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Population Ecology
Population Characteristics
Population Density:
–
The number of organisms per
unit area
Spatial Distribution:
–
–
Dispersion: The pattern of
spacing a population within an
area
3 main types of dispersion
•
•
•
–
Clumped
Uniform/Even
Random
The primary cause of
dispersion is resource
availability
• Aside from the examples listed, give me
an example of each of the types of
dispersion
–Clumped
–Uniform
–Random
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Population Limiting Factors
Population growth rate
– How fast a given population grows
– Factors that influence this are:
•
•
•
•
Emigration: the number of individuals moving out of a
population
Immigration: the number of individuals into a
population
Natality: Birth rate
Mortality: Death rate
• Give me a factor that influences population
growth and explain what that factor is.
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Population Limiting Factors
• Density-independent factors
– Factors that limit population size, regardless
of population density.
– These are usually abiotic (non-living) factors
– They include natural phenomena, such as
weather events
• Drought, flooding, extreme
heat or cold, tornadoes,
hurricanes, fires, etc.
• Give some examples of density-independent
factors.
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Population Limiting Factors
• Density-dependent factors
– Any factor in the environment that depends on the
number of members in a population per unit area
– Usually biotic factors
– These include
•
•
•
•
Predation
Disease
Parasites
Competition
• What are some differences between densitydependent and density-independent factors?
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Understanding Exponentials
• Think about this:
– An employer offers you two equal jobs for one hour
each day for fourteen days.
– The first pays $10 an hour.
– The second pays only 1 cent a day, but the rate
doubles each day.
– Which job will you accept?
Population Ecology: Population Growth Rate
Understanding Exponentials
Now, how much would
your employer owe you
if you stayed at this job
for another 2 weeks?
Job 2 lags for a long
time before
exponential growth
kicks in!
What would happen if this
type of growth took place
within a population?
Population Limiting Factors
• Population growth models
– Exponential growth model
• Also called geometric growth or J-shaped growth.
– Limits to exponential growth
• Population Density (the number of individuals per unit of
land area or water volume) increases as well
• Competition follows as nutrients and resources are used
up
– The limit to population size that a particular
environment can support is called carrying capacity
• What are some limits to population growth?
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What population do you think this is?
So, what do you think is going to
happen to the human population?
• We will probably reach our carrying capacity.
• Our growth rate will start to look like most
organisms, which is the Logistic Growth Model
Population Limiting Factors
• Population growth models
– Logistic Growth Model
• Often called the S-shaped growth curve
• Occurs when a population’s growth slows or stops
following exponential growth.
• Growth stops at the population’s carrying capacity
• Populations stop increasing when:
– Birth rate is less than death rate
(Birth rate < Death rate)
– Emigration exceeds Immigration
(Emigration > Immigration)
• What causes the logistic growth model to form?
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Population Limiting Factors
•
Population growth models
– Logistic Growth Model
The S-curve is not as pretty as the image looks
1. Carrying capacity can be raised or lowered. How?
Example 1: Artificial fertilizers have raised k
Example 2: Decreased habitat can lower k
2. Populations don’t reach k as smoothly as in the logistic
graph.
•
•
Boom-and-Bust Cycles
Predator-Prey Cycles
Population Limiting Factors
•
Species that follow this
pattern usually
1. Have a Boom-and-Bust Life
Cycle
2. Have short life spans
3. Produce many offspring
4. Smaller organisms
•
i.e., fruit flies, mice, locusts
5. Don’t maintain a population
near carrying capacity
6. Controlled by densityindependent factors
•
They are called r-strategists
(for rate of increase)
• What animals are r-strategists?
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Population Limiting Factors
•
Species that follow this
pattern usually
1. Have long life spans
2. Produce few offspring that
have a better chance of
living to a reproductive
age
3. Are larger organisms
•
i.e., elephants
4. Maintain a population at
or near k
5. Controlled by densitydependent factors
•
They are called kstrategists (for karrying
kapacity)
• What type of animals are k-strategists?
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