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Populations in Ecosystems
•Growth of Populations
•Long-Term Survival of Species
•Impact of Environmental Change
•Populations Respond to External Factors
A group of the same kind of
organism that lives in the same
place is called a ….
POPULATION
CHARACTERISTICS OF POPULATIONS
Three important characteristics of a population
are its:
• Geographic distribution or range.
• Population density – the number of
individuals per unit area.
• Growth rate
Density
• Number of individuals per unit area
• Varies depending on species and its ecosystem
US Population Density 1990
High or Low Density?
High or Low Density?
POPULATION GROWTH
Three factors can affect population size:
• The number of births.
• The number of deaths.
• The number of individuals that enter or leave
the population.
A population can grow when
its birthrate is greater than
its death rate.
Exponential Growth
• Under ideal conditions with unlimited resources, a
population will grow exponentially.
• Exponential growth occurs when the individuals in a
population reproduce at a constant rate.
• The population becomes LARGER and LARGER
until it approaches an infinitely large size.
Turn to the person next to you and explain how
the two graphs are similar OR different?
Exponential Growth
Do populations really grow
EXPONENTIALLY??
Can populations continue to grow
exponentially forever?
What do you think?
Answer…..is….NO!!
• As resources become limited (limiting
resource), the growth of a population slows or
stops.
• The result looks more like
the S-Shaped graph 
Logistic Growth
Carrying Capacity
• The environment
can only HOLD or
CARRY so many
individuals
• Largest number of
individuals that a
given environment
can support =
Carrying Capacity
What Limits Growth of a
Population?
LIMITING FACTORS
Factor that causes population growth
to DECREASE
Density Dependent
Limiting Factors
• Density-dependent factors operate only when a
population is large and dense.
• Examples:
•
•
•
•
competition
predation
parasitism
disease
Density Independent
Limiting Factors
• Density-independent affect all populations in a
given area in similar ways, regardless of the
population size.
• Examples:
• unusual weather like droughts
• natural disasters like hurricanes, forest fire
• seasonal cycles
• certain human activities
What is Environmental Stability?
How do ecosystems maintain
stability despite constant limiting
factors?
What is ecosystem stability?
• The vast majority of natural ecosystems
experience regular environmental
change, or disturbances.
• Most ecologists describe ecosystem
stability as the ability of an ecosystem
to maintain its structure and function
over long periods of time despite
disturbances.
How do Ecosystems maintain
stability with all of these limiting
factors?
Biodiversity
• The sum total of the genetically based variety of all
the organisms in the biosphere is called biodiversity.
• Biodiversity is one of
our greatest natural
resources. Species of
many kinds have
provided us with
many foods, industrial
products, and
medicines.
Environmental Adaptations
• Populations have become adapted to live in
specific conditions as a result of their
environment.
• For Example:
– Desert cacti
– Palm trees
– Snow Owls
What threatens BIODIVERSITY and
ECOSYSTEM STABILITY?
Threats to Biodiversity
• Human activities can cause decreases in the
amount of biodiversity by:
– Habitat alteration
-- Pollution
– Improper hunting
--Introduced species
Introduced Species
• Throughout history humans have
transported apparently harmless
plants or animals which have
become invasive in a new habitat.
• Invasive species (also called exotic
species or nonnative species) are
those that rapidly increase their
populations due to lack of natural
predators and/or parasites
thereby giving it an advantage
over native species.
ASIAN CARP INVASION!!!!!!
http://abcnews.go.com/Nightline/video/beware-asian-carpinvasion-12329740
How does environmental change
affect ecosystem stability?
• Changing environmental conditions can
cause the decline of local biodiversity.
• Ecosystems that are less stable may not be
able to respond to a normal
environmental disturbance.
How does natural environmental
change affect ecosystem stability?
• Fires, heavy storms, and natural climate
change can cause major changes in local
populations of plants and animals.
• A decline in natural biodiversity can make
an ecosystem less stable.
Something to think about for the
future….
• How will the Earth’s increasing average
temperatures affect ecosystem structures
and functions?
• Scientists are not yet sure how predicted
changes in global climate within the next
several decades will affect ecosystem
stability worldwide.