Transcript Populations

Populations
5-1
Characteristics of Populations
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Three important characteristics of a population
are its geographic distribution, density, and
growth rate.
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Geographic distribution describes the area a
population lives in
Population density is the number of individuals
that live in an area
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Population Growth
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Three factors affect population size:
 The
number of births
 The number of deaths
 The number of individuals that enter or
leave the population
Population growth
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Populations grow if more individuals are born
than die in any period of time
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Immigration - the movement of individuals into
an area, can cause a population to grow
Emigration - the movement individuals out of a
population, can cause a population to decrease
in size
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Exponential Growth
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If a population has unlimited space and food,
and is protected from predators and disease,
then the population size will increase
exponentially
Exponential growth occurs when the individuals
in a population reproduce at a constant rate.
Under ideal conditions with unlimited resources,
a population will grow exponentially
Exponential Growth
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Bacteria reproduce by splitting in half, if the bacteria
have a doubling time of 20 minutes, then within 20
minutes the 1st bacterium with divide to produce 2
bacteria.
After 20 more minutes the 2 bacteria divide to form 4.
After 20 minutes 4 becomes 8, in one hour there is 64,
in two more hours there is 512, in just one day the
colony of bacteria will be
4,720,000,000,000,000,000,000 !!!!!!
Exponential Growth
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A graph of exponential growth forms a J-shaped
curve
Logistic Growth
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Obviously exponential growth can not continue
for very long.
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As resources become less available, the growth
of a population slows or stops.
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Logistic growth occurs when a population’s
growth slows or stops following a period of
exponential growth
Logistic Growth
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Population growth may slow or stop when
birthrates and death rates occur at the same rate.
Population growth will slow when immigration
decreases, and emigration increases, or both.
Carrying capacity is the largest number of
individuals of a population that a given
environment can support
Logistic Growth
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A graph of logistic growth forms an S-shaped
curve.
5-2
Limits to Growth
 Limiting
Factors
 Density-Dependent Factors
 Density-Independent Factors
Limiting Factor

Any factor that caused population growth to
decrease.
Competition
 Predation
 Parasitism and disease
 Drought and other climate extremes
 Human disturbances
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Density-dependent factors
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Density-dependent factors are limiting factors
that depends on population size.
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The larger the population the more it is affected
Examples: competition, predation, parasitism
and disease
Predator-Prey Relationship
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The regulation of a population by predation
Density-independent factors
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Density-independent factors affect all
populations in similar ways, no matter what the
size is
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Examples: weather, natural disasters, seasonal
cycles and certain human activities
5-3
Human Population Growth
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Just like the populations of many other
organisms, the human population is increasing
with time
Demography
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Demography is the study of human populations
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Birthrates, death rates, and the age structure of a
population help predict why some countries
have high growth rates while other countries
grow more slowly
Demographic transition
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Demographic transition a dramatic change in
birth and death rates which slows a countries
growth rate
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Age structure diagrams are models used to
predict future growth of countries
Age structure diagram
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Which countries has gone through demographic
transition?