Population Ecology
Study of the factors that affect the
population dynamics of any species!
Current Population Clock
Overview of Chapter 8
 Principles of Population Ecology
 Reproductive Strategies
 The Human Population
 Demographics of Countries
 Demographics of United States
Principles of Population Ecology
 Population Ecology
– Study of populations and why their numbers change
over time
– Important for
 Endangered species
 Invasive species
 Population
– Group of individuals of same species living in the
same geographic area at the same time
Population Dynamics:
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Several factors are included:
Population density
Birth rate
Death rate
Growth rate
Age structure
Resources (quantity, type, quality)
How a Population Responds to the
Environment Depends Upon Several
Interactions:
• How individuals compete for food
• How disease, predation, and other environmental
pressures affect population
• Reproductive success (or failure)
• Management of populations for ecosystem health
and/or human needs
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•
•
forestry management – tree populations,
agronomy – pest and crop populations
wildlife management – animal and fish populations
Population Density
 the number of individuals of a species per unit
of area or volume at a given time
 Population density can change dramatically in
different habitats, or in the same habitat over a
short distance
 Population density can indicate a potential
pollution source in a river or stream
Population Density
 Population density
– The number of individuals of a species per unit area
or volume at a given time
– Ex: minnows per Liter of pond water
 Ovals below have same population, and
different densities
Birth Rate
 the rate at which individuals produce offspring
 Birth rate in humans: (b) = the number of
births per 1000 people per year (human terms)
Death Rate
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= the rate at which organisms die
Death rate in humans: (d) = the number
of deaths per 1,000 people per year
Growth Rate
 the rate of change (increase or decrease) in a
population;
 Formula: Growth rate (r) = b-d
Growth rate in humans is also called the
“natural increase” in the population
Example Growth Rate Problem:
 Population of 40,000 people; 800 births per
year; 400 deaths per year
– b = 800/40,000 = 0.02
– d = 400/40,000 = 0.01
– r = 0.02 – 0.01 = 0.01 or 1.0% per year population
growth rate
 r is positive if people are born faster than they
die; r is negative is people die faster than they
are born
Dispersal
 the movement of individuals from one location
to another, which affects the population at
either location
 Immigration (i) – individuals entering a
population
 Emigration (e) – individuals leaving a
population
Global Populations vs. Local
Populations…
 Global populations depend purely on birth and
death rates
 Local populations depend on birth and death
rates plus immigration and emigration rates
Example Growth Rate Problem 2:
 Population of 250,000; 1400 births per year;
900 deaths per year; 100 immigrants and 200
emigrants yearly
 b = 1,400/250,000 = 0.0056
 d = 900/250,000 = 0.0036
 i = 100/250,000 = 0.0004
 e = 200/250,000 = 0.0008
 r = b – d + i –e = 0.0056 – 0.0036 + 0.0004 –
0.0008 = 0.0016
 **Note: This is equivalent to 0.16% growth
Birth Rate & Death Rate
Calculations…Points to Consider
 NOTE: Sometimes birth and death rates are
simply given as percentages, which indicates
how many people were born (or died) out of
100. Either way of reporting (rate or number
per 1,000 living) is acceptable.
 Example: Birth rate of 1.1% means 1.1 people
were born for every 100 people living in area
under consideration.
 Typically reported as 11 births per 1,000
people living.
Example Growth Rate Problem 3:
Note: NOT on your packet…
 Birth rate = 1.1% (NOTE: this would be 1.1 per
100; or 11 per 1,000)
 Death rate = 0.8%
 Immigration = 0.08%
 Emigration = 0.04%
 Growth rate = 1.1 – 0.8 + .08 - .04
= 0.34%
This means that 0.34 people are added to the
population for every 100 living; or 3.4 people
per 1,000 living.
Calculating Future Populations
 Extending population growth into the future utilizes a
physical constant e = 2.7183
 Formula:
Population (final) = Population (initial) x e (r * t)
Pf = Pi x e (r * t)
Where r is the growth rate IN PERCENT expressed in
DECIMAL form, and t is the time (number of years)
NOTE: On a calculator, you plug in the r x t first, then raise
2.7183 to that number, then multiply result by the initial
population!
Calculating Future Populations
 EXAMPLE:
– 2002 Pakistan population = 143.5 million
– Pakistan population growth rate is 2.1%
– 2010 population:
 P = 143.5 million x 2.7183 (.021 x 8 yrs)
 P = 143.5 million x 2.7183 (0.168)
 P = 143.5 million x 1.182937939
 P = 169.8 million
Estimating Animal Populations:
 Most of the time, determining animal
populations in an ecosystem setting is difficult
unless the animal is sessile (immobile) and
easy to count.
 Tag & Recapture is often the preferred method
used to estimate animal populations
– Works by catching an initial sample, “tagging” with
some identification mark or tag, releasing them,
and recapturing them later.
– A simple ratio is used to determine final population.
Biotic Potential:
 the maximum rate at which a population can
increase
 Life History Characteristics that Influence
biotic potential:
– Age at which reproduction begins
– Amount of life that reproduction is possible
– Number of reproductive periods per lifetime
– Number of offspring per reproductive event
 Larger organisms generally have smallest
biotic potential, while microorganisms have the
highest
Life history traits – characteristics of an individual
that influence survival and reproduction
Age at maturity
House Mouse
African elephant
2 months
11 - 20 years
Atlantic Salmon
3-6 years
Life history traits – characteristics of an individual
that influence survival and reproduction
Number of offspring produced
House Mouse
5-8 young every
month
African elephant
1 calf every
3-8 years
Atlantic Salmon
1,500 to 8,000
eggs once
Life history traits – characteristics of an individual
that influence survival and reproduction
Number of reproductive events
House Mouse
African elephant
~6-12
~3 - 10
Atlantic Salmon
1
Life history traits – characteristics of an individual
that influence survival and reproduction
Lifespan
House Mouse
~2 years
African elephant
60 - 70 years
Atlantic Salmon
3-6 years
Exponential Population Growth:
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Populations with a constant reproductive
rate will have an accelerating population
growth under optimal conditions
doubling of population occurs in
successively shorter intervals
Numbers of Bacteria During 10 hour
Period (growing exponentially!)
Environmental Resistance:
 limits to exponential population growth occur
when population reaches a size that allows
environmental limits to take effect
 Environmental limits include:
– Space
– Food
– Exposure to toxins, etc.
– Increase in population of predators
Carrying Capacity:
 Carrying Capacity (K) = the largest population
that can be maintained for an indefinite period
of time in a particular environment
 Carrying capacity changes with changes in the
environment, either natural or artificial
– Droughts, pollution, excess rainfall, etc.
Carrying Capacity
S-Shaped Population Curves…
 Graphs of populations influenced by
environmental limitations show a characteristic
S shape curve
 Shows an initial exponential growth, followed
by slowed growth and then a flattening of the
curve as environmental limits are reached
S-Shaped Growth Curve (paramecia):
Example 1: bacterial population crash due
to toxic waste accumulation
Reproductive Strategies:
 Nature forces organisms to make tradeoffs in
the expenditure of energy
 Only some energy can be used for
reproduction, since each organism must
uptake nutrients to grow, hunt for food, etc.
 “r-selected” and “k-selected” species have
developed over time in response to energy
requirements
r – Selected Species:
 Those species that have traits that favor
growth rate strategies (“r-strategists)
 Typical r-selected strategies:
– Small body size
– Early maturity
– Short life span
– Large broods
– Minimal required parental care
– Live in unpredictable or temporary environments
– Opportunists (like mosquitos, insects, weeds, etc.)
K-Selected Species
• Called “K-strategists”
• Species that try to maximize the chances
of survival, especially in environments
where the number of individuals (N) is
near the carrying capacity (K) of the
environment
K-Selected Strategies…
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•
•
•
•
Long life span, slow development
Late reproduction
Large body size
Low reproductive rate
Examples: redwood trees, animals
requiring long parental care (Tawny owls
pair for life!)
Survivorship:
• Life tables are constructed by ecologists
to indicate the relative chances of survival
at any time during the life of the organism
• Each organism is classified as a Type I,
Type II, or Type III Survivorship organism
Survivorship = the probability that a given individual
in a population will survive to a particular age.
Type I Survivorship: the
young and those at
reproductive age have
a high chance of living
Type II Survivorship: the
probability of survival
does not change with
age
Type III Survivorship: the
probability of death
greatest early in life,
those that survive have
high survival rate until
old age
Factors That Affect Population
Size
Density-Dependent Factors:
 A change in population density initiates
some factor, which subsequently has a
counter-effect on the population
 Examples of D-D Factors:
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Disease, predation, competition for resources
Predator-prey relationships involve
dynamics of density-dependent factors.
Predator-Prey on Isle Royale…
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From 1960 to the mid 1980’s…this is a simple predator-prey dynamic relationship
Mid 1980’s the wolf population crashed…later confirmed to be the result of a deadly
canine parvovirus disease…
This allowed moose to flourish…but by mid-1990’s there were so many that they
literally overgrazed their main food source (ash and aspen)…which led to a sudden
collapse, which then triggered a die-off of the already struggling wolves by 1998
(only 14 individuals were counted!)
Population Cycles – Hares/Lynx Example
Example 2: Population cycling due
to organism interaction
Start: both have low population density
 Hares – high food, low predators = pop
increase over next generations
 Lynx – as hares increase, more food = pop
increase
 Hares – when high pop density, increased
competition for food and increased predation =
low birth rate & high death rate = sharp pop
decrease
 Lynx – when high pop density and few hares,
low food = low birth rate & high death rate =
sharp pop decrease
Population Cycles
Density-Independent Factors…
 Any
environmental factor that affects the
size of a population but is not influenced
by changes in population density
 Usually abiotic factors, such as random
severe weather, fires, timing & severity of
winter season, etc.
 Cause distress to a population
Human Population
 Right now at over 6.6 billion and
counting…
 Scientific advances have enabled us to
improve the productivity of the land…so
food production has kept pace…not a
limiting factor for much of the world
 However, have we reduced the ability of
the land to sustainably feed future
generations?
 Exponential
growth so far
 Should “level
out”
sometime
during the
21st century
 S-curve may
be
developing
already
 What will
cause the Scurve????
The Human Population Clock…
100 million more people from 2009 to
2010…let’s see why:
 Increase is NOT due to a rise in the birth rate
(b). World birth rate has declined slightly.
 Increase is due to a DRAMATIC decrease in
the death rate (d).
– Greater food production
– Better medical care
– Improvements in water quality and sanitation
World Growth Rate
Actually has started to decline!
Growth Rate (r) was at 2.2% in 1960’s,
now estimated at 1.2% (2006)
S-curve coming soon…
Zero Population Growth: when the birth
rate = death rate; should occur by the
end of the 21st century (2100)
Role of the U.N. (United Nations)
UN periodically provides estimates of
population growth rates in each country,
including factors influencing births and
deaths.
UN estimates that human population will
most likely be 9.1 billion by 2050. (range
between 7.7 and 10.6 billion)
Earth’s “carrying capacity” unknown…but
studies by Van Den Bergh and Rietveld
(Netherlands) suggest 7.7 billion is our upper
limit!
Will humans slow down pop. growth, or will
there be widespread human suffering and
death as Earth becomes uninhabitable????
Demographics
Branch of sociology that deals with
population statistics
Separates countries into categories
based on stage of development, which
relates to population growth potential
Highly Developed, Developing, LessDeveloped nations
Demographics…
Highly Developed Countries include:
– U.S., Germany, Canada, France, Sweden,
Australia, Japan, U.K., etc.
– Highly industrialized
– Low birth rates
– Very low infant mortality rates
– Longer life expectancies
Demographics…
Developing Countries – includes moderately
developed and less-developed
Moderately developed countries include:
– Thailand,
etc
– Moderate
– Moderate
– Moderate
Mexico, Turkey, Peru, Brazil, Chile,
birth rates
infant mortality rates
level of industrialization
Demographics…
Less Developed Nations Include:
– Cambodia, Bangladesh, Ethiopia, Niger, etc.
– Highest birth rates
– Highest infant mortality rates
– Shortest life expectancies
– Lowest average per-capita GNI PPP’s (gross
national income in purchasing power party divided by midyear
population)
Doubling Time:
 Doubling Time = amount of time it takes for
the population to double in size
– Usually calculated for each country
 Formula for Doubling Time (td)
– td = 70/r
 Shorter the doubling time, the less developed
the country likely is
 Typical Doubling Times:
– Laos = 30 yrs.;
Ethiopia = 28 yrs.
– France = 175 yrs.; Turkey = 47 yrs.
Fertility:
 Replacement-Level Fertility = the number of
children a couple must produce in order to
“replace” themselves (usually considered 2.1, since
some infants & children die before reaching maturity)
 Total Fertility Rate is currently 2.8 worldwide
Demographic Stages:
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1.
2.
3.
4.
Four stages are observed, based
upon European history
Pre-industrial stage
Transitional stage
Industrial stage
Postindustrial stage
Pre-industrial stage
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High birth and death rates
Modest population growth
High infant mortality rate
Famines, plagues, wars common
Transitional Stage
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Lowered death rate due to advances
in health care, better food supply,
water sanitation measures
Birth rate still high
Population grows rapidly
Industrial Stage
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Decline in birth rates, still relatively
low death rate
Population growth rate slows a bit
Increase in wealth usually
accompanies a decrease in birth
rates
Postindustrial Stage
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Low birth and death rates
Better educated and more affluent
population…tend to desire smaller
families
Examples: U.S.A., Canada, Australia,
Japan, Europe
Demographic Stages based on
birth and death rates…
Age Structure of Countries
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Age structure = the number and proportion
of people at each age in a population
Age structure helps in determining future
growth rates
Age structure diagrams provide a quick way
to look at figures, and determine what is
happening to a population
Age Structure Diagrams:
Nigeria, United States, and Germany
Age Structure Diagrams:
United States Population Issues:
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Largest population of all “highly developed” nations
U.S. is over-populated due ONLY to immense overconsumption of resources by individuals
Immigration rate is among the highest, adding to
population growth well beyond birth rate
1 million legal immigrants yearly, plus illegal
immigrants estimated at 300,000 yearly
(conservative estimate…)
No formal population policy exists (as it does in
China and other countries, for example)
United States Population Issues:
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Immigration and Nationality Act (1952) now
called the Immigration Reform and Control Act
(IRCA):
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Abolished national quotas on immigration
Gives preference to those with family members
already here, those who can fill vacant jobs, and
refugees seeking asylum
Most immigrants now come from Mexico,
Phillipines, Vietnam, Dominican Republic, and
China
Many immigrants are poor, have few skills
Education of Immigrants into US
Immigration & Environmental
Questions
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Arguments/Questions in favor of immigration restrictions:
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Arguments/Questions against immigration restrictions:
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Is population of U.S. already too large?
More immigrants means more pollution, less literate public,
resource depletion?
Immigrants adopt the polluting lifestyle of current U.S. residents?
Immigrants generally do not get involved in the political
process…creates a more apathetic population
Morally unacceptable to refuse immigrants?
Immigrants typically lead simpler lifestyles, so they contribute only
marginally to environmental stress?
Creates a global consciousness & more diverse population
Immigrants will gladly work in jobs most Americans would prefer
not to…so they are necessary to some degree????
Where do you stand????