Dispersion
• Even Spacing
Advantages
• Reduces energy wasted in competition
• Individuals hold territories
Random Dispersion
• Organisms distributed in no real pattern
Not common – only occur where:
• Resources are found evenly throughout an
area
• No significant much competition
Watch Video
• Population Spacing
Reproductive Strategies
*R strategists
*K strategists
R strategists






Short life span
Small body size
Reproduce early age
Have many young
Little/no parental care
Ex: cockroaches,
weeds, bacteria
K strategists
 Long life span
 Large body size
 Later reproductive age
 Have few young
 Provides parental care
 Ex: humans, elephants
Factors that affect
Population Size and Growth
1. Births - number of individuals born
2. Deaths – number of individuals who died
3. Immigration- movement of
individuals into a population
4. Emigration- movement of
individuals out of a population
Factors that affect Population Growth
INCREASES TO POPULATION
• Births
• Immigration (In)
DECREASES TO POPULATION
• Deaths
• Emigration (Exit)
Population Growth Rate is determined by:
Birth Rate + Immigration Rate
minus
Death Rate + Emigration Rate
Rate is amount per time period.
Example:
100 babies are born/mth +
- 14 die/mth
+
4 Immigrate in/mth
1 Emigrate = 86/mth
Growth rate = 89/mth
Factors That Affect Population
Growth
Immigration
Natality
+
+
Population
Size
Emigration
-
Mortality
Population Growth
• Rate of population growth is directly determined
by the amount of resources available
•
•
•
•
•
Food
Water
Space
Nesting/Breeding Sites
Available Mates
Exponential Growth
• Occurs when resources
are abundant and
predators may be
lacking
• J shaped growth curve
• Very few limits to
growth
• Often occurs when
species moves to
previously uninhabited
area.
• Ex. Invasive Stinkbugs
Logistic Growth
• Most environments
have limited resources
which limit population
growth
• A typical growth pattern
is slow (lag) growth,
period of exponential
growth, then a leveling
as resources deplete
and pop. gets to its
carrying capacity
• S – shaped curve
Human Population
Exponential Growth
Carrying Capacity
• Maximum number of individuals of a species
that the environment can support
Carrying Capacity is dependent on
environmental changes
Overshoot and Population Crashes
• Overshoot occurs when a population
exceeds the long term carrying capacity of
its environment.
• The consequence of overshoot is called a
collapse, a crash or a die-off.
Limiting Factors
• Many factors affect the carrying capacity
• The factor that has the greatest effect in
restricting the population growth is called the
limiting factor.
Limiting factor
Ex. 20 Rabbits live where they
have enough water, nesting spaces,
cover but only enough food for 10
rabbits. What’s the limiting factor?
Limiting factor – nitrogen fertilizer
2 Groups of Limiting Factors
• Density–dependent limiting factors
• Density–independent limiting factors
Factors that Population Growth
Density-dependent factorsAffect varies depending on population density.
Greater population density, factor has greater affect.
Smaller population density, factor has lessor affect.
Density Dependent Limiting Factors
• Competition – greater population density
causes resources used faster
• Predation – Greater population density makes
it easier on predators to get prey
• Parasitism – When organisms live more closely
together/ densely packed, it allows for easier
spread of parasites and disease
Density-independent factorsFactor whose affects does not vary with
population density
Ex. Temperature
Drought
Natural disasters – fire, volcano, tsunami,
Habitat destruction by humans
Watch Video
• Population growth
Human Population Growth
What does the future hold for human
population growth?
• When will human
population growth
outstrip the available
resources
• Are we already living
unsustainably???
Human Population Growth
Births
Deaths
Natural
increase
Year
130,013,274
56,130,242
73,883,032
Month
Day
10,834,440
4,677,520
6,156,919
356,201
153,781
202,419
6,408
8,434
Time unit
Hour
14,842
Minute
247
107
141
Second
4.1
1.8
2.3