Download Population and Growth Patterns

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text pages
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440–444
36 39
436–439
SECTION
and
14.4 Population
Growth Patterns
SC.912.L.17.5
KEY CONCEPT Populations grow in predictable patterns.
Changes in a population’s size are determined by
immigration, births, emigration, and deaths.
The size of a population is usually changing. If there are plenty of
resources such as food and water, a population may grow. If there are
few resources, the population may decrease in size. Four factors affect
the size of a population: immigration, births, emigration, and deaths.
• Immigration is the movement of individuals into a population from
another population.
• Births increase the number of individuals in a population.
• Emigration is the movement of individuals out of a population and
into another population.
• Deaths decrease the size of a population.
Circle two factors that increase a population’s size.
Population growth is based on available resources.
The rate of growth for a population depends on the resources available.
Two different types of population growth are exponential growth and
logistic growth.
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LOGISTIC GROWTH
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EXPONENTIAL GROWTH
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Exponential growth occurs when a
Logistic growth begins with a period
population size increases greatly over a
period of time. This type of population
growth occurs when there are plenty
of resources available.
of slow growth, followed by exponential growth. As the population grows,
resources become more and more limited. The population eventually levels off
at a size the environment can support.
McDougal Littell Biology
The carrying capacity of an environment is the maximum number
of individuals of a particular species that the environment can support.
If the environment changes, the carrying capacity can change, too. For
example, a disease might destroy a population’s main food source. This
situation may cause the population to crash. A population crash is a
large decrease in the size of a population over a short time period.
If there
are unlimited
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If there are
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growth is likely to occur?
Ecological factors limit population growth.
Many factors affect the carrying capacity of an environment for a population. The factor that has the greatest effect on limiting population
growth is called the limiting factor. For example, if a lack of space is the
biggest factor limiting population growth, then space would be the limiting factor. Two main categories of limiting factors are density-dependent
factors and density-independent factors.
Density-Dependent Limiting Factors
Visual Connection
Density-dependent limiting factors are limiting factors that are affected
by the population density, or the number of individuals living in a
given area.
• Competition The more dense the population is, the greater the
competition among individuals is for resources such as food.
• Predation The density of the prey population affects the number of
predators that can survive. If there is an increase in the number of
prey, then the area can support more predators. If there is a decrease
in the number of prey, then the number of predators will drop, too.
• Parasitism and disease Parasites and disease spread more quickly
through dense, or crowded, populations. This can result in a decrease
in the population size.
See graph in student
text, pg. 443
Density-Independent Limiting Factors
Density-independent limiting factors are limiting factors that affect the
size of any population, no matter what density of individuals live in
an area.
• Unusual weather Weather can affect the size of a population regardless of its density.
• Natural disasters Volcanoes, hurricanes, and other natural disasters
can wipe out populations regardless of density.
Interactive Reader
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• Human activities Destruction of habitat, pollution, and the introduction of nonnative species are all human activities that affect
population size regardless of density.
What is the difference between a density-dependent and a
density-independent limiting factor?
14.4
Vocabulary Check
immigration
emigration
exponential growth
logistic growth
carrying capacity
Mark It Up
Go back and highlight
population crash
each sentence that
limiting factor
has a vocabulary
density-dependent limiting factor
word in bold.
density-independent limiting factor
Circle the correct term from each pair to complete the sentences below.
1. Immigration / emigration is the movement of organisms out of one
population into another population.
2. A limiting factor that does not depend on the number of individuals in
an area is a density-dependent / density-independent limiting factor.
3. A type of population growth that begins slowly, then grows quickly,
and finally levels off is exponential growth / logistic growth.
4. If the carrying capacity of an environment drops suddenly, it can result
in a population crash / limiting factor.
14.4
The Big Picture
5. List two factors that increase a population’s size and two factors that
decrease a population’s size.
6. How does the availability of resources affect whether a population’s
growth is exponential or logistic?
7. Give one example of a density-dependent limiting factor and one
example of a density-independent limiting factor.
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McDougal Littell Biology