Chapter 3
Ecosystems: What Are They and How Do They Work
Summary
1.
Ecology is the study of connections in nature.
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.
Outline
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 half of all biodiversity. Already more than half of this area has been destroyed, and degradation
is increasing. This will cause a reduction in biodiversity, an increase in climate change, and changes in
regional weather patterns.
B. Ecology is the study of connections in the natural world.
1. An organism is any form of life. The cell is the basic unit of life in organisms.
2. Organisms are classified as either eukaryotic or prokaryotic based on the presence or absence of a
membrane-bound nucleus.
3. Organisms are classified into species, which groups organisms similar to each other together.
4. Sexually reproducing organisms are classified as a species if, under natural conditions, they can
potentially breed with one another and produce live, fertile offspring.
5. The tiny microbes rule the world; they are unseen by the naked eye but keep the natural world
operating.
6. About 1.4 million species have been identified, 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 species occupying a specific area.
Genetic diversity explains why these individuals may not behave nor look exactly alike. The habitat is the
place where a population or an individual usually lives. Its distribution or range is the area over which a
species may be found.
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D. A community represents populations of different species living and interacting in a specific area. A
biological community 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
insects in maintaining the environmental balance.
E. An ecosystem 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 layers 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 about 11 miles above sea level.
3. The stratosphere lies 11–30 miles above the troposphere and filters the sun’s harmful radiation.
4. The hydrosphere consists of earth’s water, found in liquid water, ice, and water vapor.
5. The lithosphere is the crust and upper mantle of the earth.
B. The biosphere includes most of the hydrosphere, parts of the lower atmosphere and upper lithosphere.
C. Ecology’s goal is to understand the interactions in the earth’s global skin of air, water, soil, and organisms.
D. Sun, cycles of matter, and gravity sustain life on earth.
1. The one-way flow of high-quality solar energy through materials and living things (as they eat)
produces low-quality energy. Energy can’t be recycled.
2. Matter cycles through parts of the biosphere.
3. Gravity causes the downward movement of chemicals as matter cycles through the earth.
E. Solar energy just passes through the earth as electromagnetic waves.
F. As solar radiation interacts with the earth, infrared radiation is produced. Greenhouse gases trap the heat
and warm the troposphere. This natural greenhouse effect makes the planet warm enough to support life.
3-3 What are the major components of an ecosystem?
A. The major components of ecosystems are abiotic (nonliving) water, air, nutrients, solar energy, and biotic
(living) plants, animals, and microbes.
B. Populations have a range of tolerance to physical and chemical environments.
1. Law of tolerance: The distribution of a species in an ecosystem is determined by the levels of one or
more physical or chemical factors being within the range tolerated by that species.
a. The limiting factor principle states that too much or too little of any abiotic factor can limit or
prevent growth of a population.
C. The major biological components of ecosystems are the producers/autotrophs that are self-feeders and the
consumers/heterotrophs.
1. Autotrophs make their own food from compounds in the environment.
2. Consumers, or heterotrophs, feed on other organisms or their remains.
a. Decomposers break down organic detritus (bacteria/fungi) into simpler inorganic compounds.
b. Omnivores feed on both plants and animals.
c. Carnivores feed on animals.
d. Detritivores feed on dead organic matter and break it down into smaller molecules.
e. Herbivores feed on plants.
3. Glucose and other organic compounds are broken down and energy is released by the process of aerobic
respiration, 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 oxygen. This process is
called anaerobic respiration, or fermentation. SCIENCE FOCUS: The importance of microbes to our
life.
5. Matter is recycled; there is a one-way flow of energy.
3-4 What happens to energy in an ecosystem?
A. Food chains and food webs 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 chain.
1. Trophic levels are feeding levels for organisms within an ecosystem.
a. Producers belong to the first tropic level.
b. Primary consumers belong to the second tropic level.
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C.
D.
E.
F.
G.
H.
c. Secondary consumers belong to the third tropic level.
d. Detritivores and decomposers process detritus from all trophic levels.
2. Food webs are complex networks of interconnected food chains.
Energy flow in a food web/chain decreases at each succeeding organism in a chain or web.
The dry weight of all organic matter within the organisms of a food chain/web is called biomass.
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 10% being typical.
The greater number of trophic levels in a food chain, the greater loss of usable energy.
The pyramid of energy flow visualizes the loss of usable energy through a food chain. The lower levels of
the trophic pyramid support more organisms.
Production of biomass takes place at different rates among different ecosystems.
1. The rate of an ecosystem’s biomass production is the gross primary productivity (GPP).
2. Some of the biomass must be used for the producers’ own respiration. Net primary productivity (NPP)
measures how fast producers can provide biomass needed by consumers in an ecosystem.
3. Ecosystems and life zones differ in their NPP.
The planet’s NPP limits the numbers of consumers who can survive on earth.
3-5 What happens to matter in an ecosystem?
A. Nutrient cycles/biogeochemical cycles are global recycling systems that interconnect all organisms.
1. These cycles include the carbon, oxygen, nitrogen, phosphorus, and water cycles.
B. The water/hydrologic cycle collects, purifies, and distributes the earth’s water in a vast global cycle.
1. Solar energy evaporates water, and the water returns as rain/snow.
2. Some water becomes surface runoff, returning to streams/rivers.
3. Water is the major form of transporting nutrients within and between ecosystems.
C. The water cycle is altered by man’s activities. SCIENCE FOCUS: the unique properties of water.
1. We withdraw large quantities of fresh water.
2. We clear vegetation and increase runoff, reduce filtering and increase flooding.
3. We add nutrients like fertilizers and modify the quality of the water.
4. The earth’s water cycle may be speeding up due to a warmer climate. This could change global
precipitation patterns and may intensify global warming (water vapor increases in the troposphere).
D. The carbon cycle circulates through the biosphere.
1. CO2 gas is an important temperature regulator on earth.
2. Photosynthesis and aerobic respiration circulates carbon in the biosphere.
3. Fossil fuels contain carbon.
4. Carbon recycles through the oceans. Oceans act as a carbon sink, but when warming occurs they release
carbon dioxide.
5. Excess carbon dioxide in the atmosphere has contributed to global warming.
E. Nitrogen is recycled through the earth’s systems by different types of bacteria.
1. The nitrogen cycle converts 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 ammonia in nitrogen fixation.
b. Special bacteria convert ammonia in the soil to nitrite ions and nitrate ions; the latter is used by
plants as a nutrient. This process is nitrification.
c. Decomposer bacteria convert detritus into ammonia and water-soluble salts in ammonification.
d. In denitrification, nitrogen leaves the soil. Anaerobic bacteria in soggy soil and bottom sediments of
water areas convert NH3 and NH4+ back into nitrite and nitrate 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 nitric oxide into the atmosphere; it can be converted to NO 2 gas and nitric
acid, and it can return to the earth’s surface as acid rain.
b. Nitrous oxide that comes from livestock, wastes, and inorganic fertilizers we use on the soil can
warm the atmosphere and deplete the ozone layer.
c. We destroy forest, grasslands, and wetland and, thus, release large amounts of nitrogen into the
atmosphere.
d. We pollute aquatic ecosystems with agricultural runoff and human sewage.
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e. We remove nitrogen from topsoil with our harvesting, irrigating, and land-clearing practices.
f. Increased input of nitrogen into air, soil, and water is affecting the biodiversity toward species that
can thrive on increased supplies of nitrogen nutrients.
F. We need to use phosphorus-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 fertilizer to encourage plant growth.
2. Phosphorus also limits growth of producers in freshwater streams and lakes due to low solubility in
water.
G. Man interferes with the phosphorous cycle in harmful ways.
1. We mine phosphate rock to produce fertilizers and detergents.
2. We cut down tropical forests and, thereby, reduce the phosphorus in tropical soils.
3. We compromise aquatic systems with animal waste 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 anaerobic decomposition occurs.
2. Some marine algae produce dimethyl sulfide (DMS). DMS acts as nuclei for condensation of water
found in clouds. This can affect the cloud cover and climate.
3. Sulfur compounds can be converted to sulfuric acid, which falls as acid deposition.
4. Burning coal 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 field research, observing and measuring the ecosystem structure and function.
B. New technologies such as remote 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 chambers to study ecosystems
(laboratory research). This allows control of light, temperature, CO 2, humidity, and other variables.
D. Field and laboratory studies must be coupled together for a more complete picture of an ecosystem.
E. Systems analysis develops mathematical and other models 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.
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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)
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trophic level (p. 58)
troposphere (p. 54)
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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 indepth 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.
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6.
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.
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
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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.
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.
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.
10. 8.
Humans trying to work with ecosystems: composting; organic gardening; land reclamation; rebuilding
degraded lands; tree-planting projects.
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