CHAPTER 3
ECOSYSTEMS: WHAT ARE THEY AND HOW DO THEY WORK?
Outline
3-1 How does the earth’s life-support system work?
A. The earth’s life-support system has four major components.
2. The troposphere is the air layer about 4–11 miles above sea level. It contains greenhouse gases
that absorb and release energy which warms the inner layer of the atmosphere.
4. The hydrosphere consists of the earth’s water, found in liquid water, ice, and water vapor.
6. The biosphere includes parts of the atmosphere, hydrosphere, and geosphere.
B. Three factors sustain the earth’s life:
2. Cycling of nutrients.
3-2 What are the major components of an ecosystem?
A. Ecologists study interactions in nature.
1. Ecology focuses on how organisms interact with each other and with their non-living
environment. They study interactions within and among these following levels of organization:
a. Population is a group of individuals of the same species living in a particular place.
b. Community is populations of different species living in a particular place, and
potentially interacting with each other.
B. Ecosystems have several important components.
1. Every organism belongs to a particular trophic level depending on its source of nutrients.
3. Consumers, or heterotrophs, get their nutrients by feeding on other organisms or their remains.
5. Consumers can be primary, secondary or tertiary consumers, depending upon their trophic level.
7. Detritivores (detritus feeders) feed on waste or dead bodies.
3-3 What happens to energy in an ecosystem?
A. Energy flows through ecosystems in food chains and food webs.
1. A food chain is a sequence of organisms, each of which serves as a source of nutrients and energy
for the next organisms. Organisms are assigned to trophic levels in a food chain.
2. A food web is a series of interconnected food chains. Food webs occur in most ecosystems.
Organisms are also assigned to trophic levels in food webs.
a. Producers belong to the first trophic level.
Instructor’s Manual for Environmental Science, 15th edition
e. Detritivores and decomposers process detritus from all trophic levels.
B. Usable energy decreases with each link in a food chain or web.
1. There is less high-quality energy available to organisms at each succeeding feeding level because
when chemical energy is transferred from one trophic level to the next, about 90% the energy is
lost as heat.
C. Some ecosystems produce plant matter faster than others do.
2. Some of the biomass must be used for the producers’ own respiration. Net primary productivity
3. Ecosystems and aquatic life zones differ in their NPP. The three most productive systems are
3-4 What happens to matter in an ecosystem?
A. Nutrients cycle within and among ecosystems.
1. Elements and compounds move through air, water, soil, rock and living organisms in
biogeochemical, or nutrient, cycles.
B. The water cycle (hydrologic cycle).
1. Solar energy evaporates water; the water returns as precipitation (rain or snow), goes through
organisms, goes into bodies of water, and evaporates again.
3. Water can be stored as ice in glaciers or in underground aquifers.
4. Unique properties of water include that it:
a. Is held together by hydrogen bonds.
b. Exists as a liquid over a wide temperate range.
2. Humans alter the water cycle in three ways.
a. Withdrawing freshwater at faster rates than nature can replenish it.
b. Clearing vegetation which increases runoff and decreases replenishment of groundwater
supplies.
c. Draining wetlands which interferes with flood control.
B. The carbon cycle.
2. Carbon circulates through the biosphere, hydrosphere, and atmosphere.
4. Fossil fuels contain carbon.
5. Humans are altering atmospheric carbon dioxide mostly by our use of fossil fuels and our
destruction of the carbon-absorbing vegetation.
C. The nitrogen cycle: bacteria in action.
2. Nitrogen-fixing bacteria convert N2 into compounds that are useful nutrients for plants and
animals.
3. The nitrogen cycle includes the following steps:
a. Specialized bacteria convert gaseous nitrogen to ammonia in nitrogen fixation.
Chapter 3: Ecosystems: What Are They and How Do They Work?
4. Human activities have more than doubled the annual release of nitrogen from the land into the rest
of the environment, mostly from the greatly increased use of inorganic fertilizers to grow crops.
This excessive input of nitrogen into the air and water contributes to pollution and other problems.
E. The phosphorus cycle.
2. The major reservoirs of phosphorus on Earth are rock formations and ocean bottom sediments.
4. Phosphorus is often the limiting factor for plant growth.
5. Human activity removes phosphate from the earth to make fertilizer and reduces phosphate levels
in tropical soils by clearing forests. Phosphate-rich runoff from the land can produce huge
populations of algae, which can upset chemical cycling and other processes
F. The sulfur cycle.
2. Hydrogen sulfide (H2S) is released from volcanoes and anaerobic decomposition of organic matter
in bogs and swamps.
3. Humans have been increasing atmospheric sulfur dioxide by burning sulfur-containing fuels,
3-5 How do scientists study ecosystems?
A. Some scientists study nature directly.
2. Remote sensing devices can gather data on the earth’s surface that can be converted into usable
forms by geographic information systems (GIS), such as computerized maps of an area that are
used to examine forest cover, water resources, air pollution emissions, coastal changes, and
changes in global sea temperatures.
B. Some scientists study ecosystems in the laboratory.
1. Ecologists use tanks, greenhouses, and controlled indoor and outdoor chambers to study
ecosystems in laboratory research. This allows control of light, temperature, CO2, humidity, and
other variables.
C. We need to learn more about the health of the world’s ecosystems.
1. Mathematical models and computer simulations can help scientists understand large and very
complex systems.
3. We need more baseline data about components and physical and chemical conditions in order to
3-1 How does the earth’s life-support system work?
CONCEPT 3-1A The four major components of the earth’s life-support system are the atmosphere (air), the
hydrosphere (water), the geosphere (rock, soil, sediment), and the biosphere (living things).
1. Distinguish among the following terms: atmosphere, hydrosphere, geosphere, and biosphere.
Instructor’s Manual for Environmental Science, 15th edition
CONCEPT 3-1B Life is sustained by the flow of energy from the sun through the biosphere, the cycling of
nutrients within the biosphere, and gravity.
1. Briefly describe how the sun, nutrient cycles, and gravity sustain life on the earth. Compare the flow of
matter and the flow of energy through the biosphere.
3-2 What are the major components of an ecosystem?
CONCEPT 3-2 Some organisms produce the nutrients they need, others get the nutrients they need by
consuming other organisms, and some recycle nutrients back to producers by decomposing the wastes and
remains of organisms.
2. Define abiotic component of an ecosystem. List three important physical factors and three important
chemical factors that have large effects on ecosystems.
4. Discuss the importance of decomposers and detritivores in nutrient cycling.
3-3 What happens to energy in an ecosystem?
CONCEPT 3-3 As energy flows through ecosystems in food chains and webs, the amount of high-quality
chemical energy available to organisms at each succeeding feeding level decreases.
1. Distinguish among trophic levels and give examples of organisms at the primary, secondary and
tertiary trophic level. Describe food chain and food web and how the two differ.
3. Distinguish between gross primary productivity (GPP) and net primary productivity (NPP). Evaluate
3-4 What happens to matter in an ecosystem?
CONCEPT 3-4 Matter, in the form of nutrients, cycles within and among ecosystems and the biosphere, and
human activities are altering these cycles.
1. Briefly describe the hydrologic cycle. Distinguish among the following: evaporation, transpiration,
precipitation and runoff. Describe the storage of water in glaciers and aquifers.
3. Summarize the major ways humans affect each biogeochemical cycle.
3-5 How do scientists study ecosystems?
CONCEPT 3-5 Scientists use both field research and laboratory research, as well as mathematical and other
models to learn about ecosystems.
1. Briefly describe the distinguishing features of three approaches ecologists use to learn about ecosystems:
field research, laboratory research, and simulations analysis.
Chapter 3: Ecosystems: What Are They and How Do They Work?
Key Terms
abiotic
aerobic respiration
atmosphere
atom
autotrophs
biogeochemical cycles
biomass
biosphere
detritus feeders
ecology
ecosystem
food chain
food web
geosphere
greenhouse effect
greenhouse gases
omnivores
organisms
phosphorus cycle
photosynthesis
population
primary consumers
producers
pyramid of energy flow
Teaching Tips
Draw relationships to the Core Case Study regarding the rainforest and their essential role in global
sustainability.
• Begin by asking students what they know about rainforests, especially any specific species, and
ask them to bring in pictures of different rainforest species including microbes. Also ask if anyone
has seen any media clips on the rainforests.
• Make a list of rainforest species in one column and a list of native species in a second column.
Discuss some differences in number and type of species found in the rainforest and your
community. Draw relationship arrows showing connections—like bee to flowers to fruit to birds
to soil—and start discussing nutrient cycles.
Ask each individual student to sketch/photo and label themselves in their habitat (home/school/work) then
produce a list of how they depend on and impact their environment. They can use the text’s labels
(producer, primary consumer, secondary consumer, and decomposer).
• Produce a home refrigerator/freezer inventory and specify for two or three items where the food
came from and the food’s nutritional value.
• List a few material items in their habitat and specify what they were made from and where they
were produced.
Instructor’s Manual for Environmental Science, 15th edition
Discussion Topics
1. How does the human body utilize biotic and abiotic materials? Describe the specific elements and
2. How does energy drive the nutrient cycles? Where does the energy come from and what happens to the
energy?
4. Explore how field and laboratory methods used in ecological research help increase our understanding
of our world by examining data that measures net primary productivity and respiration rates; analyzes
1. Organize a class trip or a virtual field trip (Google Earth) to a natural area such as a forest, grassland,
2. Bring a self-sustaining terrarium or aquarium to class and explain the structure and function of this
3. Find works of literature, art, and music that show human attachment to and destruction of natural
ecosystems.
4. Have your students debate the issue of saving the rainforests from a social, economic, and ecological
perspective. Are there indigenous societies living in the rainforests that are being displaced? Will
sustaining the current rainforest be enough? Make suggestions and support these suggestions.
5. Define an ecosystem to study on campus. As a class project, analyze the abiotic and biotic components
of the ecosystem. Draw food webs to show the relationships among species in the ecosystem. Project
1. Do you feel you are part of an ecosystem? What role do you fill?
2. Do you hold any particular feelings for producers? Consumers? Decomposers?
3. Do you feel there will always be enough matter and energy for the survival of all individuals of all
species? Will nature be able to continually absorb “waste products” from human societies?
4. How do you feel when you think of a coyote eating a rabbit? How do you feel when you think of
humans eating hamburgers?
5. Should you do anything to preserve the rainforests?
Chapter 3: Ecosystems: What Are They and How Do They Work?
© 2016 Cengage Learning
6. Why would anyone embrace a stewardship philosophy?
Laboratory Skills
Wells, Edward. Lab Manual for Environmental Science. 2009. Lab #5: Food Webs.
Additional Videos
Animals: New Species Discovered in Papua New Guinea (online video from Discovery News, 2010)
http://news.discovery.com/videos/animals-new-species-discovered-in-papua-new-guinea.html
Web Resources
Food Webs
Interactive exploration of terrestrial and aquatic food webs.
http://www.gould.edu.au/foodwebs/kids_web.htm
Digital Integration
Correlation to Global Environment Watch
Aquifers Greenhouse Gas Emissions
Carbon Markets Organic Agriculture and Organic Foods
Ecosystems Ozone Depletion
Correlation to Explore More
Ecology
Instructor’s Manual for Environmental Science, 15th edition
Suggested Answers to End of Chapter Questions
Answers will vary but these represent phrases from this chapter. The following are examples of the material
that should be contained in possible student answers to the end of chapter questions. They represent only a
summary overview and serve to highlight the core concepts that are addressed in the text. It should be
anticipated that the students will provide more in-depth and detailed responses to the questions depending
on an individual instructor’s stated expectations.
Review
Core Case Study
1. What are three harmful effects resulting from the clearing and degradation of tropical rain forests?
See page 40.
• It will reduce the earth’s vital biodiversity by destroying or degrading the habitats of many of the
unique plant and animal species found in these forests, thereby causing their premature extinction.
Section 3-1
1. What are the two key concepts for this section?
a. The four major components of the earth’s life-support system are the atmosphere (air), the
2. Define and distinguish among the atmosphere, troposphere, stratosphere, hydrosphere, geosphere, and
biosphere.
a. The atmosphere is a thin spherical envelope of gases surrounding the earth’s surface.
b. The troposphere is the inner layer of the atmosphere, extending only about 17 kilometers (11
3. What three interconnected factors sustain life on the earth?
4. The one-way flow of high-quality energy from the sun, through living things in their feeding
interactions, into the environment as low-quality energy (mostly heat dispersed into air or water at a
low temperature), and eventually back into space as heat (Figure 3-3).
Chapter 3: Ecosystems: What Are They and How Do They Work?
6. Gravity allows the planet to hold onto its atmosphere and helps to enable the movement and cycling of
chemicals through the air, water, soil, and organisms.
8. Solar energy flows to the earth. About one-third of the incoming solar radiation is reflected back into
space by clouds, particles in the atmosphere, and the earth’s surface. Another fifth of the incoming
9. What is the greenhouse effect and why is it important?
10. Water vapor, carbon dioxide, and methane are greenhouse gases found in the troposphere. They
absorb and release energy that warms the lower atmosphere. Without these gases the earth would be
too cold for the existence of life as we know it.
Section 3-2
1. What is the key concept for this section?
2. Define ecology.
3. Define organism, population, and community and give an example of each.
a. An organism is an individual living being. A deer is an organism.
b. Populations are groups of individuals of the same species living in a particular place. A
4. Distinguish between the living and nonliving components in ecosystems and give two examples of
each. See page 43.
5. What is a trophic level?
6. Distinguish among producers (autotrophs), consumers (heterotrophs), decomposers and detritus
feeders (detritivores), and give an example of each.
a. Producers, sometimes called autotrophs (self-feeders), make the nutrients they need from
compounds and energy obtained from their environment through a process called
7. Distinguish among primary consumers (herbivores), carnivores, secondary consumers, tertiary
consumers and omnivores and give an example of each.
a. Primary consumers, or herbivores (plant eaters), are animals that eat producers, feeding
mostly on green plants. Examples are caterpillars, deer, and zooplankton.
8. Explain the importance of microbes.
a. Microbes that decompose dead and decaying plant and animal materials are vital to all
9. What is aerobic respiration?
10. What two processes sustain ecosystems and the biosphere and how are they linked?
a. This linkage between photosynthesis in producers and aerobic respiration in producers,
Section 3-3
1. What is the key concept for this section?
2. Define and distinguish between a food chain and a food web.
3. Explain what happens to energy as it flows through a food chains and a food web.
a. Each trophic level in a food chain or web contains a certain amount of biomass. In a food
4. Distinguish between gross primary productivity (GPP) and net primary productivity (NPP) and
explain their importance.
a. Gross primary productivity (GPP) is the rate at which an ecosystem’s producers (usually
plants) convert solar energy into chemical energy in the form of biomass found in their
tissues.
Chapter 3: Ecosystems: What Are They and How Do They Work?
5. What are the two most productive land ecosystems and the two most productive aquatic
ecosystems?
Section 3-4
9. What is the key concept for this section? What happens to matter in an ecosystem? What is a nutrient
cycle? Explain how nutrient cycles connect past, present, and future life. Summarize the unique
properties of water. Describe the hydrologic cycle, or water cycle. What three major processes are
involved in the water cycle? What is surface runoff? Define groundwater. What is an aquifer? What
percentage of the earth’s water supply is available to humans and other species as liquid freshwater?
Explain how human activities are affecting the water cycle. Explain how clearing a rain forest can
affect local weather and climate. Describe the carbon, nitrogen, and phosphorus cycles, and explain
how human activities are affecting each cycle.
See pages 50–57.
• The key concept for this section is:
o Matter, in the form of nutrients, cycles within and among ecosystems and the biosphere,
and human activities are altering these cycles.
• Matter in an ecosystem.
• A nutrient cycle – The elements and compounds that make up nutrients move continually through
air, water, soil, rock, and living organisms within ecosystems in cycles called biogeochemical
cycles, or nutrient cycles.
• How nutrient cycles connect past, present and future – Nutrient cycles connect past, present, and
future forms of life. Some of the carbon atoms in your skin may once have been part of an oak
leaf, a dinosaur’s skin, or a layer of limestone rock. Your grandmother, rock star Bono, or a
hunter–gatherer who lived 25,000 years ago may have inhaled some of the same nitrogen
molecules that you just inhaled.
• Unique properties of water include that it:
Is held together by hydrogen bonds.
• The hydrological cycle, or water cycle, collects, purifies, and distributes this supply of water, as
shown in figure 3-14. Its three major processes are evaporation, precipitation and transpiration.
• Most precipitation falling on terrestrial ecosystems becomes surface runoff. This water flows into
streams, rivers, lakes, wetlands, and oceans, from which it can evaporate to repeat the cycle. Some
precipitation seeps into the upper layers of soils where it is used by plants, and some evaporates
from the soils back into the atmosphere. Some precipitation also sinks through soil into
underground layers of rock, sand, and gravel called aquifers, where it is stored as groundwater.
Some precipitation is converted to ice that is stored in glaciers, usually for long periods of time.
Instructor’s Manual for Environmental Science, 15th edition
▪ Clearing vegetation which increases runoff and decreases replenishment of
groundwater supplies.
▪ Draining wetlands which interferes with flood control.
• Clearing vegetation can alter weather patterns by reducing transpiration, especially in dense
tropical rain forests where vegetation is the primary source of local rainfall. Cutting down the forest
• Human activity removes large amounts of phosphate from the earth to make fertilizer and reduces
phosphate levels in tropical soils by clearing forests. Topsoil that is eroded from fertilized crop
fields, lawns, and golf courses carries large quantities of phosphate ions into streams, lakes, and
oceans. There they stimulate the growth of producers such as algae and various aquatic plants.
Phosphate-rich runoff from the land can produce huge populations of algae, which can upset
chemical cycling and other processes.
Section 3-5
10. What is the key concept for this section? Describe three ways in which scientists study ecosystems.
Explain why we need much more basic data about the structure and condition of the world’s
ecosystems. Distinguish between the Holocene and Anthropocene eras. List nine planetary boundaries
that scientists have identified. Which three of these boundaries have already been exceeded, according
to these scientists? What are this chapter’s three big ideas? How are the three scientific principles of
sustainability showcased in tropical rain forests?
See page 57-58.
• CONCEPT 3-5 Scientists use both field research and laboratory research, as well as
mathematical and other models to learn about ecosystems.
• Three approaches ecologists use to learn about ecosystems: field research, laboratory research,
and ecosystem models.
• We need more baseline ecological data about living components and physical and chemical
conditions in order to evaluate ecosystems are functioning, to see how they are changing, and to
develop effective strategies for preventing or slowing their degradation.
Chapter 3: Ecosystems: What Are They and How Do They Work?
• Here are this chapter’s three big ideas:
o Life is sustained by the flow of energy from the sun through the biosphere, the cycling of
nutrients within the biosphere, and gravity.
o Some organisms produce the nutrients they need, others survive by consuming other
Critical Thinking
1. How would you explain the importance of tropical rainforests (Core Case Study) to people who think
that such forests have no connections with their lives?
Students might focus on the role these forests play as carbon sinks, tying up carbon that might
otherwise contribute to climate change. Additionally, the biodiversity in rainforests affects the lives of
2. Explain why: (a) the flow of energy through the biosphere depends on the cycling of nutrients, and (b)
the cycling of nutrients depends on gravity (Concept 3-1B).
(a) The earth is closed to significant inputs of matter and has a fixed supply of nutrients that must be
recycled to support life. Energy flows through living things in their feeding interactions, starting with
3. Explain why microbes are so important. What are two ways in which they benefit your health or
lifestyle? Write a brief description of what you think would happen to you if microbes were eliminated
from the earth.
Microbes that decompose dead and decaying plant and animal materials are vital to all ecosystems.
Their importance is often ignored, but without them life would not exist. They consist of many
4. Make a list of the food you ate for lunch or dinner today. Trace each type of food back to a particular
producer species. Describe the sequence of feeding levels that led to your feeding.
5. Use the second law of thermodynamics to explain why many poor people in less-developed countries
live on a mostly vegetarian diet.
The second law of thermodynamics states that in any energy transformation, the energy quality will
6. How might your life and the lives of any children or grandchildren you might have be
affected if human activities as a whole continue to intensify the water cycle?
Answers may vary but it’s important to realize that humans are intervening in the natural
working of the water cycle in two important ways: 1) polluting water; 2) using more water
than is being recharged.
What would happen to an ecosystem if (a) all of its decomposers and detritus feeders were
eliminated, (b) all of its producers were eliminated, and (c) all of its insects were eliminated?
Could a balanced ecosystem exist with only producers and decomposers and no consumers
such as humans and other animals? Explain.
7. For each of the earth’s nine major planetary boundaries (Figure 3.A), describe how our
exceeding that boundary might affect (a) you, (b) any child you might have, and (c) any
grandchild you might have.
With all of the planetary boundaries, time will only intensify the long-range and possibly
Global Environment Watch Exercise
Search for “nitrogen cycle” and look for information how humans are affecting the nitrogen cycle.
Specifically look for impacts on the atmosphere and human health from emissions of nitrogen oxides
and the harmful ecological effects of the runoff of nitrate fertilizers into rivers and lakes. Make a list of
these impacts and use this information to review your daily activities. Find three things that you do
regularly that contribute to these impacts.
Answers will vary but could include:
(1) burning fuel – ways to contribute include driving a car, using electric appliances, purchasing goods
that required energy to be produced
(2) using inorganic fertilizers – ways to contribute include purchasing food that is not grown
organically, fertilizing the garden or lawn, and improper maintenance of septic systems.
Data Analysis
Recall that net primary productivity (NPP) is the rate at which producers can make the chemical energy that
is stored in their tissues and that is potentially available to other organisms (consumers) in an ecosystem. In
Figure 3.15, it is expressed as units of energy (kilocalories, or kcal) produced in a given area (square meters,
or m2) over a period of time (a year). Look again at Figure 3.12 and consider the differences in NPP among
various ecosystems.
Then answer the following questions:
1. What is the approximate NPP of a tropical rain forest in kcal/m2/yr?
• About 9000
Which terrestrial ecosystem produces about one-third of that rate?
• Savannah
Which aquatic ecosystem has about the same NPP as a tropical rain forest?
2. Early in the 20th century, large areas of temperate forestland in the United States were cleared to make
way for agricultural land. For each unit of this forest area that was cleared and replaced by farmland,
about how much NPP was lost?
3. Why do you think deserts and grasslands have dramatically lower NPP than swamps and marshes?
4. About how many times more NPP do estuaries produce, compared to lakes and streams? Why do you
think this is so?
