Download 5–2 Limits to Growth

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Section 5–2
Page 124
5–2 Limits to Growth
BI 6.c. Students know how fluctuations in population size in an ecosystem are determined by the relative rates of birth,
immigration, emigration, and death.
1 FOCUS
N
Objectives
5.2.1 Identify factors that limit
population growth.
5.2.2 Differentiate between
density-dependent and
density-independent limiting
factors.
Vocabulary Preview
To prepare students for the new
Vocabulary terms density-dependent
and density-independent, ask them
to recall the definition of population
density that they learned in Section
5–1. (The number of individuals per
unit of area)
Key Concept
• What factors limit population
growth?
Vocabulary
limiting factor
density-dependent limiting
factor
predator-prey relationship
density-independent limiting
factor
ow that you know a few things about population growth,
think again about the sea otter example in the beginning of
the previous section. When a sea otter population declines,
something has changed the relationship between the birthrate
and the death rate, or between the rates of immigration and
emigration. For instance, in part of the sea otter’s range, the
death rate of sea otters is increasing because killer whales are
eating the otters. Predation by killer whales creates a situation
that reduces the growth of the sea otter population.
Limiting Factors
Reading Strategy:
Predicting Before you read,
preview the diagram below.
Predict how each factor might
limit the growth of a population.
As you read, note whether your
predictions were correct.
Recall from Chapter 3 that the primary productivity of an
ecosystem can be reduced when there is an insufficient supply
of a particular nutrient. Ecologists call such substances limiting
nutrients. A limiting nutrient is an example of a more general
ecological concept: a limiting factor. In the context of populations, a limiting factor is a factor that causes population
growth to decrease. Some of the limiting factors that can affect a
population are shown in Figure 5–5.
Reading Strategy
Suggest that students record their
predictions in writing, leaving space
below each one to note whether the
prediction was correct or incorrect. In
the case of incorrect predictions,
have students correctly note how
those limiting factors affect a population’s growth.
왔 Figure 5–5 Many different factors can limit
population growth. Some of these factors are
shown below. Inferring How might each of these
factors increase the death rate in a population?
2 INSTRUCT
Limiting Factors
Use Visuals
Figure 5–5 For each of the limiting
factors shown in the diagram, have
students suggest examples that are
already familiar to them—for example, weeds and crop plants
competing for light, space, and nutrients in a vegetable garden; the
predator-prey relationship of a toad
eating a moth; and so on. As students cite examples, have a volunteer
list them on a large sheet of paper.
Save the list for use again later in
Evaluate Understanding, page 127.
SECTION RESOURCES
Print:
Tim
124
Chapter 5
r
• Teaching Resources, Lesson Plan 5–2,
Adapted Section Summary 5–2, Adapted
ve
Worksheets 5–2, Section SummarySa5–2,
e
Worksheets 5–2, Section Review 5–2,
Enrichment
• Reading and Study Workbook A, Section 5–2
• Adapted Reading and Study Workbook B,
Section 5–2
• Issues and Decision Making, Issues and
Decisions 48
Technology:
• iText, Section 5–2
• Transparencies Plus, Section 5–2
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Page 125
Density-Dependent
Factors
왘 Figure 5– 6 The panda is one of the most
critically endangered species in the world today.
Populations are declining, in large part because
pandas depend on bamboo for food, which grows
only in certain forests because of habitat destruction.
Inferring How might the panda population be saved?
A resource base that is limited can also affect the long-term
survival of a species. As shown in Figure 5–6, pandas depend on
bamboo for food. Bamboo grows in certain kinds of temperate
forests in China. Since the time that these forests have been
cleared for timber and farmland, panda populations have fallen
dramatically and have become isolated in small pockets of
remaining forest.
Density-Dependent Factors
A limiting factor that depends on population size is called a
density-dependent limiting factor. Density-dependent
factors become limiting only when the population density—the
number of organisms per unit area—reaches a certain level.
These factors operate most strongly when a population is large
and dense. They do not affect small, scattered populations as
greatly.
Density-dependent limiting factors include
competition, predation, parasitism, and disease.
Competition When populations become crowded, organisms
compete with one another for food, water, space, sunlight, and
other essentials. For example, puffins must compete for limited
nesting sites. Competition among members of the same species
is a density-dependent limiting factor. The more individuals
living in an area, the sooner they use up the available resources.
Likewise, the fewer the number of individuals, the more
resources are available to them and the less they must compete
with one another.
Competition can also occur between members of different
species. This type of competition is a major force behind evolutionary change. When two species compete for the same resources,
both species are under pressure to change in ways that decrease
their competition. Over time, the species may evolve to occupy
separate niches. That is because, as you may recall, no two species
can occupy the same niche in the same place at the same time.
How does competition affect growth?
Materials bean seeds, 2 paper
cups, potting soil
Procedure
1. Label two paper cups 3 and 15.
Use a pencil to make several
holes in the bottom of each
paper cup. Fill each paper cup
two-thirds full with potting soil.
Plant 3 bean seeds in cup 3, and
plant 15 bean seeds in cup 15.
2. Water both cups so that the soil
is moist but not wet. Put them
in a location that receives bright
indirect light. Water the cups
equally as needed.
3. Count the seedlings every other
day for 2 weeks. CAUTION:
Wash your hands with soap and
warm water before leaving the lab.
Objective Students will be able to
determine that crowding is a limiting factor in plant growth.
Skill Focus Observing
Materials bean seeds, 2 paper
cups, potting soil
Time 15 minutes for initial setup,
followed by observation and recording for 2 weeks
Strategy Remind students that the
only variable that should be different between the two cups is the
number of seeds planted. Ask: What
variables should you keep the
same in both cups? (The depth at
which the seeds are planted, the
amount of water the cups are given,
the amount of sunlight the cups
receive, temperature)
Expected Outcome The
uncrowded seedlings will thrive. The
crowded seedlings will show limited
growth, and some may die.
Analyze and Conclude The
seedlings in cup 15 will be smaller
and less robust than those in cup 3,
and some may die.
Analyze and Conclude
Observing What differences did
you observe between the two cups?
What is a density-dependent limiting factor?
Answers to . . .
A factor that limits
population growth only when the population’s density reaches a certain level
UNIVERSAL ACCESS
Less Proficient Readers
Have students make a compare/contrast table of the
limiting factors discussed in the
section. Column heads might
include Factor, Definition, and
Example. In the factor column,
students should use subheads to
divide the limiting factors into
density-dependent and densityindependent factors.
English Language Learners
Have students pronounce the
word dependent, and then discuss common usages of the
word. Explain that the prefix inmeans “not,” and thus the word
independent means “not
dependent.” This section introduces factors that do and do not
depend upon the density of a
population.
Advanced Learners
Encourage interested students to
further research the relationship
between predator and prey on
Isle Royale, as mentioned on
page 126. These populations of
wolves and moose have been
studied extensively, and students
should readily find relevant
resources. Have them report
their research to the class.
Figure 5–5 Each of the factors that
limit population growth could cause
deaths of individuals. Accept all reasonable responses.
Figure 5– 6 The panda population
might be saved if the habitat on which
they depend—bamboo in certain
forests—is prevented from being
destroyed or is restored in areas that
had been cleared.
Populations
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Wolf and Moose Populations on Isle Royale
5–2 (continued)
Figure 5–7 Make sure students
understand that two separate sets of
data are plotted on the graph: the
blue line represents the numbers of
wolves labeled on the graph’s left
vertical axis, and the red line represents the numbers of moose labeled
on the right vertical axis.
Number of Wolves
Use Visuals
60
2400
50
2000
40
1600
30
1200
20
800
10
400
0
1955
0
1960
1965
1970
Make Connections
Mathematics Present the following
math problems: In order to survive, a
50-kilogram wolf needs to eat about
2700 kilograms of moose per year.
The average mass of a moose is
about 385 kilograms; males have
more mass, females less. Ask: How
many “average” moose does a wolf
need to eat each year? (About 7) If
there are 8 wolves in a pack, how
many moose does the pack need to
eat each year? (About 56)
왖 Figure 5–7 The relationship between
moose and wolves on Isle Royale illustrates
how predation can affect population
growth. In this example, the moose
population was also affected by changes in
food supply, and the wolf population was
also affected by disease. Interpreting
Graphics How are the increases and
decreases in the moose population related
to the changes in the wolf population?
1985
1990
1995
Wolves
Predation Populations in nature are often controlled
by predation. The regulation of a population by predation
takes place within a predator-prey relationship, one of
the best-known mechanisms of population control. The
relationships between sea otters and sea urchins and
between sea otters and killer whales are examples of
predator-prey interactions that affect population growth.
A well-documented example of a predator-prey relationship is the interaction between wolves and moose on
Isle Royale, an island in Lake Superior. The graph in
Figure 5–7 shows how periodic increases in the moose
population—the prey—on Isle Royale are quickly followed
by increases in the wolf population—the predators. As the
wolves prey on the moose, the moose population falls. The
decline in the moose population is followed, sooner or
later, by a decline in the wolf population because there is
less for the wolves to feed upon. A decline in the wolf
population means that the moose have fewer enemies, so
the moose population rises again. This cycle of predator
and prey populations can be repeated indefinitely.
growth of a population. Parasitic organisms range in size
from microscopic, disease-causing bacteria to tapeworms
30 centimeters or more in length. These organisms are
similar to predators in many ways. Like predators, parasites take nourishment at the expense of their hosts, often
weakening them and causing disease or death. The wasp
cocoons in Figure 5–8, for example, can weaken or kill
many caterpillars.
Build Science Skills
Chapter 5
1980
Parasitism and Disease Parasites can also limit the
Density-Independent
Factors
126
1975
Year
Moose
For: Population Dynamics
activity
Visit: PHSchool.com
Web Code: cbe-2059
Students can interact with the
art online.
Applying Concepts Ask: Does the
graph in Figure 5–7 show a crash
in either population? (Yes; the wolf
population from 1980 to 1982 and the
moose population from 1995 to 1996)
Read the caption and then ask: What
combination of density-dependent
and density-independent factors
may have caused the crash in the
wolf population? (Decline in the
moose population; unusually deep winter snows could have made it difficult
for the wolves to hunt; parasites or disease could have weakened or killed the
wolves.)
Number of Moose
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Chapter 5
왗 Figure 5–8 This larval sphinx moth has been attacked by a
parasitic wasp. The wasp inserted its eggs beneath the moth’s
skin. After hatching, the wasp larvae fed on their host internally
until they appeared as white cocoons on its back. Predicting
How might the wasp larvae affect the sphinx moth population?
TEACHER TO TEACHER
Use a short video clip of a forest fire, such as the
National Park Service’s video of fires in
Yellowstone Park. Then, read one or two actual
news articles that describe the fire and its effects.
Divide the class into groups of three or four,
and assign each group the task of developing a
management plan to study the ecological damage caused by the fire and to help reestablish
plant and animal populations in the burned area.
Students should research the following information: the types of populations living in the area
before and after the fire; the initial and current
size of each population; resources now available
to the populations and resources no longer available; and possible steps to help reintroduce
populations. Give each group an opportunity to
explain its plan to the class.
—Brenda Waldon
Biology Teacher
Clayton County Public Schools
Morrow, Georgia
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3 ASSESS
Density-Independent Factors
Evaluate Understanding
Density-independent limiting factors affect all
populations in similar ways, regardless of the population
size.
Unusual weather, natural disasters, seasonal cycles, and certain human activities—such as
damming rivers and clear-cutting forests—are all
examples of density-independent limiting factors. In
response to such factors, many species show a characteristic crash in population size. After the crash, the population
may soon build up again, or it may stay low for some time.
For some species, storms or hurricanes can nearly
extinguish a population. For example, thrips, aphids, and
other insects that feed on plant buds and leaves might be
washed out by a heavy rainstorm. Extremes of cold or hot
weather also can take their toll on a population, regardless of the population’s density. A severe winter frost, for
example, can kill giant saguaro cactuses in the Arizona
desert. In some areas, periodic droughts can affect entire
populations of vegetation, as shown in Figure 5–9. Such
events can, in turn, affect the populations of consumers
within the food web.
Environments are always changing, and most populations can adapt to a certain amount of change.
Populations often grow and shrink in response to
such changes. Major upsets in an ecosystem,
however, can lead to long-term declines in certain
populations. Human activities have caused some
of these major upsets, as you will soon read.
Display the list of examples that the
class created at the beginning of this
section (Use Visuals, page 124). Call
on students at random to identify
each example as density-dependent
or density-independent.
Reteach
Have students work in groups of
three, with each student responsible
for writing a brief description of how
one of the three density-dependent
limiting factors discussed in the text
can limit a population’s growth. Let
the group members share their
descriptions and offer corrections
and improvements.
왘
Figure 5–9 A drought can result in the
abrupt decrease of a population, regardless of
its size.
Droughts and other natural
disasters are density-independent limiting
factors.
5–2 Section Assessment
1.
Key Concept List three
density-dependent factors and
three density-independent
factors that can limit the growth
of a population.
2. What is the relationship between
competition and population size?
3. If an entire lynx population
disappears, what is likely to
happen to the hare population
on which it preys?
4. Identify how a limited resource
can affect the size of a population. Give an example that
illustrates this situation.
5. Critical Thinking Applying
Concepts Give an example of
a density-independent limiting
factor that has affected a human
population. Describe how this
factor changed the human
population.
Interdependence in
Nature Study the factors
that limit population growth
as shown in Figure 5–5.
Classify each factor as either
biotic or abiotic. Refer to the
information on biotic and abiotic factors in Section 4–2.
Density-dependent limiting factors, such as competition,
predation, parasitism, and disease,
can be classified as biotic factors.
Density-independent limiting factors, such as drought and other
climate extremes as well as human
disturbance to ecosystems, can be
classified as abiotic factors. Some
students may argue that human
disturbance should be considered
a biotic factor for the reason that
humans are organisms interacting
with other organisms. At the same
time, however, human disturbances such as building roads,
filling wetlands, or clearing forests
cause large-scale changes in the
physical environment that should
be considered abiotic factors.
If your class subscribes to the
iText, use it to review the Key
Concepts in Section 5–2.
5–2 Section Assessment
1. Density-dependent: competition, predation,
parasitism and disease; density-independent:
unusual weather, natural disasters, seasonal
cycles, human activities
2. When populations become larger and more
crowded, organisms must compete with one
another for food, water, space, sunlight, and
other essential resources.
3. The hare population would probably undergo explosive growth.
4. Accept all reasonable responses. Students
might mention any of the densityindependent factors as limiting a resource. A
limited resource limits the size of a population.
5. Accept all reasonable responses. Sample
answer: A prolonged drought, with its associated crop loss, could cause deaths,
financial hardship, and emigration to other
countries.
Answers to . . .
Figure 5–7 As the moose population
increased, the wolf population
increased. Decreases in the moose population were followed by decreases in
the wolf population.
Figure 5–8 Predation by wasp larvae
would reduce the growth rate of the
sphinx moth population.
Populations
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Does the Gray Wolf Population
Need Protection?
Encourage students to research current nature publications to gather
more detailed information on both
sides of this issue. If students live in
an area where ranchers or farmers
suffer losses due to wolf predation,
suggest that they interview affected
people and research local newspaper
articles to learn more.
You may wish to let students work
in several small groups, with some
groups representing ranchers and
others representing conservationists
and other stakeholders. Give the
groups time to discuss their points of
view and their reasoning. Then, hold
a mock meeting in which each side
presents its view to the “government
official.” (You may want to role-play
the official yourself.)
Research and Decide
1. Students’ lists will vary and may
include researched information as
well as information included in this
feature.
2. Accept opinions on both sides of
the issue so long as students defend
their choices with reasonable explanations.
Students can research gray wolf
protection on the site developed
by authors Ken Miller and Joe
Levine.
W
olves were once widely distributed around
the world, occupying almost every habitat
except tropical jungles. Today, however, wolves
occupy only a fraction of their former range. In
1973, the Endangered Species Act was passed by
the U.S. Congress to protect declining populations
of gray wolves from becoming extinct. At the time,
there were only about 400 wolves in the lower 48
states. By 2002, the population had swelled to an
estimated 4000 individuals scattered mostly
throughout the Rocky Mountains and Great Lakes
areas.
Classifying the status of animals is a judgment
call. In some cases, the judgment is easy. For
instance, the California condor population now
includes only a few remaining members and is
clearly in great danger. With other species, such as
the gray wolf, the situation is much more complex.
How should the gray wolf be classified—and therefore managed—in the United States?
The Viewpoints
Ranchers are concerned that, at the current growth
rate, wolves will encroach on their livestock. Many
feel strongly that landowners should have the right
to protect themselves from potential losses. The protection of wolves currently costs the U.S. government over $200,000 per year. If the wolves could be
legally hunted and trapped, the money that would
be saved could be used to help protect other, more
endangered species.
Research and Decide
1. Analyzing the Viewpoints To make an
informed decision, learn more about this issue
by consulting library or Internet resources. List
the pros and cons of each option as they relate
to both humans and wolves. Consider the different perspectives of landowners, conservationists, and other interested groups.
2. Forming Your Opinion Decide whether the
federal government should change the status of
the gray wolf. Write a persuasive statement to
support your decision.
Keep the Endangered Classification
People who want to keep the gray wolf’s status as an
endangered species say that most of its former habitat in the 48 contiguous states is unsuitable because
of human encroachment. Proponents of this view cite
the fact that only after gray wolves were given protection under the Endangered Species Act did the
wolf population in the United States begin to
increase. There is concern that persecution by people
and loss of habitat will confine gray wolves to more
remote areas, or reduce their habitat even further,
unless federal protection continues.
Reclassify the Wolf and Remove
Federal Protection
Opponents of the endangered species classification
counter that in states like Minnesota, the gray wolf
population is growing at a rate of 4 to 5 percent
each year. These people are confident that, because
the populations are increasing at a healthy rate,
the wolves no longer need federal protection.
For: Links from
the authors
Visit: PHSchool.com
Web Code: cbe-2052
BACKGROUND
The gray wolf profiled
The gray wolf is a subspecies of Canis lupus and a
member of the dog family, Canidae. Adult gray
wolves are 1.5–1.8 meters long and stand 66–81
centimeters at the shoulder. Adult males average
31.8–45.4 kilograms; adult females average
24.9–38.6 kilograms. Wolves have long legs, and
their bodies are suited for traveling great distances. Wolves prey on everything from large
ungulates, such as moose and elk, to small
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Chapter 5
rodents, such as field mice. Although wolves were
once common in what is now the United States,
populations of gray wolves are now found only in
Alaska, Upper Michigan, Wisconsin, Minnesota,
Wyoming, Montana, Idaho, and Washington
State. Their decline was a result of loss of habitat,
loss of prey, and hunting by humans. Bounties on
wolves were once common. Wolf recovery in the
United States is primarily the result of the
Endangered Species Act.