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Inputs
increase
decrease
population Immigration Population Emigration Outputs
population
size
size Births Deaths size
FIGURE 17.3 The human population as a system. We can think of the human population as a
system, with births and immigration as inputs and deaths and emigration as outputs.
PRACTICE
Visual Representations (SP2) and success of plant and animal species. Many of these same Births, Deaths, and Migration
(5 minutes) Models are powerful tools to factors influence human populations as well. Habitat size, When demographers look at population trends in indi-
help students understand environmental resource availability, population size, birth and death rates, vidual countries, they consider the inputs and outputs.
fertility, life expectancy, and migration are factors that influ-
processes. Ask students to answer the ence population size on Earth as a whole and within indi- As FIGURE 17.3 shows, inputs include both births and
immigration, which is the movement of people into a
following questions about Figure 17.3, to vidual countries. In order to understand the impact of the country or region from another country or region. Outputs,
human population on the environment and carrying capac-
help them understand system modeling. ity, we must first understand what drives human population or decreases, include deaths and emigration, which is the
movement of people out of a country or region. When
• Identify the system that is represented change. The study of human populations and population inputs to the population are greater than outputs,the growth
by the model. (Human population) trends is called demography, and a scientist in this field is rate is positive. Conversely, if outputs are greater than inputs,
called a demographer. In many ways, this field is parallel to
• Identify the inputs to the system. the studies done for plants and animals. But because human the growth rate is negative.
Demographers use specific measurements to determine
(Immigration and births) choice plays a much larger role in reproduction rates, the yearly birth and death rates. The crude birth rate (CBR)
study of demography is often more complicated. By analyz-
PRACTICE the seven skills outlined in the Course and Exam
• Identify the outputs from the system. ing specific data such as changes in population size, fertil- is the number of births per 1,000 individuals per year. The
(Emigration and deaths) ity, life expectancy, and migration, demographers can offer crude death rate (CDR) is the number of deaths per
1,000 individuals per year. Worldwide, there were 19 births
insights — some of them surprising — into how and why
Description and make CONNECTIONS to enhance learning.
• Identify the process or change that is human populations change and what can be done to influ- and 7 deaths per 1,000 people in 2020. Migration is not
happening in the system. (Change in ence rates of change. factored in for the global population because, even though
population size) people move from place to place, they do not leave Earth.
Thus, in 2020, the global population increased by 12 people
Social and Cultural Factors in Population per 1,000 people. This rate can be expressed mathematically
Copyright 2023 BFW Publishers. Pages Not Final. For Review Purposes Only. Do Not Share.
Change as a percentage (CBR and CDR are expressed per 1,000 so
we divide by 10 to express as per 100):
We can view the human population as a system with inputs 160
and outputs, like all biological systems. However, human [ CBR − CDR]
population trends are influenced by a wide range of social 120 9
and cultural factors that are not relevant to most other rate in percent 10
biological systems, such as access to health care, birth con- Number of hares Globalpopulationgrowth = [ 19 − 7] 6
80
trol and education, economic (thousands) = Number of lynx (thousands)
PRACTICE prosperity, and geopolitical
conflicts. 40 10 3
Data Analysis (SP5) [ 12]
12%
(5 minutes) Show students Figure 0 = × 100 =. 0
16.7 and ask them to use the graph to 1850 1860 1870 1880 10 1890 1900 1910 1920 1930
Demography The study of human populations and population
Year
trends. summarize the effect of predator–prey When we consider demographic trends for a specific coun-
relationships on population sizes. (Prey

FIGURE 16.7 Population oscillations in lynx and hares. Both lynx and hares exhibit repeated

Demographer A scientist in the field of demography. try, we must look beyond its birth and death rates because
oscillations of abundance, with the lynx population peaking 1 to 2 years after the hare popula-
populations can increase with an increase
Immigration The movement of people into a country or a country may experience population growth, stability, or
tion. When hares are not abundant, there is plenty of food, which allows the hare population to
in available food. As prey populations
in available food. As prey populations
increase. As the hare population increases, there are more hares for lynx to eat, so then the lynx
region, from another country or region. decline as a result of migration. Net migration rate is the
increase, the food supply increases for
increase, the food supply increases for
population increases. As the hare population becomes very abundant, they start to run out of
Emigration The movement of people out of a country or
food and the hare population dies back. As hares become less abundant, the lynx population
the predator. The availability of food—
All seven science practices from the predator. The availability of food— difference between immigration and emigration in a given
subsequently dies back. With less predation and more food once again available, the hare popu-
region.
year per 1,000 people in a country. A positive net migra-
prey—allows the predator populations to
prey—allows the predator populations to
lation increases again, and the cycle repeats. (Data from Hudson’s Bay Company.)

Crude birth rate (CBR) The number of births per 1,000
increase. As predator numbers increase,
the Course and Exam Description are increase. As predator numbers increase, tion rate means there is more immigration than emigration,
individuals per year.
and a negative net migration rate means the opposite. For
spreading of the virus. As wolf predation on moose declined,
resources, or disease. In human populations, which we will
prey populations decrease. Without
prey populations decrease. Without
Crude death rate (CDR) The number of deaths per 1,000
consider in the next two modules, overpopulation can con-
the moose population grew rapidly until it ran out of food
highlighted throughout the TE. With the prey as food, predator populations example, approximately 1 million people immigrate to the
individuals per year. the prey as food, predator populations
tribute to famine, conflict (war), and the spread of infectious
and experienced another large dieback that began in 1995.
United States each year, and a much smaller number emi-
decrease, and the cycle starts again.)
decrease, and the cycle starts again.)

FIGURE 16.8 shows the changes in both populations over a
diseases. There are frequent examples in recent decades of
Net migration rate The difference between immigration and
these activities, tips, and resources, grate. With a U.S. population of 330 million, these rates are
period of more than 50 years. By 2018, only two wolves
famine, conflict, and the spread of disease. For example, a
emigration in a given year per 1,000 people in a country.
drought in East Africa in 2011 and 2012 led to food short-
equal to 3 immigrants per 1,000 people.
students will gain frequent reliable ages, famine, conflict, and the congregating of people in Wolves struck by
refugee camps. Because of crowded conditions,
infectious diseases and malnutrition flourished,
exposure to the skills they’ll be tested ■ Populations resulting in an estimated 260,000 deaths. 50 canine virus 2,500
We can find one more example in the natural
UNIT 3
192
on come exam day. world as a result of a natural experiment in Michigan 40 2,000
during the last century that was partially described
in Unit 1. As you saw with the moose-wolf Isle
Royale, Michigan, example from Module 1, 30 1,500
wolves and moose have coexisted on Isle Royale
for several decades and recorded estimates of their 1,000 Moose
04_FriedlandRelyea4e_40928_Unit 3_172_225.indd 192
PRACTICE TEACH population sizes go back to 1959. However, prior to Wolves 20 14/10/22 11:32 AM
their coexistence, moose lived on the island without
wolves and would go through population fluctua- 10 500
tions as they over-browsed the vegetation and expe-
Concept Explanation (SP1) Teaching Tip 0
rienced diebacks due to food shortages. In the very
cold winter of 1948, ice connected the mainland
(5 minutes) Ask students to respond to the (5 minutes) Help your students review the current 1969 1979 1989 1999 2009 2019
1959
with Isle Royale and wolves crossed the ice bridge
Year
that formed after weeks of cold temperatures and
following questions to help them understand how statistics that they need to know by directing them FIRST PASS
populated the island. This started a predator control FIGURE 16.8 Predator control of prey populations. As the population of

on moose, and ultimately led to a reduction in the wolves on Isle Royale succumbed to a canine virus, their moose prey expe-
populations are dynamic: to the population clock on the U.S. Census Bureau’s fried_es3e_19_06_19.06 - February 11, 2018
moose population. Then, as we learned in Module 1, rienced a dramatic population increase. The moose population ultimately
starting in the late 1970s and early 1980s, the
grew so large that they exceeded their carrying capacity and then experi-
wolf population declined sharply, probably as a o quickly
• What factors increase the size of a population? website. This website will allow students t enced a large dieback. In 2018, only two wolves remained and the moose
population began to once again increase. (Data from R. O. Peterson and J. A. Vucetich,
result of a deadly canine virus. Its transmission
(Births and immigration increase the size of a review current statistics. Unit 3 Web Resources
Ecological Studies of Wolves on Isle Royale: Annual Report 2016 –2017, School of Forest Resources and
was probably facilitated by the relatively large
Environmental Science, Michigan Technological University.)
numbers of wolves at the time that allowed the
population.) contains a link to this website. Be sure to look up
• What factors decrease the size of a current statistics because population numbers will
■ Populations
186 UNIT 3
population? (Deaths and emigration decrease change from one year to the next. It is not neces-
the size of a population.) sary for students to know exact numbers; however,
(The wolves experienced a die-off event due to
04_FriedlandRelyea4e_40928_Unit 3_172_225.indd 186
,
PRACTICE
permafrost is an impermeable,permanently frozen layer of they will need to memorize estimates of the U.S. 14/10/22 11:32 AM
disease—a density-dependent factor. The moose
soil that prevents water from draining and roots from pen- • Write a formula to describe these changes. population and world population. Ask students to
population soared because predators were
+ )
Visual Representations (SP2)
etrating. Permafrost, combined with the cold temperatures ( +−B( I ) ( DE = change in population size. create a notecard with this information to study and
and a short growing season, prevents deep-rooted plants (births + immigration) – (deaths + emigration)) (5 minutes) Show students Figure 16.8 and ask absent. As the moose population increased, its
them to write a paragraph describing the figure.
such as trees from living in the tundra. use in practice calculations. food source became scarce—a density-
dependent factor—and the population crashed.)
While the tundra receives little precipitation, there is In their paragraph they should include answers to
the following:
enough to support some plant growth. The characteristic 192 TRM Unit 3 Web Resources Conclude the exercise by asking students to
Unit 3 Populations
plants of this biome, such as small woody shrubs, mosses, CONNECT • Describe why each of the populations discuss the role of predators in an ecosystem.
(Predators can regulate prey populations in a
heaths, and lichens, can grow in shallow, waterlogged experienced a die-off event.
soil and can survive short growing seasons and bitterly Making Connections • Describe the relationship between the two community, thereby preventing the prey from
becoming too abundant and decreasing the popu-
cold winters. At these cold temperatures, chemical reac- die-off events. lation’s food supply to an unsustainable level.)
tions occur slowly, and as a result, dead plants and animals (10 minutes) When covering the tundra’s • Describe the role of density-dependent and
decompose slowly. This slow rate of decomposition results growing season and permafrost, refer density-independent factors.
in the accumulation of organic matter in the soil over
time with relatively low levels of soil nutrients. Common to the topic of global warming, which is
animals in the Arctic tundra include muskoxen ( Ovibos covered more broadly in Unit 9. To spark
moschatus), Arctic foxes ( Vulpes lagopus), and polar bears student curiosity about future topics and
( Ursus maritimus ). 30°N 186 Unit 3 Populations
The major human impact on the tundra biome is the provide them with a reference point when
threat of warming global temperatures melting the perma- Equator you reach Unit 9, note the following:
frost, which would then decompose dead plants and animals Environmental Science is a very interconnected
at a faster rate than usual. Such decomposition would release 30°S • As atmospheric concentrations of course. Utilize the making connections features
additional greenhouse gases. We will discuss this in more CO 2 increase, Arctic regions become
detail in Unit 9. warmer and permafrost starts to thaw. to draw connections between different units of
®
Thunder Bay, Ontario, Canada • Thawing permafrost causes anaerobic content, other AP courses, and other regions of the
®
AP Exam Tip 40 400 decomposition and the release of
30 300 methane, a potent greenhouse gas. world. These features will help students prepare for
®
On the AP Environmental Science exam you may be 20 200
asked to identify a biome based on a climate diagram. 10 100 • The release of methane leads to the cumulative nature of the FRQs on the exam.
has two
the diagram
Realize that
Realize that the diagram has two y axes, one for tem-
Realize that the diagram has two y y y axes, one for tem- Average temperature (°C) Average precipitation (mm) even more global warming and
perature and one for precipitation. Look at temperature 0 0 therefore initiates a positive feedback
first. Identify if there is a winter period where it goes –10
below freezing. This will narrow down your options for –20 loop where permafrost melts at an
terrestrial biomes. Next, look at precipitation. Does this –30 increasing rate. Students will review
place look wet, average, or dry? This will make it easier J F M A M J J A S O N D feedback loops later in this unit and
to determine each terrestrial biome. Month throughout the course. If students
have not come across this term
SECOND PASS
FIGURE 2.6 Taiga biome. Also known as the boreal forest
Friedland_2e_Fg12.06 - May 7, 2014
biome, it is made up primarily of coniferous evergreen trees that before, you may need to explain
Taiga can tolerate cold winters and short growing seasons. (Bill Brooks/ the term. Features of the Teacher’s Edition TE-xvii
Alamy)
The taiga biome, also referred to as a boreal forest biome,
shown in FIGURE 2.6, consists primarily of coniferous a result of the slow rate of decomposition and the low
(cone-bearing) evergreen trees that can tolerate cold win-
ters and short, cool growing seasons in the summer. Ever- nutrient content of the needles, taiga soils are covered in
a thick layer of organic material, but the soils are poor in
green trees appear green year-round because they drop
only a fraction of their needles each year. Taigas are found nutrients.
While the taiga biome generally receives a bit more pre-
°
between about 50 N and °60 N in Europe, Russia, and cipitation than the tundra biome, the climate characteristics
North America. This subarctic biome has a very cold cli-
mate, and plant growth is more constrained by temperature
than by precipitation. 01_friedlandrelyeaTE4e_47542_FM_i_liii.indd 17 27/01/23 1:12 PM
As in the tundra, cold temperatures and relatively low Permafrost An impermeable, permanently frozen layer of soil.
precipitation make decomposition in taigas a slow pro- Taiga A forest biome made up primarily of coniferous
cess. In addition, the waxy needles of evergreen trees con- evergreen trees that can tolerate cold winters and short
growing seasons. Also known as boreal forest.
tain compounds that are resistant to decomposition. As
MODULE 2 ■ Terrestrial Biomes 33
02_FriedlandRelyea4e_40928_Unit 1_002_093.indd 33 • Which biome still has rain during the 14/10/22 10:14 PM
Teaching the AP ® Tip
summer, but much less than rain during other
(10 minutes) There are several distinctive parts of the year? (Temperate rainforest)
climate graphs that are often asked about on • Which biome has low rain and cold
®
the AP Environmental Science Exam. To help temperatures year round? (Tundra)
students identify these, ask them to examine the • Which biome has a lot of rain year round and
climate graphs on pages 32 to 40 and answer high temperatures? (Tropical rainforest)
the following questions:
• Which biome has almost no rain during the
summer months? (Woodland/Shrubland)
Module 2 Terrestrial Biomes 33