Chapter 4 Ecosystems and Communities
Big Idea: Interdependence in Nature
How do abiotic and biotic factors shape ecosystems?
Anchor: BIO.B.4.11
Anchor:BIO.B.4.1.2
Anchor:BIO.4.2.4
Anchor:BIO.B.4.2.5
Teacher emphasizes how the light from the sun creates
conditions for the different biomes. The biotic
components of the ecosystem adapt to those abiotic
conditions.
Powerpoint Lesson Overview 4.1 Climate
Chapter Mystery The Wolf Effect
Wolf interaction work sheet. Optional segment The
wolves of Yellowstone. Teacher asks question What do
you know about wolves? Video segment #4 From Lava
to Life Untamed Science. Teachers can continue with
eco-columns to demonstrate abiotic and biotic
interactions, habitat, niche, competition, limiting
factors, cycles, trophic levels, predation and herbivory,
decomposition photosynthesis, cellular respiration,
food chains and food webs.
Study workbook B pg. 43 Ecosystems and Communities
Interdependence in Nature Q: How do abiotic and biotic
factors shape ecosystems.
Chapter 4 Lab Abiotic Factors and Plant Selection pg. 29
lab book optional or better lab.
For each chapter teacher builds a word wall for
vocabulary in chapter.
4.1 Climate
Key Questions: What is climate?
What factors determine Global Climate?
Demonstration :Flashlight globe demonstrate seasons.
Hands on learning Tilt of the Earth pg. 83 teachers
adition.
Objectives :4,1.1 Differentiate between weather and
climate. 4,1.2 Identify the factors that influence climate.
You Tube tornadoes.
Preview the pages:1. Have students find vocabulary
words from text. 2. Preview the diagram The
greenhouse effect. 3. Preview the visual winds and
currents.
Study workbook B 4.1 Climate
Lesson objectives :Differentiate between weather and
climate.
Identify the factors that influence climate.
The Greenhouse effect Optional trip to the Ridley High
Greenhouse. Optional students read article
Pennsylvania and the Greenhouse Effect.
Teacher covers pg. 82-83 in text cover terms weather,
climate, and microclimates. Key question: What is
climate? A regions climate is defined by year-after-year
patterns of temperature and precipitation.
Text pg. 82 Factors that affect Climate. Solar Energy
and the Greenhouse Effect. Overhead transparency The
Greenhouse Effect. Latitude and solar energy (flashlight
demonstration seasons Hands on Learning pg. 83 Tilt of
the Earth. The tilt of the Earth’s axis causes the
distribution of sunlight to change over the course of the
year.
Teacher asks: What causes Earth’s three types of
climate zones? (uneven distribution of heat caused by
the curvature of the Earth) Why are there cooler and
warmer seasons in temperate regions? (Earth’s axis is
tilted, which causes solar radiation to hit Earth at an
angle . The angle varies from summer to winter.
Heat transport in the Biosphere pg 84 text. Teacher
demonstrates convection circulation. Optional lab or
worksheet Heat absorption between soil and water.
Speed Bump pg. 84 How does solar energy cause wind?
What causes most ocean currents?
Use the Winds and Currents visuals to determine the
directions winds and currents move. Ask Do the
southeast winds move toward the west or toward the
east? (west)
Ask: In what direction do cold surface currents in the
Northern Hemisphere generally move?(southward
toward the equator) In what direction do warm surface
currents in the Northern Hemisphere generally move? (
northward toward the polar region) Explain that these
currents redistribute heat in the Northern Hemisphere,
moving warm water from the tropical region north
toward the polar region.
Wrap-Up Activity pg.84 Have students use colored
pencils to draw a diagram showing how the sun’s
energy causes wind. Instruct them to use yellow arrows
to show sunlight reaching the Earth’s surface, red to
show heated air rising, and blue to show cooler air
sinking. (Check Understanding pg. 84 1-4. Climate
powerpoint Review
4.2 Niches and Community Interactions
pg. 85 text
Key Questions: 1. What is a niche?
2. How does competition shape communities?
3. How do predation and herbivory shape communities?
4. What are the three primary ways that organisims
depend on each other?
Concept Map Use highlighted vocabulary words to
create a concept map. Students complete concept map
in workbook B Lesson 4.2.
Preview the Tolerance graph, pg. 85. Use an example,
such as the amount of sunlight falling on a population of
plants, to explain the concept of optimum range. Below
the optimum range there is too little light for most
plants to grow. Above the optimum range there is too
much sunlight, and plants may die. The plant
population is largest when the amount of sunlight is in
the optimum range. (you tube competitive exclusion
acorn barnacles)
Students define and discuss habitat. (a general place
where an organism lives)
Chapter Mystery pg. 47 in workbook B The Wolf Effect.
Key Question: What is a niche? ( A niche is the range of
physical and biological conditions in which a species
lives. It includes the way the species obtains what it
needs to survive and reproduce.)
Competition: pg. 85 text
Competition when organisms try to use a limited
resource in the same place at the same time. Have
students identify competition in Eco-column ex. Plants
compete for light and water and nutrients in soil.
Animals compete for food, mates. Competition can
occur among members of the same species
(intraspecific competition ) and between members of
different species. (interspecific competition)
The Competitive Exclusion Principle pg. 86 text
Key Question: Why can’t two species share the same
niche in the same habitat? (If two species share the
same niche, competing for the same resources, one
species will always win.) Guide students as the
interpret the Competitive Exclusion Graph pg. 86 text.
Discuss limiting factors. Gupy Graph work sheet
demonstrating carrying capacity- limiting factors.
Key Question: How does competition shape
communities?
( Competition causes species to divide resources. It
helps determine the number and kinds of species in a
community. It also determines the niche each species
occupies.
Predation, Herbivory, and Keystone Species. Text pg. 86
Teacher covers predator-prey relationships and
herbivore-plant relationships. Can revisit the Mystery
about wolves. Ask: How did the reintroduction of
wolves into Yellowstone bring the ecosystem back into
balance?
Talk about Carrying capacity (guppy graph) and limiting
factors.
Keystone Species: Text pg. 86 video clip sea otters, sea
urchins and the kelp forest.
Key Question : How do predation and herbivory shape
communities? (Predators can affect where prey
populations can survive. Herbivores can help
determine where populations of certain plants can
survive.)
Symbioses pg. 87 text
Any relationship in which two species live closely
together. Biologists recognize three main classes of
symbiotic relationships in nature: mutualism,
parasitism, and commensalism. Students can identify
mutualistic relationships in Eco-columns. Optional
video clips on symbiotic relationships. (Zombie Snails)
Key Question: What are the three primary ways that
organisms depend on each other? ( The three main
classes of symbiotic relationships are mutualism,
parasitism, and commensalism.)
Check understanding pg. 87 text.
Mystery Clue pg. 87 wolves of Yellowstone interaction
with producers.
Wrap Up Activity: Students work in pairs to create a
chart comparing the different types of relationships
between organisms discussed in this lesson. The chart
should include the following relationships: predator –
prey, herbivore –plant, mutualism, parasitism, and
commensalism. Include drawing. Eco-journal entry.
Workbook B pg. 48 Concept map.
4.3 Succession
activity pg
Do ecosystems return to “normal” following a
disturbance?
Compare succession after a natural disturbance with
succession after a human-caused disturbance.
Activate Prior Knowledge: Ask, what kind of natural
disasters have you heard about? What happened to
plants and animals as a result of those disasters?
Succession Powerpoint (Pearson ) Notes from
powerpoint.
Workbook B pg. 49 Build Vocabulary The chart shows
key terms from the lesson with their definitions.
Complete pg. 90 in textmeaning of each term.
Workbook b. pg. 50 Primary and Secondary Succession.
Untamed Science video segment #4 From Lava to Life.
Teacher emphasizes Why Succession Occurs pg. 89 text.
Use the building Vocabulary pg. 89 text.
Answer Key Question : How do communities change
over time? (Ecosystems change over time, especially
after disturbance. As some species die out, new species
move in.) Eco-journal competition. Complete pg. 51
Workbook B Compare/Contrast table on Primary and
secondary Succession. Cover Climax Communities pg.
51 Workbook B Teacher asks key question: Do all of
the same species remain in a community that is going
through ecological succession? ( No; because conditions
keep changing. With each change, it becomes suitable
for different species.) Teacher covers Pioneer species
and adaptations that enable them to thrive. Ex Lichens.
Teacher reviews differences between primary and
secondary succession.
Ask Speed Bump questions pg. 89. What often happens
to the number of species in a community as succession
progresses? (The number of species increases.)
What causes succession? ( As organisms change the
environment they live in, other organisms are able to
compete and survive.) Question; How has Man
changed the Environment? Cover pg. 90 in text
Succession after Human-Caused Disturbance discuss
what happens when the tropical rainforest is cleared for
farming.
Find the main idea for the Studying Patterns of
Succession. Ask: How did researchers learn about
primary succession on Mount St. Helens and Krakatau?
(By comparing initial conditions and the stages of
succession in different communities. Ask What is the
main idea of this section? (Scientist study succession by
observing and comparing stages of succession in
different communities.)
Key Question; What would be needed for an ecosystem
to return to its original climax community after a
disturbance? ( The same conditions that originally
produced the community.) Key Question; Do
ecosystems return to “normal” following a disturbance?
( In healthy ecosystems, secondary succession often
reproduces the original climax community after a
natural disturbance. But ecosystems may or may not
recover from widespread disturbance caused by
humans.
Students complete Check Understanding pg. 90 1-6.
Wrap-up Activity (optional) Perform a quiz show;
Have teams write questions about the about the lesson
content on one side of a card and answers on the other
side. Organize a quiz show to see which team gets the
most correct answers. (White-Board)
4.4 Biomes
Opener: Teacher demonstrates the seasons using a
globe and a flash light. Emphasize that one factor that
impacts biomes is the amount of light energy an area
receives . Other factors including an area’s nearness to
an ocean or a mountain range. (Optional Map annual
solar impact of North America)
Key Question; What abiotic and biotic factors
characterize biomes?
Opener: Preview the biome map. Study the names of
the biomes carefully. Workbook B pg. 52 Biomes. As
students read pg. 91-92 in text. Complete the table in
lesson 4.4 by filling in the missing information.
Preview: Preview the vocabulary terms by having
having students find them in the text. Invite students
to discuss any terms with which they are familiar.
Activity 2: Use powerpoint 4.4 to review this
information. Preview the images and climate graphs for
the major biomes. Point out that the Y axis for each
graph has two separate scales-one for temperature and
one for precipitation. Make sure students understand
that the higher the red line, the higher the temperature,
and the taller the bar, the greater the rainfall amount.
Point out also that the X and Y axes for all the graphs are
the same, so that data can be easily compared.
Optional :Biome Project Each group is assigned a
biome. Groups research Abiotic factors like climate and
soil type along with biotic factors like plants and
animals. Students also research problems associated
with each biome.. (example deforestation of tropical
rain forest.) Project should include dominant plant
species and animal species and identify the adaptations
necessary for survival in that biome.
Active Reading: Teacher ask , What is the single most
important factor that influences the kind of biological
community present in an area? (climate)
Have students define abiotic and biotic factors. (Abiotic
factors are physical, nonliving factors that shape an
ecosystem. Biotic factors are all of the living parts of the
ecosystem.)
Direct students attention to the map of biomes pg. 91
text. Ask: which kind of tropical forest covers the most
area? (rain forest) Does the desert biome appear on all
the Earth’s continents? (yes) Have students find their
own location on the map. Then challenge them to find
two other places on the map that are in the same biome.
Ask In what ways would you expect ecosystems in those
areas to be similar to our ecosystem? (Answers may
vary but should address similarities of climate and
similarities of plant and animal life.)
Compare and contrast: Have students compare the
descriptions of the tropical rain forest and the tropical
dry forest biomes. Ask Which of these two biomes has
rain all year round? (tropical rain forest) Ask Why are
animals in the tropical rain forest unlikely to practice
estivation? (there’s plenty of water year round.)
Extend student’s knowledge by telling them that tall
trees growing in the poor, shallow soil of the tropical
rain forest often have buttress roots for support. In
order to get enough sunlight, epiphytic plants grow on
the branches of tall plants rather than in the soil.
Teacher option- segments of Living planet to survey the
different biomes of the world.
Optional chapter 4 lab page 29 lab book Abiotic Factors
and plant Selection. Perhaps a miny garden by
windowsill. Or perhaps courtyard flower boxes.
Active Reading: Teacher provides students with
additional details about the adaptations of plants in
grasslands and deserts. Explain that some grasses in
tropical grasslands have a high silica content that makes
them less appetizing to grazing herbivores. Also, unlike
most plants , grow from their bases, not their tips. This
means that they can continue to grow after being
grazed. Some desert plants like cacti , have an
adaptation that allows them to conserve moisture
during photosynthesis , when plants open their pores to
take in carbon dioxide and release oxygen. These
plants, called C-4 plants open their pores only at night,
preventing loss of moisture on hot, dry days.
Ask; What abiotic factors affect the ability of organisms
to survive in deserts? (low precipitation, extreme
temperature changes) What are some ways that plants
have adapted to these abiotic factors? ( Plants store
water in their tissues, some only open pores at night
some have spines for protection because the animals
know they have moisture within them.)
Ask; What is a key biotic factor that affects the ability of
plants to survive in grasslands.pg.93 Ask What is a key
biotic factor that affects the ability of plants to survive
in grasslands? (heavy grazing by animals) Ask what are
some adaptations that allow plants to survive heavy
grazing? ( Grasses grow from the base , not tips. Some
grasses have a high silica content that makes them
unappetizing to herbivores.
Make inferences (tell students that coniferous trees
have needlelike leaves that are coated in a waxy
substance. Ask them how these adaptations might help
conifers survive in temperate forest. (these adaptations
minimize water loss.)
Opener: Find images of desert plants cacti, plants with
large and small leaves grazing animals in grasslands.
On overhead For discussion: ask . What adaptations
help plants survive with little water in the desert?
(small leaaf surface area, ability to store water in
tissues)
Why is it an advantage for plants growing in limited
light to large leaves? (The large leaves capture more
sunlight than smaller leaves would.)
Why is predation a constant threat for animals in
temperate grasslands? (The animals are not able to
quickly find a place to hide from predators.) Why aren’t
all plant and animal communities in the same biome
exactly alike? ( Differences in elevation, soil conditions,
and exposure to wind and other weather factors can
cause different habitats within the same biome.)
Add to word wall terms ( canopy, understory, humus,
taiga, and permafrost)
As students read the description of each biome , have
them return to the biomes map and globe and locate the
areas on Earth where that biome is found. Teacher
emphasizes how the angle of sunlight hitting that part
of the Earth has an impact on the biotic factors.
Draw conclusions: Have students use the descriptions
of the temperate woodland and temperate forest
biomes to draw conclusions. Ask :Why do oily plants
increase the risk of fire? ( Plant oils fuel a fire once it
gets started.)
Ask: Why does forest soil contain more rich humus
than woodland soil does? ( A forest has more trees ,
and therefore more leaves that drop to the ground and
decay.)
Analyzing Data: Page 95 text Explain to students that
they will analyze temperature and precipitation data
for two ecosystems to determine which biome each
ecosystem belongs to.
Ask: Do you think these two ecosystems belong to the
same biome? Why or why not? ( No because one has
much higher summer temperatures than the other.
Have students complete check understanding pg 95.
For question 4, remind students that classifying
involves sorting things into groups or categories.
Explain that the question is asking which characteristics
are used to sort ecosystems into biomes.
Wrap-up : Play 20 questions
Have a student secretly select one biome. Then, have
the student answer questions from other students
whose goal is to identify the biome.
4.5 Aquatic Ecosystems pg. 96-99 text
Activate Prior Knowledge Ask: What are some bodies
of water in our area? How would you describe each
one. Teacher shows pictures of fresh water ecosystems.
Little Crum Creek, Ridley Lake, Delaware River,
Delaware Estuary, Delaware Bay,and The Atlantic
Ocean. Teacher might emphasise the idea of a
Watershed (A body of land that sheds its water to one
place.) and that we here in Ridley live in the Delaware
River Watershed.
Key Question: What factors affect Life in aquatic
ecosystems? (Aquatic organisims are affected primarily
by the waters depth, temperature, flow, and the
availability of nutrients.)
Optional : Monitoring Water Quality on The Little Crum
Creek. Students make a hypothesis about what they
think the water quality is on the LCC. Students measure
pH, temperature, flow rate, depth, turbidity and
macroinvertabrate study.
Or Pond Study at the Ridley High Outdoor Education
Center .
Workbook B 4.5 Aquatic Ecosystems
Key Question: What are the major categories of
freshwater ecosystems? (Freshwater ecosystems can
be divided into three main caegories: rivers and
streams, lakes and ponds and freshwater wetlands.)
Optional: Trip to the Ridley Wetlands designed to filter
runoff from Ridley High. Green booklets and questions
for the Ridley Wetland. Also we can demonstrate
wetlands at the pond.
Teacher shows photograph of photic zone.
Ask: Why do phytoplankton grow only in the photic
zone?
(Phytoplankton need sunlight for photosynthesis,
sunlight does not penetrate below the photic zone.
Optional: Students can observe pond water under
microscope and observe algae and populations of
protozoa. Elodea can also be observed while discussing
photosynthesis to observe chloroplasts and cytoplasmic
streaming.
Ask: How does the presence of dissolved nutrients
affect aquatic organisims? ( Different species need
different kinds and amounts of nutrients. Nutrient
levels in the water help determine which species can
live there.
Find the Main Idea: Have students draw a two-column
chart. Have them label the columns Main Idea and
Details.
Have them read Lakes and ponds. Ask what is the main
idea of the paragraph? (Food webs in lakes and ponds
are often based on plankton, algae, and plants.)
Have students read paragraph again . Write details
about the main idea in the right column.
Key Question: What are three helpful things that
freshwater wetlands do? ( They purify water, prevent
flooding, and serve as breeding grounds for many
organisims.) 97. Build word wall with terms ,(benthos,
plankton, wetland, estuary canopy, understory humus
taiga permafrost, symbiosis, mutualism, parasitism,
commensalism. Climate, microclimate greenhouse
effect.)
From Workbook B 4.4 Biomes pg 52-55
Estuaries. Opener show pictures of rivers along big
cities, New York, Pittsburg and Philadelphia. Ask Why is
it that all these big cities are built along rivers?
Discussion should lead to the fact that the rivers are
commercially valuable. Ask : What does the word
commerce mean? And How do people use fish and
shellfish?
After this show the picture of the deep water
hatchetfish pg. 98. Teacher google images deep water
fish. Demonstrate adaptations like bioluminescence.
Ask What kind of adaptations do you think the
hatchetfish and other deep sea dwellers that allow to
live aphotic zone? (adaptations that allow it to live
with high pressure, frigid temperatures, and total
darkness. Ask: What might make a fish species
ecologically important? ( It may be a species that plays
a key role in a food web. It may be a keystone species. (
A species that is vital to the functioning of the
ecosystem..)
Direct students to the ocean zones diagram pg. 99 text.
Ask: What is the relationship between ocean depth and
distance from shore? ( Depth typically increases with
distance from shore.) Be sure the students understand
that the diagram is divided horizontally as well as
vertically. Check understanding by pointing to one of
the organisms and asking students which zone it
occupies. Point out that the diagram is not drawn to
scale.
Key Question: How do ecologists usually classify
marine ecosystems? ( Ecologists divide the ocean into
zones based on depth and distance from shore.) Have
students look at the continental shelf in the ocean zone
diagram. Ask: Which marine ecosystems have benthos
within the photic zone? (Intertidal zone, coastal ocean)
Teacher You Tubes ( Intertidal zone and class observes
the organisims and their adaptation.)
Students complete 4.5 Workbook B Aquatic
Ecosystems pg. 56-57
Ask: How does food availability in the photic zone
compare with food availability in the aphotic zone.?
Again bring students attention to the fact that the
sunlight is the energy source that drives almost all
marine ecosystems. ( Algae and phytoplankton, which
are important food sources, can only live in the photic
zone because light is available for photosynthesis.
Optional students can observe algae fronm the RHS
pond and set up experiment measuring oxygen and its
relationship with light. Emphasize that freshwater
runoff from the land increases nutrient levels in
intertidal and coastal zones.
Lab Manual B The Effect of ferilizer on Algae pg 25-28
lab manual.
Ask: What is life like for organisms that live in the
intertidal zone? ( Because of ocean tides (short video
clip explaining the Moons impact on ocean tides) they
are alternately submerged in water and exposed to air.
They are also subject to waves crashing onto shore.
Ask: Why is the coastal ocean highly productive? ( It is
in the photic zone, and freshwater runoff from land
brings in plenty of nutrients for living organisims.
Then show the exception (You tube food webs around
deep ocean volcanic vents. Ask students how this food
web is sustained without light. Explain that in this case
the producer is bacteria which is using hydrogen sulfide
to produce food (chemosynthesis)
Teacher emphasizes that the coastal zone is so
productive because of the abundance of light and
nutrients.
Optional: Students are given a map of the Delaware
River Estuary. Students brainstorm about the problems
associated with water as it travels through Ridley on the
way to the Atlantic. Nonpoint pollution, oil spills etc.
Students then read article explaining the relationship
between The population of Horseshoe crabs and the
migratory shorebirds of the western hemisphere.
Students can do research and model how the birds and
crabs impact one another. Student’s write a 5
paragraph essay explaing the relationship between the
horseshoe crabs and the migratory shore birds. The
pressure on the horseshoe crabs and that impact on the
birds. Students should include graphs comparing the
populations. And describe by how A Ridley students
actions ( nonpoint pollution) can impact this delicate
relationship.
Have students complete check understanding pg. 99 15.
Workbook B Chapter Review pg. 58 and self test pg. 5960
Pg. 100 Pre-Lab: Abiotic Factors and Plant Selection
Lab Manual B pg. 29-34 Abiotic Factors and Plant
Selection.
Pg 100-105 Chapter Review.
Chapter 5 Populations
Key Question: What factors contribute to changes in
populations?
Have students observe picture of crabs pg. 106.
Remind students that a population consists of
organisims belonging to the same species.
Teacher asks : Why are the red crabs on the move? (
They are leaving the forest to reach the sea, where they
breed.) Ask: Why do all the adult crabs take this trip at
the same time? ( They have to make the trip to
participate in breeding.) Teacher explains that it is the
goal of all living things to reproduce. For a population
to survive , the organisms must reproduce.)
Introduce the concept of limiting factors. Factors that
organisms require to live also determine how a
population changes. Ask: What do both you and a red
crab need to live? (food, water, oxygen shelter)
Ask: What types of abiotic factors and community
interactions might affect the number of organisims
living in a place? ( Abiotic factors such as rainfall and
temperature of an area and community interactions
such as competition predationand parasitism. )
Emphasize to students that abiotic factors determine
where particular organisms live, and explain that
changes in abiotic conditions affect populations.
Teacher here may discuss mans impact on the carbon
cycle which is bringing about climate change and how
this may impact a particular population. Community
interactions such as those between prey and predators ,
help determine whether a population grows, shrinks,
or stays the same. ( predatore prey graph work sheet)
Workbook B 5 Populations pg. 61-64
Teacher copies graphic organizer pg. 107 on board.
Ask: How might interdependence in nature relate to the
questions in the graphic organizer ( Changes in a
population depend on many factors in nature. In turn ,
changes in populations can affect these factors.
Read chapter Mystery A plague of rabbits. Which can
demonstrate what happens when a speicies exceeds the
carrying capacity of an environment. ( Graphing
Exercise guppies in a fish tank.)
Show video clip Untamed Science Census Consensus
Ask: What growing populations have you observed?
(fish in a tank, weeds in the yard, people in a town.)
Populations in their eco-columns.
Ask: How will the chapter question, What factors
contribute to changes in populations? Why did the
rabbit population grow so much in Australia?
5.1 How Populations Grow
Teacher discusses Populations, Geographic Range,
Density and Distribution, Growth Rate, age structure.
(from Math activity on Exponential Growth) (Age
Structure histograms from bscs.)
Key Question: How do ecologists study populations?
Ecologists study a population’s range, density, growth
rate, and age structure.
Teacher builds word wall with terms (age structure,
immigration, emigration, exponential growth, logistic
growth, carrying capacity, limiting factor, density-
dependent limiting factors, density-independent
limiting factor, demography and demographic
transition)
Teacher covers population growth, birthrate and death
rate, immigration and emigration. Answer Key
Question : What factors affect population growth?
(Population growth is affected by birthrate and death
rate. It can also be affected by immigration and
emigration.)
Teacher begins the concept of Exponential Growth with
classic (What would you rather have. 10$ a week or 1
cent and then double that every week) Graph showing J
curve. Optional Hands on Learning Record Change in a
population pg 109 in text. Using colored squares.
Ask: What do we call the process by which organisms
leave their population? (emigration) How does this
compare to immigration? ( it is the opposite. Through
immigration, organisms join a population. )
Ask: Which factor causes a population to shrink:
emigration or immigration? (emigration)