Download Population Growth

LESSON
3
Population Growth
Guiding Question: What factors determine whether, and how, a
population’s size changes?
• Describe the factors that influence a population’s
growth rate.
• Explain exponential growth and logistic growth.
• Explain how limiting factors and biotic potential
affect population growth.
Reading Strategy Create a KWL chart for each of the headings in this lesson. Before you read, fill in what you know and
what you want to know. After reading, fill in what you learned.
Vocabulary survivorship curve, immigration, emigration,
migration, exponential growth, limiting factor, carrying
capacity, logistic growth, density-dependent factor,
density-independent factor, biotic potential
Why are there so many bacteria and so few
4.3 LESSON PLAN PREVIEW
Inquiry Model the effects of
different factors that determine
population growth.
Real World Apply the concept
of limiting factors to a classroom environment.
Differentiated Instruction Support less proficient readers
by guiding them to use headings to find information.
whales? Place a few bacteria on a nutrient-rich petri dish, and a few
hours later, each will have generated a colony of millions. But place
a few whales in the middle of the ocean, and it takes a year or more
for any new whales to be born. Why the difference?
4.3 RESOURCES
In Your Neighborhood Activity,
Migrating Populations • Scientific
Method Lab, Yeast Population Growth
• Real Data Online • Real Data Math
Worksheet • Lesson 4.3 Worksheets
• Lesson 4.3 Assessment • Chapter 4
Overview Presentation
GUIDING QUESTION
FOCUS Name several factors that
can affect a population’s growth,
such as disease, unusually large
numbers of births, and limited food
resources. Have students predict
how each of these factors might
affect a population’s growth.
110 Lesson 3
Proteus bacteria colonies and two
humpback whales
Factors That Determine
Population Growth
A population’s growth rate is determined by births,
deaths, immigration, and emigration.
In the simplest terms, populations increase in size when more
individuals enter the population than leave it. Likewise, populations
decrease in size when more individuals leave it than enter it. Two
sets of factors influence the ratio of individuals entering and leaving
a population: births and deaths, and immigration and emigration.
Birth- and Death Rates Population size, density,
distribution, sex ratio, and age structure can all influence the rates at which individuals within a population are born and die. The rate at which individuals are
born is called natality. The rate at which individuals die
is called mortality. Natality and mortality are usually
expressed as the number of births or deaths per 1000
individuals over a given time. All else being equal, when
a population’s birthrate is greater than its death rate, the
population size increases. When its death rate is greater
than its birthrate, the population size decreases.
Individuals of different ages
have different probabilites of dying. To show how the
likelihood of death varies with age, population ecologists use graphs called survivorship curves. Figure 8
shows the three basic types.
Organisms with a type I curve, such as humans,
have higher mortality at older ages. Most individuals survive at young ages, and the likelihood of dying
increases with age. If you were to follow a thousand
10-year-old and 80-year-old humans for a year, you
would find that at year’s end more 80-year-olds had
died than 10-year-olds.
Amphibians such as the golden toad show a different survivorship pattern. Golden toads produced
large numbers of young that suffered high death
rates. This pattern, in which death is less likely (and
survival more likely) at an older age than at a very
young age, defines a type III survivorship curve. A
type II survivorship curve indicates a population with
equal mortality at all ages. Many bird species have type
II curves.
▶ Survivorship Curves Recall
that a population’s age structure describes the relative
number of individuals within various age groups. The
age structure of a population influences how relative
birthrates and death rates affect its size. Consider a
population following a type I survivorship curve. If the
population is made up of mostly young, reproductive
or pre-reproductive individuals, there will likely be
more births than deaths, and the population size will
increase. In a population of mostly older individuals,
there are likely to be more deaths than births, and the
population size will decrease. A population with an
even age distribution will likely remain stable.
▶ Age Structure and Population Growth Reading
Checkpoint
hich type of survivorship curve describes
W
populations whose mortality is highest at
young ages?
Survivorship Curves
Number of Survivors
Type I
Type II
Type III
Young
Old
Age
Figure 8 Survivorship Curves In a type I
survivorship curve, individuals are most likely to
die when they are old. In a type III survivorship
curve, mortality is highest for young members
of the population. In a type II survivorship curve,
mortality remains constant throughout an
individual’s lifetime.
ANSWERS Reading Checkpoint Type III
Population Ecology 111
Figure 9 Seasonal Migration
and Population Size The size of
the turkey vulture population in
Pennsylvania changes over the course
of the year. Much of the change has to
do with seasonal migration into and
out of the area.
Immigration and Emigration In addition to births and deaths,
population size can also change because of individuals moving into or out
of a population. Immigration is the arrival of individuals from outside a
given area. Emigration is the departure of individuals from a given area.
Immigration and emigration can have dramatic effects on a population’s
size, especially when it comes to humans. Millions of people move around
the world each year, driven by necessity, opportunity, or a desire to make
new connections and see new places.
Sometimes, organisms make brief movements into and out of an area as
part of a seasonal routine. Migration is a seasonal movement into and out
of an area. Many animals, including fishes, mammals, and birds, migrate.
Every year, for example, the turkey vulture population near Hawk Mountain Sanctuary in Kempton, Pennsylvania, increases in late summer and
early autumn. Part of the population increase is due to local births. Most
of the increase, however, is due to birds arriving from the north as part of
their annual migration cycle. In the early winter, the entire local population leaves the area as the migration continues south. Then, in early spring,
they come back. Some stay, and some continue to breeding grounds farther
north. The cycle starts again when temperatures begin to drop.
Real Data
Turkey Vultures
1. Interpret Graphs Describe the annual trend in
turkey vulture sightings along the survey route.
2. Apply Concepts What factors might be increasing the vulture population’s size? What factors
decrease population size?
3. Infer Turkey vultures arrive from the north
onto sanctuary lands and reside there for a
while before migrating south. When do you
think the vultures from the north arrive? When
do you think they all leave?
112 Lesson 3
Turkey Vultures Per Survey, 1992–2008
Average number
of vultures per survey
Hawk Mountain Sanctuary in Kempton, Pennsylvania, is a protected area for birds of prey. Scientists at the sanctuary monitor bird populations
by conducting roadside surveys. Scientists drive
slowly along a set route and count the birds they
spot. The graph at right shows the average number
of turkey vultures surveyed along a 48-kilometer
route near the sanctuary early and late month
throughout the year.
40
35
30
25
20
15
10
5
0
Early month
Late month
J
F
M
A
M
J J A
Month
S
O
N
D
Data from Hawk Mountain Data Archives
4. Perform Error Analysis What is one potential
source of error when conducting a roadside
survey?
Births
Deaths
Population size
increases
Immigration
Population size
decreases
Golden toad population
Emigration
How Populations Grow
BIG QUESTION
How do changes in population
size relate to environmental
conditions?
Perspective Ask students to consider
the environmental conditions that
support the exponential growth of
a single population. Then, ask them
to discuss how exponential growth
relates to resource availability. Suggest that students consider not only
the resources used by the growing population, but also how the
resources used by other populations
in the ecosystem may change.
Figure 11 Exponential Growth
Populations growing by a fixed percent
experience exponential growth. Every
incremental increase in number is
larger than the one before it. The Scots
pine population grew exponentially in
Great Britain following the last ice age.
Population Size
Exponential Growth
Time
114 Lesson 3
Populations can grow exponentially or logistically.
Growth rates tend to change depending on the resources available to the
organisms in the population. Population ecologists recognize two basic
patterns of population growth: exponential and logistic.
Exponential Growth Based on a population of 1000 individuals, if
the population grows by 10 percent per year, there will be 1100 individuals in the population next year (1000 + 100). The year after that, there will
be 1210 (1100 + 110), and then 1331 (1210 + 121) the next year. Notice
that even though the growth rate (10%) remains the same, each increase
in population size is larger than the one before it. Only 100 individuals were added in the first year. But 110 were added in Year 2, and 121
were added in Year 3. Each year, a 10% increase means more and more
individuals are added to the population. When a population increases by
a fixed percentage each year, it is said to undergo exponential growth.
Changes in population size are shown with population growth curves.
The J-shaped curve in Figure 11 shows exponential growth.
Normally, exponential growth occurs in nature only when the starting population is small and environmental conditions are ideal. Most
often, these conditions occur when organisms are introduced to a new
environment. Mold growing on a piece of bread and bacteria colonizing
a recently dead animal are examples. But species of any size may show
exponential growth under the right conditions. A population of the Scots
pine (Pinus sylvestris) grew exponentially when it began colonizing the
British Isles after the end of the last ice age. Receding glaciers had left
conditions ideal for their expansion.
Logistic Growth When it happens, exponential
Logistic Growth
growth rarely lasts long. After all, if a single species
increased exponentially for many generations, it would
take over the planet. Instead, most populations are eventually constrained by limiting factors. Limiting factors
are characteristics of the environment that limit population growth. Limiting factors determine a population’s
carrying capacity. Carrying capacity is the largest population size a given environment can sustainably support.
Logistic growth describes how a population’s initial
exponential increase is slowed and finally stopped by
limiting factors. Figure 12 shows the S shape of logistic
growth. Notice that the population size increases sharply
at first, but then begins to level off as the effects of limiting factors become stronger. Eventually, population size
stabilizes around its carrying capacity.
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row
th
ne
nti
a
Time
Figure 12 Logistic Growth The logistic growth curve
shows how population size may increase rapidly at first,
then grow more slowly, and finally stabilize at the carrying
capacity.
Population Growth in Nature The logistic curve
is a simplified model, and real populations, like those
shown in Figure 13, can behave differently. Some may
fluctuate, or cycle, indefinitely above and below the carrying capacity. Others may rise quickly, overshoot the
carrying capacity, and then crash.
Carrying capacities are not fixed. As limiting factors
in an environment change, so does its carrying capacity.
Plants in the understory of a dense forest, for example,
may be limited by the amount of sunlight available. If a
large tree dies, however, and sunlight pours in through a
gap overhead, then the carrying capacity for understory
plants in the area may increase.
Fluctuating
Rise and Crash
2500
Population size
Population size
6000
4000
2000
0
20
60
Time (day)
2000
1500
1000
500
0
1910
100
Data from Huffaker, C.B. 1958. Experimental studies on predation: Dispersion
factors and predator-prey oscillations, Hilgardia 27: 343–383.
(a)
Limiting factors:
• Water
• Space
• Food
• Predators
• Disease
Ex
po
Population Size
Carrying
capacity
Stabilized
population
size
1920
1930
1940
Time (year)
1950
Data from Scheffer, V.C. 1951. Rise and fall of a reindeer herd,
Scientific Monthly 73: 356–362.
(b)
Figure 13 Population Growth in Nature Population growth in nature
does not often follow an idealized logistic curve. (a) The population size of
some organisms, such as mites, fluctuates around its carrying capacity. (b)
Some populations grow rapidly and use resources too quickly, causing their
numbers to crash suddenly. Reindeer introduced to the Bering Sea island of
St. Paul showed this pattern.
Population Ecology 115
Limiting Factors and Biotic Potential
Limiting factors and biotic potential regulate a population’s
growth.
ANSWERS Reading Checkpoint Severe
weather can affect a population
regardless of the population’s
density.
Lesson 3 Assessment
1. –3/1000; smaller
2. During exponential growth, the
population continues to grow by
a fixed percent. Logistic growth
starts out as exponential growth,
but then limiting factors cause
the growth rate to slow down and
even out. Logistic growth is more
common in nature.
3. A limiting factor is any environmental characteristic that limits
population growth. Biotic potential is the maximum offspring an
organism can produce under ideal
conditions.
4. Answers will vary.
Figure 14 Density Dependence
Competition is a density-dependent
limiting factor. Here, great blackbacked gulls are fighting over a fish.
The more gulls, the more intense the
competition.
Limiting factors slow population growth either by decreasing birthrates
or immigration, increasing death rates or emigration, or some combination of these events. Some limiting factors have more of an effect in dense
populations. Other limiting factors affect all populations in the same way.
Density-Dependent Factors Recall that high population density
increases competition for resources such as food and water. Competition,
as shown in Figure 14, is a density-dependent factor because its influence changes with population density. The higher the population density, the less food and water will be available per individual. In turn, this
causes competition for those resources to intensify. Predation and disease
are two other examples of density-dependent factors.
Density-Independent Factors Density-independent factors are
limiting factors whose influence is not affected by population density.
Catastrophic events such as floods, fires, and landslides are considered
density-independent factors. It does not matter if the original population
was dense or not. The result is always the same: a dramatic and sudden
reduction in population size.
A change in the region’s climate brought devastation to the golden
toads of Monteverde. In the spring of 1987, unusually warm and dry
conditions caused the breeding pools used by the toads to dry up almost
completely. In the process, nearly all of the eggs and tadpoles within the
pools were killed. This climate change was a density-independent factor
that caused the golden toad population to crash. As you’ll recall, by 1990,
the toads were extinct.
Reading
Checkpoint
hy is severe weather considered a density-independent
W
factor?
(a)
(b)
Biotic Potential Limiting factors from an organism’s environment
provide only half the story of population regulation. The other half comes
from the characteristics of the organism itself. For example, organisms
differ in their biotic potential, or maximum ability to produce offspring
in ideal conditions.
Many factors influence biotic potential, including gestation and generation time. Gestation time is how long it takes for an embryo or fetus to
develop and be “born.” The span from an organism’s birth to the time it
has its own offspring is called generation time. Generation time is largely
controlled by how long it takes an animal to reach sexual maturity. The
number of offspring born at a time also affects biotic potential.
Cabezon, or “scorpion fish,” and orangutans (Figure 15) vary greatly
in their biotic potential. Once female cabezon are mature, at about 3 to 5
years old, they can release 50,000 to 100,000 eggs every year. Once fertilized, the eggs take just 12 to 16 days to hatch. Clearly, scorpion fish have
a very high biotic potential. Orangutans, however, have a very low biotic
potential. Females are not sexually mature until they are about 10 years
old, and they give birth to a single baby only about once every 8 years.
Populations of individuals with high biotic potential recover more quickly
from declines than those of individuals with low biotic potential.
Figure 15 Biotic Potential
Organisms differ in their biotic
potential. (a) Cabezon, like many
fish species, have a very high biotic
potential. In just one year, a female
can release tens of thousands of eggs.
(b) Female orangutans, however,
usually only have three or four
offspring in their lifetime.
3
1. Calculate A population has a birthrate of 10/1000,
a death rate of 9/1000, an immigration rate of
3/1000, and an emigration rate of 7/1000. What is
the population’s growth rate? Is the population getting larger or smaller?
2. Compare and Contrast What is the difference
between exponential growth and logistic growth?
Which is more common over long terms in nature?
3. Apply Concepts In your own words, define limiting factor and biotic potential.
4.
You are a population
ecologist studying white-tailed deer populations in
your state. Populations have been growing exponentially for some time, and food is becoming a
limiting factor. Many deer are dying of starvation,
and others are in bad health. What do you recommend to state officials? Should people intervene
and try to limit deer populations through relocation or hunting? Or should they do nothing and
wait for the population to regulate itself? Explain
your reasoning.
Population Ecology 117