Population Ecology
Elizabeth Petrie
Jill Stittleburg
Sherry Wiedman
Today’s objectives:
• Define carrying capacity
• Describe how and why population growth
rates change as the population
approaches carrying capacity
• Discuss the “sixth extinction” and
measurement of extinction rates
• Analyze human causes of extinction
Changes in Population Size
• What causes populations to change in
size?
• What are some factors that may limit
population growth?
• Is there a point at which the population
cannot grow anymore? Why or why not?
• Population growth is a function of birth rate
and death rate-
If birth rate > death rate, population
increases
If death rate > birth rate, population
decreases
If birth rate = death rate, then the
population stablizes
Limiting Factors• An environmental factor that prevents a
population from increasing.
Lack of :
food
water
shelter
space
There are 2 types of limiting factors:
• Density dependent factors
• Density independent factors
Density- dependent factors
• Limiting factors that increase in intensity
as the population increases in size.
• Will reduce birth rates or increase death
rates
Ex. limited food supplies
increased risk of infectious disease
toxins due to increased waste levels
Density-independent factors
• Influence population rates regardless of
their density
Ex. weather events: drought,
floods, extreme temperatures
human interventions
Carrying capacity
• The maximum population that can be
supported indefinitely in a given
environment
• Populations that are experiencing
exponential growth eventually reach
carrying capacity
Fruit Fly Population Growth
Population Growth
• When conditions are good, a population
can grow exponentially
• The effects of declining resources
gradually slow growth rate
• Growth rate declines to zero as it
approaches carrying capacity
• If a population grows beyond the carrying
capacity, a population crash may occur
14.1: The Sixth Extinction
• Vocabulary
– Extinction: complete loss of a species
– Biodiversity: entire variety of living organisms
– Endangered Species Act: law passed in 1973
with the purpose of protecting and
encouraging the population growth of
threatened and endangered species
Endangered Species Act (ESA)
• Written and passed because humans seemed
to be triggering the rapid rate at which species
were being lost
–
Ex: passenger pigeon, whooping crane, bald eagles,
elephant seals
• Critics of ESA say that saving all species is
unrealistic goal because extinction is a natural
process
–
The 10 million species living today represent only
1% of all species that have ever existed
Measuring Extinction Rates
• Earth’s history highlighted by 5 mass extinctions thus far
– Mass extinction: period of a few thousand or few
hundred years in which 50%-90% of living species
lost
From http://www.jkaneart.com/massextinctiongraph.htm
Causes of Mass Extinction
• First five thought to be caused by natural
global changes: changes in sea level and
climate fluctuations, changes in ocean and
land forms—continental drift, asteroid
impact
• The sixth mass extinction (that scientists
currently believe we are experiencing) is
being caused by human activity
Human causes of extinction:
• Habitat Destruction
• Habitat Fragmentation
• Introduced Species
• Overexploitation
• Pollution