Download How Populations Change in Size

CorrectionKey=B
SECTION 1
SECTION 1
How Populations
Change in Size
Objectives
Focus and Motivate Describe the three main
properties of a population.
Describe exponential population
growth.
Two hundred years ago, no quagga mussels inhabited Lake Michigan, and
blue whales numbered 275,000 in our oceans. Today, nearly a billion quagga
mussels disrupt the ecology of Lake Michigan, and blue whale numbers are barely
recovering from a low of 2,000 individuals reached under the pressure of whaling.
These are opposite extremes of environmental problems expressed at the level
of populations, where the balance between births and deaths can lead either to
stability or major changes.
What Is a Population?
A population is the set of individuals within a species living in the same
place at the same time. All the bass in an Iowa lake make up one population. Figure 1.1 shows other examples of a population and part of a
population. The adults within a population form a reproductive group
because, by definition, organisms breed with members of their own
population. For example, bass in one lake will breed with each other and
will not breed with bass from other lakes.
Describe how the reproductive
behavior of individuals can
affect the growth rate of their
population.
Explain how population sizes in
nature are regulated.
Before beginning this section, review
with your students the Objectives listed
in the Student Edition. This section
introduces the general characteristics of
populations, explores how populations
can grow at different rates, and explains
why there are natural limits to population growth.
Key Terms
population
density
dispersion
growth rate
reproductive potential
exponential growth
carrying capacity
Classroom Catalyst
Have students write down the definition
of a population in their science journals.
(A population is all members of the
same species that live in the same place
at the same time and breed with each
other.) Have them record examples of
populations in their neighborhood.
(Humans, squirrels, trees, grass, weeds,
mice, cats, microbes, etc.) Ask them to
draw or describe where these populations fit in an individual-to-ecosystem
hierarchy.
FIGURE 1.1
(r) ©Paul & Paveena Mckenzie/Oxford Scientific/Getty Images; (l) ©Sylvain Sonnet/Photographer’s Choice/Getty Images
Objectives
Populations All the palm trees on an island is a population, and a school of fish in a
body of water is part of a population.
Homework
Favorite Populations Ask students to
research information about populations
of their favorite plant or animal. Have
them answer the following questions:
“Where in the world can you find
populations of this organism? What
kinds of resources are limiting to its
growth? How are the individuals
dispersed within their habitat? How do
the organisms find each other to mate?”
Differentiated Instruction
group activity
EV_CNLESE904016_C08S1.indd 197
Two Types of Growth Present the following
scenario to groups of students: “You have just
been offered a job that will last one month.
You have two salary options. You can either
receive $10 a week with a $5 per week raise
every week, or you can receive one penny for
your first day on the job, and then double the
previous day’s pay for each of the remaining
30 days.” Ask students to determine which
salary option they would prefer. Provide
students with calculators to calculate their
salaries. (The “double penny” option yields a
Chapter 8:
Understanding Populations
197
much higher salary. The weekly payment
10/30/2012 6:32:26 PM
option would yield $70 for the month, while
the “double penny” option would yield over
$10 million.) Tell students that populations
may increase in size in either of these two
ways, and that this chapter will describe
situations in which these two types of growth
may occur.
Teacher Notes
In some populations, adults may spend
much of the year apart and then come
together for breeding. For example,
elephant seals from the California
population spend most of their year
spread across the North Pacific foraging.
An important question in environmental science is whether individuals are
from the same population. For example,
are all the tiger sharks in the Indian
Ocean part of a population or are there
separate breeding groups in different
parts of the ocean (and therefore
multiple populations)?
Chapter 8: Understanding Populations 197
p
printcode=a
FIGURE 1.2
Population Density Populations may have very different sizes, densities, and
dispersions. Flamingos (right) are usually found in huge, dense flocks, whereas most
snakes (left) are solitary and are dispersed randomly.
Teach
Misconception Alert!
QuickLab
Skills Acquired
• Collecting Data
• Organizing and Analyzing Data
Answers
1. Sample results:
Year
1 2 3 4 5 6 7 8 9 10
Starting 5 6 7 9 11 13 15 18 22 27
Births
2 2 3 4 4 5 6 8 9 11
Deaths
1 1 1 2 2 3 3 4 4 5
Ending
6 7 9 11 13 15 18 22 27 33
Properties of Populations
QUICKLAB
QUICKLAB
Population Growth
Procedure
1. Model the change in size of
a population by applying the
following equation: change in
population size = births – deaths.
2. Start with 100 g (3.5 oz) of dry
beans. Count out five beans to
represent the starting population of
a species.
3. Assume that each year 20 percent
of the beans each have two
offspring. Also assume that 20
percent of the beans die each year.
4. Calculate the number of beans
to add or subtract for 1 year.
Round your calculations to whole
numbers. Add to or remove
beans from your population as
appropriate.
5. Continue modeling your population
changes over the course of 10
years. Record each change.
Analysis
1. Make a graph of your data.
Describe the changes in your
population.
Populations may be described in terms of size, density, and dispersion,
as shown in Figure 1.2. Population size is the total number of individuals,
whereas density is the number of individuals per unit area or volume,
such as the number of bass per cubic meter of water in a lake. A population’s dispersion describes the arrangement of its individuals in space. A
population’s dispersion may be even, clumped, or random.
How Does a Population Grow?
A population gains individuals with each new offspring or birth and loses
them with each death. The resulting population change over time can be
represented by the equation below. The percentage change in the size of
a population over a given period of time is that population’s growth rate.
The growth rate is the birth rate minus the death rate.
Over time, the growth rate of a population changes because birth rates
and death rates increase or decrease. “Growth” rates can be positive,
negative, or zero. For a population’s growth rate to be zero, the average
number of births must equal the average number of deaths. A population
would remain the same size if each pair of adults produced exactly two
offspring, and each of those offspring survived to reproduce. If the adults
in a population are not replaced by new births, the growth rate will be
negative and the population will decrease.
(tr) ©Norman Tomalin/Bruce Coleman, Inc./Photoshot; (l) ©Design Pics/Jack Goldfarb/Getty Images
Populations Are Difficult to
Contain The strict ecological definition
of a population is tricky to apply to
organisms in natural ecosystems. In the
strictest sense, a population is only
those members of a species that
actually are interbreeding. Also, populations are in constant flux as individuals
reproduce, die, or migrate in or out of a
given area. Thus, the term population
may commonly be used, for convenience, to refer to the number of
members of a species within a defined
area at a given time (whether or not
they interbreed). A common example of
this use is when referring to “the human
population of the United States.”
198
Unit 3: Populations
Differentiated
Instruction
GROUP ACTIVITY
EV_CNLESE904016_C08S1.indd 198
Classroom Density To strengthen students’
concept of population density, ask students
to mark or rope off a corner of the room that
is 2 meters on each side. Have 12 students
stand within the area, and ask the class to
calculate the density of that population of
students (12 students/4 m2 = 3 students/m2).
Now ask those 12 students to double their
area (4 m × 4 m = 16 m2).
198 Unit 3: Populations
Ask the class to calculate the new density
(12 students/16 m2 = 0.75 students/m2). Ask,3/21/2012
“Which population has the higher density and
why?” (The first one, because there were more
students per unit of space.)
10:27:41 AM
CorrectionKey=B
How Fast Can a Population Grow?
A female sea turtle may lay 2,000 eggs in her lifetime.
Figure 1.3 shows newly hatched sea turtles leaving
their nests for the ocean. If all of them survived, the
turtle population would grow rapidly. But many
young turtles are eaten by crabs or fish, and others
starve. All populations experience deaths, but death
rates can differ among species and populations. To
understand the fastest hypothetical growth rate, scientists first consider what might happen when death
rates are very low.
FIGURE 1.3
Reproductive Potential Most organisms have a
reproductive potential that far exceeds the number of their
offspring that will survive. Very few of these baby sea turtles will
survive long enough to breed.
Reproductive Potential
A species’ biotic potential is the fastest rate at which
its populations can grow. This rate is limited by
the maximum number of offspring that each member of the population can produce, which is called
its reproductive potential. Some species have much
higher reproductive potentials than others. A bacterium can produce 19
million descendants in a few days or weeks. A pair of bowhead whales
would take hundreds of years to leave that many descendants!
Reproductive potential is higher when individuals produce more
offspring at one time, reproduce more often, and reproduce earlier in life.
Reproducing earlier in life has the greatest effect on reproductive potential. Reproducing early shortens the generation time, the average time it
takes a member of the population to reach the age when it reproduces.
Small organisms, such as bacteria, have short generation times. Some
bacteria can reproduce when they are only twenty minutes old. As a
result, their populations can grow quickly. In contrast, large organisms,
such as elephants and humans, become sexually mature only after a
number of years. The human generation time is about 20 years, so humans have a much lower reproductive potential than bacteria.
FIGURE 1.4
Population Growth Population
growth is graphed by plotting population
size over a period of time. Exponential
population growth will look like the curve
shown here.
CRITICAL THINKING
Explain Under what conditions
does exponential population
growth take place?
Exponential growth occurs in nature only when populations have
plenty of food and space, and have little or no competition or predators.
For example, populations of quagga mussels imported into the United
States initially underwent exponential growth. Similar population explosions occur when bacteria or mold grow on a new source of food.
Chapter 8:
EV_CNLESE904016_C08S1.indd 199
700,000
Number of individuals
©David Hughes/Bruce Coleman, Inc./Photoshot
800,000
Populations sometimes undergo exponential growth, which means
they grow faster and faster. For example, if a pair of dogs gives birth to
6 puppies, there will be 6 dogs in one generation. If each dog in that
generation mates and has a litter of 6 puppies, there will be 36 dogs in
the next generation. The following generation will contain 216 dogs, and
so on. If the number of dogs is plotted on a graph versus time, the graph
will have the shape shown in Figure 1.4.
600,000
500,000
400,000
300,000
200,000
100,000
0
0
4
8
12
16
Number of months
Understanding Populations
Reproductive Strategies Ask students
to consider the following statement:
“Animals have different strategies for
maximizing reproductive success.” The
sea turtles in Figure 1.3 use a lot of
energy to produce many offspring at a
time, but put little or no energy into
offspring care. Songbirds, on the other
hand, usually use a small amount of
energy to produce 2–3 eggs, but then
spend a great deal of energy ensuring
that those few offspring survive. Ask
students to give examples of how other
animals maximize their reproductive
success. Have students discuss how the
evolution of reproductive strategies in
those animals relates to their parental
behaviors. (Animals that do not protect
their broods gain evolutionary success
by producing many offspring; animals
that produce fewer offspring gain
success by spending more energy per
offspring.)
TEACH FROM VISUALS
Exponential Growth
Exponential Growth
Classroom Discussion
20
199
1/4/2013 2:25:33 PM
Exponential Growth Use Figure 1.4 to
reinforce the meaning of exponential
population growth. Ask students to
describe the curve and explain how it
shows exponential growth. (The curve
rises more and more steeply, meaning
the population increases by greater
amounts during each time period.) Ask,
“What would the graph look like if it
showed linear (or arithmetic) growth?” (It
would show a straight line, increasing by
the same amount during each time
period.) Ask: “How does exponential
growth relate to reproductive potential?” (Most organisms have the potential
to reproduce “multiples” of themselves,
thus creating exponential growth rates.
Exponential growth is a mathematical
description of nearly-unlimited population growth.)
Answers
Critical Thinking
Exponential population growth takes
place when the populations have plenty
of food and space, and have little or no
competition or predators.
Chapter 8: Understanding Populations 199
p
printcode=a
What Limits Population Growth? Have
students study Figure 1.5 and explain
how the environment affects population
growth. (As the population uses up
limited resources, its numbers fluctuate
and stabilize around the carrying
capacity.) Then ask students to explain
what caused the initial population crash.
(There were more organisms than the
resources could support. The population
exceeded the carrying capacity.)
Islands are good places to study
carrying capacity because islands have
clear boundaries. The Pribilof Islands
off the coast of Alaska were the site
of a well-studied population explosion
and crash. In 1911, 25 reindeer were
introduced on one of the islands. By
1938, the herd had grown to 2,000
animals. The reindeer ate mostly
lichens, which grow back very slowly.
By 1950, there were only 8 reindeer
alive on the island.
What Limits Population Growth?
Because natural conditions are neither ideal nor constant, populations
cannot grow forever and rarely grow at their reproductive potential.
Eventually, resources are used up or the environment changes, and deaths
increase or births decrease. Under the forces of natural selection in a given
environment, only some members of any population will survive and reproduce. Thus, the properties of a population tend to change over time.
Carrying Capacity
The blue line in Figure 1.5 represents a population that seems to approach
a particular size over time. This theoretical limit, the dashed yellow line,
is called carrying capacity. At high densities, populations move toward
lower birth rates or higher death rates (this is called density dependence).
Carrying capacity is the population size where birth rates and death rates
are equal. Another definition of carrying capacity for a particular species
is the maximum population that its ecosystem can support indefinitely.
A population may increase beyond its carrying capacity, but it cannot stay at an increased size for long. If a population is larger than the
carrying capacity, it may use up its resources, and fewer individuals will
survive to reproduce. Carrying capacity is difficult to predict or calculate.
However, it can be estimated by looking at average population sizes or by
observing a population crash after a certain size has been exceeded.
Misconception Alert!
Population Change Includes
Migration Another possible element in
the equation for population change is
migration. Populations can increase by
births and when individuals move into a
population (immigration). Populations
can decrease by deaths and when
individuals move out of a population
(emigration).
The history of rabbits in Australia demonstrates both exponential
growth and carrying capacity. Originally, there were no rabbits in the native
ecosystems of Australia. When rabbits were introduced there in 1859,
their numbers increased rapidly because they had plenty of vegetation to
eat, no competition, and no predators. But eventually, disease and starvation caused the rabbit population to crash. Over time, the vegetation
recovered, and the rabbit population increased again.
FIGURE 1.5
Carrying Capacity An example
of carrying capacity is shown by the
dashed yellow line in the graph (right).
When rabbits were introduced into
Australia (below), their population
quickly exceeded the carrying capacity
of the area. Rabbits have eaten all the
vegetation around this water hole.
Population overshoots
carrying capacity
Population runs out of
resources and declines
Carrying
capacity
Population recovers
and stabilizes
Exponential
growth
Population crashes
©Bettmann/Corbis
TEACH FROM VISUALS
Carrying Capacity of Islands
Population size
Teach continued
ECOFACT
Time
200
Unit 3:
Populations
Differentiated Instruction
teach with technology
inclusion
Island Carrying Capacities Have students
research a specific population of organisms
that was introduced to or invaded an island.
(Suggest Australia, Guam, Hawaii, or
Madagascar). Ask students to write an essay
that focuses on population trends of that
animal over time, and the way resources
control those trends. Ask them to use past
data to predict future population trends.
On an index card, have each student draw two
penny-sized circles. On a second card, the
student should double the number of circles.
Continue doubling the number of circles on
each of the next cards until no more complete
circles can fit on a card. Tell students the
circles represent members of a population and
the white space on the cards represent the
available resources. Students should be able to
discuss what problems would occur when the
white space (resources) is depleted and how it
would affect the population.
EV_CNLESE904016_C08S1.indd 200
200 Unit 3: Populations
3/15/2012 12:04:42 PM
printcode=a
FIGURE 1.6
Classroom Discussion
Competition Members of a population often compete with each other. These plants
(below) are growing over each other as they compete for light. These wolves (right) are
competing for food and for social dominance.
Introduced Species Ask students, “How
might an introduced species disrupt an
ecosystem?” Suggest that students use
Figure 1.5 and the example of rabbits in
Australia as a basis for discussion. (Native
species may not be adapted to compete
with an introduced species, to defend
against it, or to capture it as prey. If an
introduced species has little competition or predation, its population can
increase rapidly. These changes may
ripple across the food web. The introduced population may eventually run
out of resources, crash, or stabilize
within the ecosystem, but the ecosystem will be fundamentally changed.)
Connect to MATH
Resource Limits
Connect to MATH
A species reaches its carrying capacity when it consumes a particular
natural resource at the same rate at which the ecosystem produces the
resource. That natural resource is then called a limiting resource for the
species in that area. For example, plant growth is limited by supplies of
water, sunlight, and mineral nutrients. The supply of the most severely
limited resources determines the carrying capacity of an environment
for a particular species at a particular time.
Growth Rate
(r) ©Charles Mauzy/Corbis; (l) ©Ronald Wittek/Photographer’s Choice/Getty Images
Competition Within a Population
The members of a population tend to use the same resources in the same
ways, so they will eventually compete with one another as the population
approaches its carrying capacity. An example is mealworm larvae in a
sack of flour. Adults of this beetle will lay their eggs in a sack of flour, and
leave. Most of the first larvae to hatch will have plenty of flour to eat and
will grow to adulthood. However, the sack has a limited amount of food,
and mealworms from eggs that were laid later may not have enough food
to survive to adulthood.
Instead of competing directly for a limiting resource, members of a
species may compete indirectly for a resource by competing for social
dominance or for a territory. A territory is an area defended by one or
more individuals against other individuals. The territory is of value not
only for the space but also for the shelter, food, or breeding sites it contains. Many organisms expend a large amount of time and energy competing with members of the same species. Some examples of competition
within species are shown in Figure 1.6.
Chapter 8:
pre-ap
EV_CNLESE904016_C08S1.indd 201
Understanding Equations Write the following
equation on the board:
∆N = rN
∆t
Explain that ∆ represents change, N is the
population size at any point in time, t is time,
and r is the population’s rate of growth. Ask
students, “What does the equation mean in
English?” (The change in population size over a
period of time is equal to the rate of growth of
the population.) “How does the r-value relate to
the equation in this chapter?” (The rate of
A growth rate is a change in a
population’s size over a specific period
of time.
growth
rate
=
change in population
time
Imagine a starting population of 100
individuals. If there were 10 births and
5 deaths in a given year, what was the
population’s growth rate for the year?
In the next year, if there were 20 births
and 10 deaths, what would the new
growth rate be? If births increased by
10 and deaths increased by 5 for each
of the next 5 years, how would you
describe the growth of this population?
Answers
Check for Understanding
Sample answer: Plants grow over each
other as they compete for light.
Connect to Math
With an original population size of 100,
and 10 births and 5 deaths, the change in
population would be +5, so the
population would increase to 105. The
next year, with 20 births and 10 deaths,
the change in population would be +10,
and the new population size would be
115. In each of the next five years, the
rate of population increase would
accelerate.
Calculations:
Year
CHECK FOR UNDERSTANDING
Describe Describe one example
of competition among members of
a population.
Understanding Populations
201
growth, r, is the difference between births and
3/15/2012 12:04:53
deaths over time.) “What does this equation
tellPM
us about populations with high r- values?” (They
are increasing rapidly.) “What is missing in this
equation?” (It ignores environmental limits and
migration.) Note: For r, biologists often use the
per-capita growth rate (average growth rate per
individual), instead of the absolute growth rate
given in the Connect to Math example.
1
2
3
4
5
6
Starting 100 105 115 130 150 175
pop.
Births
10
20
30
40
50 60
Deaths
5
10
15
20
25
Ending
pop.
30
105 115 130 150 175 205
Growth +5 +10 +15 +20 +25 +30
rate
%
change
5.0 9.5 13.0 15.4 16.7 17.1
Chapter 8: Understanding Populations 201
printcode=a
Quiz
1. In what ways are disease and predation density-dependent? (In a dense
population, increased physical contact
and waste products mean that disease
could spread more easily. Dense prey
populations make it easier for
predators to find prey.)
2.What are some examples of resources
that could determine carrying
capacity in an ecosystem? (water,
sunlight, amount of soil, or any item
that regenerates at a fixed rate and is
consumed by all members of a
species)
Density-Dependent Change
The way a disease spreads through
a population is affected by the
population’s density. These pine trees
have been infected by a disease carried
by the southern pine beetle. This
disease has spread rapidly through
timber forests in the United States.
FIGURE 1.8
Density-Independent Change
Weather events usually affect
every individual in a similar way,
so such events are considered
density-independent regulation.
Alternative
Assessment
Carrying Capacity and Energy Flow Ask
students to think about energy flow in
ecosystems. Have them draw an energy
pyramid for grass, gazelles and lions.
(Grass forms a large base level, with
gazelles in the middle, and lions as a
small top level.) Ask students, “How
does the loss of energy at each trophic
level affect the carrying capacity of
these populations?” (The carrying
capacity is lower at the top of the
pyramid. A lion population needs a larger
number of gazelle prey, which need to
feed on a very large amount of grass.)
Rates of birth or death in a population may be density dependent or
density independent. Density-dependent deaths occur more quickly in a
crowded population than in a sparse population. Limited resources, predation, and disease often result in higher rates of death in dense populations than in sparse populations. The pine trees in Figure 1.7 are infected
with a disease that is spreading in a density-dependent pattern. Many of
the same kind of pine tree are growing close to each other, so a diseasecarrying beetle easily spreads the disease from one tree to another.
When a cause of death is density independent, a certain proportion
of a population dies regardless of the population’s density. This type of
regulation affects all members of a population in a general or uniform
way. Severe weather and natural disasters are often density-independent
causes of death. The winter storm shown in Figure 1.8 froze crops and
fruiting trees regardless of the density of plants in the area. Populations
can show alternating periods of exponential growth and population
crashes with density-independent death rates. Many species of animals
in unpredictable environments show this pattern of population change.
Carrying Capacity Comics Have
students design a comic strip to show
what might happen when a species
exceeds its carrying capacity.
Reteach
Patterns of Population Change
Section 1 Formative Assessment
Reviewing Main Ideas
1. Compare two populations in terms of size,
density, and dispersion. Choose any populations
you know of.
2. Describe exponential population growth.
3. Describe three methods by which the
reproductive behavior of individuals can affect
the growth rate of a population.
4. Explain how population sizes in nature are
regulated.
Critical Thinking
5. Making Predictions How accurately do you
think the future size of a population can be
predicted? What information might be needed
to make a prediction?
6. Compare and Contrast Read the description
of the populations of rabbits in Australia
and reindeer in the Pribilof Islands. List the
similarities and differences between these two
histories.
202
Unit 3: Populations
Answers
to Formative Assessment
1. Answers may vary. Students should mention
size, density, and dispersion.
2. Exponential growth will increase by a
multiplicative factor (may double or triple
with each generation), while linear growth
merely adds the same number to each
generation.
3. Populations can have increased growth rates
if individuals reproduce earlier in life,
reproduce more often, or produce more
offspring at a time.
4. Interactions with the environment will
change the characteristics of a population
over time. The population may change in
size, density, dispersion, or niche.
EV_CNLESE904016_C08S1.indd 202
202 Unit 3: Populations
(cl) ©Wendell Metzen/Bruce Coleman, Inc./Photoshot; (t) ©Science Photo Library/Alamy
Assess and Reteach
FIGURE 1.7
5. Answers may vary. To predict future population size, one would need to know all the 3/15/2012
environmental factors that might act on the
population, including predators, prey, abiotic
factors, climate, etc. One might only be able
to accurately predict population size if all of
these factors were stable.
6. Answers may vary. Students should describe
how each population exceeded its resources,
then crashed.
12:05:00 PM