Chapter 3
Ecosystems: What Are They and How Do They Work
Key Terms
abiotic (p. 57)
aerobic respiration (p. 59)
anaerobic respiration (p. 59)
aquatic life zones (p. 56)
atmosphere (p. 54)
autotrophs (p. 58)
biological community (p. 53)
biomass (p. 62)
biomes (p. 55)
biosphere (p. 53)
biotic (p. 57)
carbon cycle (p. 67)
carnivores (p. 59)
chemosynthesis (p. 59)
community (p. 53)
consumers (p. 59)
decomposers (p. 59)
detritivores (p. 59)
distribution (p. 53)
ecological efficiency (p. 62)
ecology (p. 52)
ecosystem (p. 53)
fermentation (p. 59)
food chain (p. 61)
food web (p. 62)
genetic diversity (p. 53)
gross primary productivity
(GPP) (p. 64)
habitat (p. 53)
herbivores (p. 59)
hydrologic (water) cycles (p.
65)
hydrosphere (p. 55)
limiting factor (p. 58)
limiting factor principle (p. 58)
natural greenhouse effect (p.
56)
net primary productivity
(NPP)
(p. 64)
nitrogen cycle (p. 68)
nutrient (biogeochemical)
cycles (p. 65)
omnivores (p. 59)
photosynthesis (p. 58)
population (p. 52)
primary consumers (p. 59)
producers (p. 58)
pyramid of energy flow (p. 62)
range of tolerance (p. 57)
secondary consumers (p. 59)
species (p. 51)
stratosphere (p. 54)
sulfur cycle (p. 70)
third and higher level
consumers (p. 59)
trophic level (p. 58)
troposphere (p. 54)
Study Guide
3-1 What is ecology?
CORE CASE STUDY. Tropical rainforests make up only 2% of the earth’s land surface, but account for
more than ________ of all biodiversity. Already more than half of this area has been
__________________, and degradation is increasing. This will cause a reduction in
____________________, an increase in climate change, and changes in regional ______________ patterns.
B. Ecology is the study of _____________________ in the natural world.
1. An __________________ is any form of life. The ________ is the basic unit of life in organisms.
2. Organisms are classified as either eukaryotic or _____________________ based on the presence or
absence of a membrane-bound _______________.
3. Organisms are classified into ______________, which groups organisms similar to each other together.
4. Sexually reproducing organisms are classified as a species if, under natural conditions, they can
potentially __________ with one another and produce live, _______________ offspring.
5. The tiny _______________ rule the world; they are unseen by the naked eye but keep the natural world
operating.
6. About 1.4 million species have been ____________________, but estimates of number of species range
from 3.6 million to 100 million.
C. A population consists of a group of interacting individuals of the same ______________ occupying a
specific area. _______________ diversity explains why these individuals may not behave nor look exactly
alike. The ______________ is the place where a population or an individual usually lives. Its distribution
or ___________ is the area over which a species may be found.
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D. A community represents populations of ________________ species living and interacting in a specific
area. A biological __________________ consists of all the populations of different species interacting and
living in a specific area; this is a network of plants, animals, and microorganisms. SCIENCE FOCUS: the
importance of ______________ in maintaining the environmental balance.
E. An ___________________ is a community of different species interacting with each other and with their
nonliving environment of matter and energy. All of the earth’s diverse ecosystems comprise the biosphere.
3-2 What keeps us and other organisms alive?
A. Various interconnected spherical _____________ make up the earth’s life-support system.
1. The atmosphere is the thin membrane of air around the planet.
2. The_______________________ troposphere is the air layer up to about 11 miles above sea level.
3. The ___________________________ lies 11–30 miles above the troposphere and filters the sun’s
harmful radiation.
4. The _________________________e consists of earth’s water, found in liquid water, ice, and water
vapor.
5. The _______________________ is the crust and upper mantle of the earth.
B. The _______________________ includes most of the hydrosphere, parts of the lower atmosphere and
upper lithosphere.
C. Ecology’s goal is to understand the __________________________s in the earth’s global skin of air,
water, soil, and organisms.
D. Sun, ______________ of matter, and gravity sustain life on earth.
1. The _______-way flow of high-quality solar energy through materials and living things (as they eat)
produces low-quality energy. Energy can’t be ___________________.
2. Matter cycles through parts of the __________________.
3. __________________ causes the downward movement of chemicals as matter cycles through the earth.
E. Solar energy just passes through the earth as __________________________________________ waves.
F. As solar radiation interacts with the earth, infrared radiation is produced. ________________________
gases trap the heat and warm the troposphere. This natural greenhouse effect makes the planet __________
enough to support life.
3-3 What are the major components of an ecosystem?
A. The major components of ecosystems are ______________ (nonliving) water, air, nutrients, solar energy,
and _____________ (living) plants, animals, and microbes.
B. Populations have a range of __________________ to physical and chemical environments.
1. Law of tolerance: The distribution of a species in an ecosystem is determined by the _____________ of
one or more physical or chemical factors being within the range tolerated by that species.
a. The limiting ______________ principle states that too much or too little of any abiotic factor can
limit or prevent _____________ of a population.
C. The major biological components of ecosystems are the ____________________/autotrophs that are selffeeders and the consumers/____________________________.
1. Autotrophs make their own __________ from compounds in the environment.
2. ___________________, or heterotrophs, feed on other organisms or their remains.
a. ______________________ break down organic detritus (bacteria/fungi) into simpler inorganic
compounds.
b. _____________________ feed on both plants and animals.
c. ________________________ feed on animals.
d. __________________________ feed on dead organic matter and break it down into smaller
molecules.
e. _______________________ feed on plants.
3. _______________ and other organic compounds are broken down and energy is released by the process
of aerobic _________________________, the use of oxygen to convert organic matter back to carbon
dioxide and water.
4. Some decomposers are able to break down organic compounds without using _____________. This
process is called ______________________ respiration, or fermentation. SCIENCE FOCUS: The
importance of ____________________ to our life.
5. Matter is ____________________; there is a _______-way flow of energy.
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3-4 What happens to energy in an ecosystem?
A. Food chains and food __________ help us understand how eaters, the eaten, and the decomposed are
interconnected in an ecosystem.
B. The sequence of organisms as they are eaten is a food __________.
1. _________________ levels are feeding levels for organisms within an ecosystem.
a. Producers belong to the __________ trophic level.
b. _________________ consumers belong to the second trophic level.
c. Secondary consumers belong to the __________ trophic level.
d. __________________________ and decomposers process detritus from all trophic levels.
2. Food webs are complex networks of ____________________________ food chains.
C. Energy flow in a food web/chain ___________________ at each succeeding organism in a chain or web.
D. The dry weight of all organic matter within the organisms of a food chain/web is called
_________________. Ecological efficiency is the term that describes the percentage of usable energy
transferred as biomass from one trophic level to another and ranges from 2 to 40% with _____% being
typical.
E. The greater number of trophic levels in a food chain, the greater loss of ________________ energy.
F. The ___________________ of energy flow visualizes the loss of usable energy through a food chain. The
lower levels of the trophic pyramid support more organisms.
G. Production of biomass takes place at different ____________ among different ecosystems.
1. The rate of an ecosystem’s biomass production is the __________ primary productivity (GPP).
2. Some of the biomass must be used for the producers’ own _____________________________. Net
primary productivity (NPP) measures how fast producers can provide biomass needed by
__________________ in an ecosystem.
3. Ecosystems and life ___________ differ in their NPP.
H. The planet’s NPP ________________ the numbers of consumers who can survive on earth.
3-5 What happens to matter in an ecosystem?
A. _____________________ cycles/biogeochemical cycles are global recycling systems that interconnect all
organisms.
1. These cycles include the carbon, oxygen, _____________________, phosphorus, and water cycles.
B. The water/hydrologic cycle collects, ____________________, and distributes the earth’s water in a vast
global cycle.
1. Solar energy _______________________ water, and the water returns as rain/snow.
2. Some water becomes ___________________ runoff, returning to streams/rivers.
3. Water is the major form of transporting _______________________ within and between ecosystems.
C. The water cycle is altered by man’s activities. SCIENCE FOCUS: the unique properties of water include
_______
1. We withdraw large quantities of _____________ water.
2. We clear vegetation and increase runoff, reduce ______________________ and increase flooding.
3. We add nutrients like _________________________ and modify the quality of the water.
4. The earth’s water cycle may be speeding up due to a ________________ climate. This could change
global _patterns and may _____________________ global warming (water vapor increases in the
troposphere).
D. The carbon ___________ circulates through the biosphere.
1. _______ gas is an important temperature regulator on earth.
2. __________________________________ and aerobic respiration circulates carbon in the biosphere.
3. Fossil ___________ contain carbon.
4. Carbon recycles through the oceans. Oceans act as a carbon _________, but when warming occurs they
release carbon dioxide.
5. Excess carbon dioxide in the atmosphere has contributed to global ___________________.
E. Nitrogen is recycled through the earth’s systems by different types of ___________________.
1. The nitrogen cycle ____________________ nitrogen (N2) into compounds that are useful nutrients for
plants and animals.
2. The nitrogen cycle includes these steps:
a. Specialized bacteria convert gaseous nitrogen to __________________ in nitrogen fixation.
Ecosystems: What Are They and How Do They Work?
23
b. Special bacteria convert ammonia in the soil to nitrite ions and ______________ ions; the latter is
used by plants as a nutrient. This process is __________________________.
c. Decomposer bacteria convert detritus into ammonia and water-soluble salts in
_________________________________.
d. In ___________________________________, nitrogen leaves the soil. Anaerobic bacteria in soggy
soil and bottom sediments of water areas convert NH3 and NH4+ back into nitrite and
_______________ ions, then nitrogen gas and nitrous oxide gas are released into the atmosphere.
3. Human activities affect the nitrogen cycle.
a. In burning fuel, we add _____________ ___________ into the atmosphere; it can be converted to
NO2 gas and nitric acid, and it can return to the earth’s surface as _________ rain.
b. Nitrous oxide that comes from _____________________, wastes, and inorganic fertilizers we use
on the soil can warm the atmosphere and deplete the ___________ layer.
c. We destroy forest, grasslands, and wetland and, thus, _________________ large amounts of
nitrogen into the atmosphere.
d. We pollute aquatic ecosystems with agricultural runoff and human _______________.
e. We remove nitrogen from topsoil with our harvesting, ____________________, and land-clearing
practices.
f. Increased input of nitrogen into air, soil, and water is affecting the _______________________
toward species that can ______________ on increased supplies of nitrogen nutrients.
F. We need to use _______________________-based fertilizers because the phosphorus cycle is much slower
in moving through the earth’s water, soil, and organisms and is often the limiting factor for plant growth.
1. Phosphorus is used as a _______________________________ to encourage plant growth.
2. Phosphorus also limits growth of producers in freshwater streams and lakes due to low
________________________ in water.
G. Man interferes with the phosphorous _____________ in harmful ways.
1. We mine phosphate rock to produce fertilizers and detergents.
2. We cut down tropical _____________________ and, thereby, reduce the phosphorus in tropical soils.
3. We compromise aquatic systems with animal ____________ runoff and human sewage.
H. Sulfur cycles through the earth’s air, water, soil, and living organisms.
1. Natural sources of sulfur are hydrogen sulfide, released from volcanoes, swamps, bogs, and tidal flats
where _________________________ decomposition occurs.
2. Some marine algae produce dimethyl sulfide (DMS). DMS acts as _________________ for
condensation of water found in clouds. This can affect the cloud cover and climate.
3. Sulfur compounds can be converted to sulfuric _________, which falls as acid deposition.
4. Burning ________l and oil, refining oil, and the production of some metals from ores all add sulfur to
the environment.
3-6 How do ecologists study ecosystems?
A. Ecologists do __________ research, observing and measuring the ecosystem structure and function.
B. New technologies such as _______________ sensing and geographic information systems (GISs) gather
data that is fed into computers for analysis and manipulation of data.
C. Ecologist use tanks, greenhouses, and controlled indoor and outdoor ___________________ to study
ecosystems (laboratory research). This allows control of light, temperature, CO 2,
_____________________, and other variables.
D. Field and laboratory studies must be ______________ together for a more complete picture of an
ecosystem.
E. Systems analysis develops mathematical and other _______________ that simulate ecosystems that are
large and very complex and can’t be adequately studied with field and laboratory research. This allows the
analysis of the effectiveness of various alternate solutions to environmental problems and can help
anticipate environmental surprises.
Summary
1.
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Ecology is the study of connections in nature.
Instructor's Manual: Chapter 3
2.
Life on earth is sustained by the one-way flow of high-quality energy from the sun, by the cycling of matter,
and by gravity.
3.
Matter, energy, and life are the major components of an ecosystem.
4.
Energy in an ecosystem decreases in amount to each succeeding organism in a food chair or web.
5.
Soil is a complex mixture of eroded rock, mineral nutrients, water, air, decaying organic matter, and billions of
living organisms. It covers most of the earth and provides nutrients for plant growth. Soils are formed by a
breaking down of rock, decomposing surface litter and organic matter. Bacteria and other decomposer
microorganisms break down some of soil’s organic compounds into simpler inorganic compounds.
6.
Matter is recycled through the earth’s ecosystem of air, land, water, and living organisms. This vast global
recycling system is composed of nutrient cycles.
7.
Scientists study ecosystems through the use of aquarium tanks, greenhouses, and controlled indoor and outdoor
chambers. Specific variables are carefully controlled, like temperature, light, carbon dioxide, and humidity.
8.
Two principles of sustainability found from learning how nature works are the law of conservation of matter
and the two laws of thermodynamics.
Ecosystems: What Are They and How Do They Work?
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Attitudes and Values
1.
Do you feel you are part of an ecosystem? What niche do you fill?
2.
Do you hold any particular feelings for producers? Consumers? Decomposers?
3.
How do you feel when you think of a coyote eating a rabbit? How do you feel when you think of humans
eating hamburgers?
4.
Do humans have a right to domesticate and eat other species?
5.
Do you feel there will always be enough matter and energy for the survival of all individuals of all species?
Will the carrying capacity of the Earth be expanded by new technologies? Will nature be able to continually
absorb "waste-products" from human societies?
6.
Do you feel any responsibility to protect natural ecosystems? Would you support the preservation of
representative ecosystems? If so, on what basis?
Additional Video Resources
Blue Planet (Video series from Discovery Channel, 2001)
Mammoth series, five years in the making, taking a look at the rich tapestry of life in the world's oceans.
http://dsc.discovery.com/convergence/blueplanet/blueplanet.html
Journey to Planet Earth (Video series from PBS, 2003)
http://www.pbs.org/journeytoplanetearth/
The Living Planet — A Portrait of the Earth (hosted by David Attenborough – four discs)
This series discusses the biomass and life in a variety of ecosystems spanning many of the environments found on
Earth.
Planet Earth (BBC Series, 5 discs)
An exploration of global ecosystems.
http://www.bbc.co.uk/nature/animals/planetearth/
Web Resources
Food Chain Curriculum Research Links
Interactive exploration of food webs and food chains.
http://www.picadome.fcps.net/lab/currl/food_chain/default.htm
The Hydrologic Cycle
Interactive exploration of various phases of the water cycle.
http://polaris.umuc.edu/cvu/envm/hydro/hydro.html
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Suggested Answers to End of Chapter Questions
The following are examples of the material that should be contained in possible student answers to the end of
chapter Critical Thinking 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.
1.
List three ways you could apply Concept 3-4B and Concept 3-5 to making your lifestyle more environmentally
sustainable.
Answers may vary, but may include adopting a vegetarian diet, eating from lower trophic levels (especially in
the case of many fish species that are top carnivores), and composting to recycle waste back into productive
systems.
2.
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 people around the
world because of the medicines that have been discovered there. And finally, weather patterns may be disrupted
when the natural holding capacity of the forest is diminished and water simply runs off.
3.
Explain why: (a) the flow of energy through the biosphere (Concept 3-2) depends on the cycling of nutrients,
and (b) the cycling of nutrients depends on gravity.
a.
b.
4.
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, the basic components of
which are recycled when plants photosynthesize, making molecules of sugars to be consumed.
Gravity holds the atmosphere close to the earth, and enables the cycling of chemicals through air, water,
soil, and organisms.
Explain why microbes are so important. List two beneficial and two harmful effects of microbes on your health
and lifestyle.
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 different types of
bacteria and fungi that secret enzymes that break down materials from other organisms into smaller
components, and this enables nutrients to be recycled through the ecosystem as they are taken up from the soil
and water by the producers. Two beneficial effects of microbes are their role in the recycling of matter and
ensuring that there is no build-up of waste in the natural world. They are also used in the production of foods
like cheese and yogurt. Two harmful effects of microbes are that they can cause diseases that can be detrimental
to an individual’s health, and they can cause food to decay and be rendered unfit for human consumption.
5.
Make a list of the food you ate for lunch or dinner today. Trace each type of food back to a particular producer
species.
Student answers will vary but could include some of the following: if a student had a burger and fries for lunch
the bread can be traced back to wheat, the meat to cows and the grain that was fed to them, the lettuce and
tomatoes to the original plants, and the fries to potatoes.
Ecosystems: What Are They and How Do They Work?
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6.
Use the second law of thermodynamics (pp. 45–46) to explain why many poor people in developing countries
live on a mostly vegetarian diet.
The second law of thermodynamics states that in any energy transformation, the energy quality will always
decrease and we will end up with less usable energy than we began with. Much of the degraded energy is lost in
the form of heat. Energy is lost at each trophic level in a food chain by as much as 90%. The earth can support
more people if they ate at a lower level on the food chain by consuming grains, vegetables, and fruits directly. If
these crops are fed to animals and pass through another trophic level, more energy is lost in the process. From
an economic perspective it is also more costly to buy meat from cattle than it is to buy the grain that was used to
feed them. People who live in rich developed countries can afford to live on a diet that is high in meat.
However, people in poorer, less developed countries cannot afford to buy meat and live primarily on a
vegetarian diet. In doing so they are behaving in a more energy efficient manner and in many cases a healthier
one too.
7.
Why do farmers not need to apply carbon to grow their crops but often need to add fertilizer containing nitrogen
and phosphorus?
The crops that farmers grow obtain the carbon that they need directly from the atmosphere in the form of carbon
dioxide. As the atmosphere is all around us, the farmer does not have to apply carbon to the fields to grow
crops. This is part of the gas phase of the naturally occurring biogeochemical carbon cycle. Although the
nitrogen cycle has a gas phase, plants cannot obtain the nitrogen they need directly from the air and must get it
in the form of inorganic nitrogen compounds produced in the soil by the nitrogen cycle. Often the farmer grows
and harvests crops at such a rate that the nitrogen demand cannot be kept up with by the nitrogen cycle, and so
additional nitrogen in the form of fertilizer has to be added to the field. Similarly, phosphate, which does not
have a gas phase in its very slow biogeochemical cycle, has to be absorbed into plant crops from the soil in the
form of phosphate ions. Again the farmer often grows and harvests crops at such a rate that the naturally
occurring levels of phosphate in the soil are diminished. A fertilizer high in phosphates then has to be applied to
the field to compensate.
8.
What changes might take place in the hydrologic cycle if the earth’s climate becomes: (a) hotter, or (b) cooler?
In each case explain how these changes might affect your lifestyle.
(a) If the climate became hotter the hydrologic cycle could be adversely affected. The cycle could speed up and
change global precipitation patterns, which in turn could affect the severity and frequency of storms, floods, and
droughts. It could also enhance global warming by moving more water vapor into the atmosphere. An
individual’s lifestyle could be affected by lack of water during droughts, too much water during floods causing
landslides and mudslides, or increased exposure to disease causing organisms such as mosquitoes that
reproduce in moist, humid climates that could result from increased rainfall in an area.
(b) If the climate became cooler the hydrologic cycle could slow down and water would take more time to pass
through this natural purification process. More freshwater could be trapped in the form of snow and ice in
glaciers for a longer period of time, removing it from use for drinking, etc. by humans. Precipitation patterns
could also change. A region may experience longer snow and ice coverage during winter, which could
adversely affect the growing season in the area reducing the agricultural output. Another region may experience
diminished rainfall, and water shortages could occur as aquifers are replenished at slower rates. Reduced water
supplies has profound effects on the lifestyle of inhabitants of a region from not having enough drinking water
to water rationing and bans on washing cars and watering lawns.
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9.
What would happen to an ecosystem if: (a) all its decomposers and scavengers were eliminated, (b) all its
producers were eliminated, or (c) all of its insects (Core Case Study, p. 50) were eliminated? Could a balanced
ecosystem exist with only producers and decomposers and no consumers such as humans and other animals?
Explain.
(a) The ecosystem would not be able to recycle matter, and wastes would build up. Eventually other species
would die as no nutrients would be released for plant growth, etc. The ecosystem would be doomed to collapse.
(b) The producers form the base of the food chain, and if they were removed then herbivores and subsequently
carnivores would eventually die out as they both depend on the producers for the energy that sustains them. The
ecosystem would also collapse in this scenario.
(c) Ecosystem collapse is inevitable if all insects were removed. Pollination would cease and plant growth
would be severely affected. Insects are intrinsically linked to two principles of sustainability (renewable solar
energy and recycling of nutrients). Insects play a vital role in implementing these two scientific principles. If
these parts of an ecosystem’s function were removed, sustainability cannot be achieved and the ecosystem
would become unbalanced and unstable.
A fully functioning ecosystem is made up of producers, consumers, and decomposers all interacting with each
other and the abiotic components of the environment. The greater the biodiversity of the ecosystem, the greater
the balance, stability, and sustainability there is in the ecosystem. Could an ecosystem function with only
producers and decomposers? Theoretically a plant could be grown and not eaten by any consumer, then die and
be decomposed by bacteria and fungi. However, neither the producers nor the decomposers would function
fully. Their role in the ecosystem would not be realized and the ecosystem would be out of balance. Consumers
are vital to the sustainable functioning of the ecosystem, whether they are the animals that eat the producers,
such as herbivores, or the animals that help decompose the producers, such as the detritus feeders. In order for a
balanced ecosystem to exist, it needs all of the interacting components—producers, consumers, and
decomposers.
10. List two questions that you would like to have answered as a result of reading this chapter.
Student answers will vary and provide a good starting point for class discussion.
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