5
Environmental Systems and
Ecosystem Ecology
Chapter Objectives
This chapter will help students:
Describe the nature of environmental systems
Define ecosystems and evaluate how living and nonliving entities interact in
ecosystem-level ecology
Outline the fundamentals of landscape ecology, GIS, and ecological modeling
Assess ecosystem services and how they benefit our lives
Compare and contrast how water, carbon, phosphorus, and nitrogen cycle through the
environment
Explain how human impact is affecting biogeochemical cycles
Lecture Outline
I.
Central Case: The Gulf of Mexico’s “Dead Zone”
A. The dead zone is a region in the Gulf of Mexico so depleted of oxygen that it
cannot support marine organisms, a condition called hypoxia.
B. In 2002, the dead zone grew to its largest size ever—22,000 square km (8,500
square miles).
C. The change from productive fishery to dead zone has occurred in the last 30
years. The spread of the hypoxic zone threatens the Gulf’s fishing industry,
one of the most productive fisheries in the United States.
D. Scientists studying the dead zone have determined that fertilizer runoff from
Midwestern farms carried through the watershed by rain and irrigation is a
major cause of the hypoxia.
E. Other important causes include urban runoff, industrial discharge, fossil fuel
combustion, and municipal sewage.
F. Approximately 400 coastal dead zones have been documented worldwide.
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II.
Earth’s Environmental Systems
A. Systems show several defining properties.
1. A system is a network of relationships among a group of parts, elements,
or components that interact with and influence one another through the
exchange of energy, matter, and/or information.
2. Systems receive inputs, process them, and produce outputs. Systems can
have many inputs, processes, and outputs.
3. Sometimes a system’s output can serve as an input to that same system in
a circular process called a feedback loop.
a. In a negative feedback loop, output driving the system in one
direction acts as input that moves the system in the other direction.
Negative feedback loops are not ―bad‖ feedback loops. They generally
stabilize a system.
b. In a positive feedback loop, the output drives the system further
toward one extreme. Positive feedback loops are usually not ―good‖;
they tend to destabilize a system. Positive feedback is rare in natural
systems not impacted by human behavior.
4. The inputs and outputs of a complex natural system often occur
simultaneously, keeping the system constantly active. But even processes
moving in opposite directions can be stabilized by negative feedback so
that their effects balance out, creating a state of dynamic equilibrium.
5. Processes in dynamic equilibrium contribute to homeostasis, where the
tendency of the system is to maintain stable internal conditions.
Sometimes processes have to be viewed over long time periods to see this
stability.
6. It is difficult to fully understand systems by focusing on their individual
components because systems can show emergent properties,
characteristics that are not evident in the system’s components.
7. Systems rarely have well-defined boundaries, so deciding where one
system ends and another begins can be difficult.
8. Systems may exchange energy, matter, and information with other
systems, or may contain or be contained within other systems.
B. Environmental systems interact.
1. Hypoxia in the Gulf of Mexico stems from excess nutrients, primarily
nitrogen and phosphorus, from the Mississippi River watershed. The
watershed includes all the major and minor tributaries which drain all of
the lands that ultimately drain to the Mississippi.
2. Excess nutrients are present in runoff from fertilized agricultural fields,
animal manure, crop residues, sewage, and industrial and automobile
emissions.
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3. The nutrients reach the Gulf, where they boost the growth of
phytoplankton; Dead phytoplankton and waste products sink toward the
bottom providing food for bacterial decomposers, which flourish.
4. The decomposers consume the oxygen in the water through cellular
respiration; bottom dwelling organisms, such as fish and shrimp, suffocate
and die from lack of oxygen.
5. The process of nutrient enrichment, algal bloom, bacterial increase, and
ecosystem deterioration is called eutrophication.
6. Fresh water is less dense than the ocean water so the oxygenated river
water remains in a stratified surface layer and mixes slowly with the Gulf
waters as it flows out to sea.
C. We may perceive Earth’s systems in various ways.
1. The lithosphere is everything that is solid earth beneath our feet.
2. The atmosphere is comprised of the air surrounding our planet.
3. The hydrosphere encompasses all water in surface bodies, underground,
and in the atmosphere.
4. The biosphere consists of all the planet’s living organisms, or biotic
components, and the abiotic portions of the environment with which they
interact.
5. All of these systems interact and their boundaries overlap. Ultimately, it is
impossible to isolate any one in discussing an environmental system.
III.
Ecosystems
1. An ecosystem consists of all interacting organisms and abiotic factors that
occur in a particular place at the same time.
A. Ecosystems are systems of interacting living and nonliving entities.
1. In the early 20th century, scientists began to recognize that biological
entities were tightly intertwined with physical and chemical ones.
2. Energy flows in one direction through ecosystems; most arrives from the
sun, powers the systems and its organisms, and exits in the form of heat.
3. Matter is generally recycled within an ecosystem. When organisms die
and decay, their nutrients remain in the system.
B. Energy is converted to biomass.
1. Autotrophs convert the sun’s energy to produce chemical bond energy in
sugars through photosynthesis. This is gross primary production.
2. Autotrophs use some of their acquired energy to power their own
metabolism by cellular respiration. The remainder is used to generate
biomass and is called net primary production. This is the energy, or
biomass available, for consumption by heterotrophs.
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3. The rate at which biomass is generated is called productivity. Ecosystems
with rapid biomass production are said to have high net primary
productivity.
4. Net primary productivity varies widely over different ecological zones
and according to nutrient availability.
C. Nutrients influence productivity.
1. Nutrients are elements and compounds that organisms consume and
require for survival.
2. A lack of any required nutrient will limit production. In marine systems,
too much nitrogen is generally limiting, and in freshwater, too much
phosphorus is limiting. Several experiments have been conducted to verify
this conclusion.
3. In natural ecosystems, some nutrients always run off land into oceans.
This nutrient input causes high primary productivity in nearshore waters
along continents.
4. When human factors, such as farming, urbanization, and industry, increase
this nutrient load, then eutrophication and hypoxia often occur. This
creates dead zones.
5. The number of dead zones has been rising globally. But the awareness of
the problem has led to runoff reduction in some locations and
improvement is evident.
D. Ecosystems interact spatially.
1. The scale of an ecosystem can vary from being tiny, such as a small
puddle, to being large, like a forest or lake. In some contexts, the entire
biosphere might be viewed as an ecosystem.
2. In areas where ecosystems meet and interact, the transitional zones are
known as ecotones and contain elements from each ecosystem.
E. Landscape ecologists study geographic patterns.
1. Landscape ecology is the broad-scale study of how landscape structure
affects the abundance, distribution, and interactions of organisms. These
structures often encompass multiple ecosystems.
2. A landscape is made up of a spatial array of patches spread over the
landscape in a mosaic and connected at ecotones.
3. The specific habitat needs of an organism will often determine its
distribution across patches and possibly its division into sub-populations
or metapopulations.
4. Conservation biologists are concerned about the fragmentation of
habitats caused by human development pressures.
F. Remote sensing helps us apply landscape ecology.
1. A geographic information system (GIS) is a common tool for landscape
ecology. This is computer software that integrates data from a number of
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sources, including satellite imagery, to provide a broad picture of
landscape use.
2. By integrating different data sets such as geology, hydrology, populations,
and development, GIS helps improve the planning of land use.
G. Modeling helps ecologists understand systems.
1. Models are simplified representations of complex natural processes.
2. Ecological modeling allows scientists to help explain and predict how
ecological systems function.
3. Models are tested by gathering new data from natural systems; scientists
use this data to refine their models.
H. Ecosystems provide vital services.
1. Healthy functioning ecosystems provide services called ecosystem
services, which support our lives and society in profound and innumerable
ways.
2. Earth systems provide natural resources and additional services, such as
water and air purification, soil production, water storage, and waste
disposal.
3. Ecosystems also provide recreational and educational opportunities.
4. One of the most important ecosystem services is the cycling of nutrients.
IV.
Biogeochemical Cycles
A. Nutrients circulate through ecosystems in biogeochemical cycles.
1. Nutrients move through the environment in cycles called nutrient cycles
or biogeochemical cycles.
2. Most nutrients travel through the atmosphere, hydrosphere, and
lithosphere, and from one organism to another, moving between pools, or
reservoirs, remaining in a reservoir for a residence time.
3. The rate at which materials move between reservoirs is termed flux; the
flux between reservoirs can change over time. Flux typically involves
negative feedback loops that promote dynamic equilibrium.
4. A reservoir that releases more material than it accepts is called a source;
one that accepts more than it releases is called a sink.
5. Human activity has changed some flux rates and generated destabilizing
positive feedback loops
B. The water cycle influences all other cycles.
1. Water is the essential medium for most biochemical reactions.
2. The water cycle, or hydrologic cycle, summarizes how water - in liquid,
gaseous, and solid forms - flows through our environment.
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3. Water carries nutrients and pollutants from terrestrial sources to rivers and
then to the oceans.
4. The oceans are the main reservoir, holding 97% of all water on Earth.
Less than 1% of planetary water is usable by humans.
5. Water moves from the hydrosphere into the atmosphere by evaporation, a
conversion of a liquid to a gaseous form. Its flux depends on wind speed
and temperature.
6. Water also moves into the atmosphere via transpiration, the release of
water vapor from the leaves of plants. Both processes are a natural form
of distillation.
7. Water returns from the atmosphere as precipitation to complete the cycle
when water vapor condenses and falls as rain or snow.
8. Some of this water is taken up by plants or animals, or is captured behind
dams for human use; most, however flows as runoff into streams, rivers,
lakes, ponds, and oceans.
9. Some precipitation and surface water soaks down to recharge underground
reservoirs known as aquifers. Aquifers are sponge-like regions of rock
and soil that hold groundwater, water found underground beneath layers
of soil.
10. The upper limit of the groundwater in an uppermost aquifer is referred to
as the water table.
11. Aquifers can hold water for a long time; many are from periods when
Earth’s glaciers melted. They also can take long periods of time, hundreds
or thousands of years, to recharge.
C. Our impacts on the water cycle are extensive.
1. We have dammed rivers and created reservoirs, increasing evaporation.
2. We have changed vegetation patterns, increasing runoff and erosion and
decreasing recharge.
3. Atmospheric pollutants have changed the chemical nature of precipitation.
4. Irrigation, industry, and other human uses have depleted aquifers and
increased evaporation, leading to water shortages in some parts of the
world.
5. Water shortages are giving rise to conflicts throughout the world and are
predicted to increase.
D. The carbon cycle circulates a vital organic nutrient.
1. Carbon is an ingredient in carbohydrates, fats, proteins, bones, and
cartilage, as well as in the shells of all living things.
2. The carbon cycle describes the routes carbon atoms take through the
environment.
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3. Producers, including plants, algae, and cyanobacteria, pull carbon dioxide
out of the atmosphere and out of surface water to use in photosynthesis.
4. During respiration, producers, consumers, and decomposers break down
carbohydrates to produce carbon dioxide and water.
5. A portion of the carbon taken in by organisms is stored in tissues. The
abundance of plants makes these a large reservoir of carbon and that level
is being intensively studied to assess its impact on global climate change.
6. Sedimentary rock is the largest reservoir in the carbon cycle.
7. Organisms that have died can settle in sediments in oceans or wetlands
and be converted by pressure to fossil fuels or rock.
8. The oceans are the second largest reservoir of carbon. They accumulate
carbon from the atmosphere, runoff, and dead marine organisms.
9. Scientists are studying whether the oceans can offset the increased carbon
dioxide in the atmosphere, but the increased acidity that results may limit
the uptake.
E. We are shifting carbon from the lithosphere to the atmosphere.
1. As we mine fossil fuel deposits and cut or burn vegetation, we remove
carbon from reservoirs and increase the net flux into the atmosphere.
2. This ongoing flux of carbon into the atmosphere is a major force behind
global climate change.
3. Atmospheric scientists remain baffled by the ―missing carbon sink,‖ the
roughly 1–2 billion metric tons of carbon unaccounted for (out of
approximately 7) that should be in the atmosphere due to fossil fuel
combustion and deforestation.
F. The phosphorus cycle involves mainly lithosphere and ocean.
1. The element phosphorus is a key component of DNA, RNA, ATP, and cell
membranes.
2. Phosphorus is primarily found in rocks, soil, sediments, and oceans; the
weathering of rocks releases phosphates into water at a very low rate of
flux.
3. The phosphorus cycle has no appreciable atmospheric component.
4. Concentrations of available phosphorus in the environment are very low;
this is often a limiting factor for producers.
G. We affect the phosphorus cycle.
1. We mine rocks for phosphorus to make fertilizers, and our sewage
discharge and agricultural runoff are high in phosphates.
2. These additions of phosphorus can cause rapid increases of biomass in soil
and eutrophication, leading to murkier waters and altering the structure
and function of aquatic ecosystems.
H. The nitrogen cycle involves specialized bacteria.
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1. Nitrogen makes up 78% of the atmosphere and is the sixth most abundant
element on Earth.
2. The nitrogen cycle involves chemically inert nitrogen gas, which most
living organisms cannot use. This makes the atmosphere the major
reservoir for nitrogen.
3. Lightning, highly specialized bacteria, and human technology are the only
ways to fix nitrogen into compounds that living organisms can use.
4. Nitrogen is frequently a limiting factor for producers and therefore limits
populations of consumers, including humans.
5. There are two ways that inert nitrogen gas becomes ―fixed‖ so that plants
can use it—nitrogen fixation and nitrification.
a. Nitrogen fixation occurs through lightning or nitrogen-fixing
bacteria that live in mutualistic relationships with many leguminous
plants.
b. Nitrification occurs through specialized free-living bacteria.
c. Denitrifying bacteria convert nitrates in soil or water to gaseous
nitrogen.
I. We have greatly influenced the nitrogen cycle.
1. Nitrogen fixation has historically been a bottleneck, limiting the flux of
nitrogen out of the atmospheric reservoir.
2. The Haber-Bosch process allows humans to synthesize ammonia,
accelerating its flux into other reservoirs within the cycle.
3. Burning forests, fields, and fossil fuels all increase the amount of
atmospheric nitrogen, as does bacterial decomposition of animal wastes
from feedlots.
4. Strategies to limit the amount of damage caused by excess nutrients in the
waterways are not successful at this time.
J. People are searching for solutions to the dead zone.
1. In 1998, Congress passed the Harmful Algal Bloom and Hypoxia
Research and Control Act, which called for an assessment of the dead
zone’s ecological and economic impacts.
2. A state and federal task force proposed reducing nitrogen flowing down
the Mississippi River by 30% by the year 2015, in order to reduce the dead
zone’s average size to 5,000 km2 (1,930 mi2).
3. To protect farmers, in 2004, Congress reauthorized the Harmful Algal
Bloom and Hypoxia Research Control Act, encouraging scientists,
farmers, and policymakers to search for innovative ways to alleviate
pollution while safeguarding agriculture.
4. In 2009, the Obama administration responded with $320 million for
programs to fight nutrient pollution in the Mississippi River Basin.
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5. The Gulf should also benefit from a separate program to protect and
restore the Mississippi River’s delta. It would boost the growth of marsh
plants, helping to restore coastal wetlands.
V.
Conclusion
A. A systems approach to understanding Earth’s dynamics is important in
helping us avoid disrupting its processes. Integrating systems perspectives
with scientific findings allows us to address issues like the Gulf of Mexico
hypoxic zone.
B. Knowledge of the ways in which biotic and abiotic systems interact, and how
energy and mass flow through the systems, is essential. An understanding of
biogeochemical cycles that describe the movement of nutrients within and
among ecosystems is also crucial because human activities are causing
significant changes in these cycles.
C. Natural ecosystems use renewable solar energy, recycle nutrients, and are
stabilized by negative feedback loops. We should take a careful look at these
systems that have withstood the test of time as models of sustainability.
Key Terms
aquifers
atmosphere
biogeochemical cycles
biosphere
carbon cycle
conservation biologists
denitrification
denitrifying bacteria
dynamic equilibrium
ecological modeling
ecosystem
ecosystem services
ecotone
emergent properties
eutrophication
evaporation
feedback loop
flux
geographic information systems
gross primary production
groundwater
Haber-Bosch process
homeostasis
hydrologic cycle
metapopulation
model
mosaic
negative feedback loop
net primary production
net primary productivity
nitrification
nitrogen cycle
nitrogen fixation
nitrogen-fixing bacteria
nutrient cycles
nutrients
patches
phosphorus cycle
pool
positive feedback loop
precipitation
productivity
reservoir
residence time
runoff
sink
source
system
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hydrosphere
hypoxia
landscape ecology
lithosphere
transpiration
water cycle
watershed
water table
Teaching Tips
1. Ask students to conduct an Internet research on the Gulf of Mexico’s ―dead zone.‖
Ask them to find out its current status, whether it has increased or decreased in size
since 2002, and what is being done to address the problem.
2. Consider dividing students into small groups, each of which will make an oral
presentation during the semester. This chapter’s group might seek out the current
research on carbon sequestration by the oceans, terrestrial capture by forests, or
direct burial of CO2 from power generation.
3. Describe human activities that are affecting the flux rates in each of the major
biogeochemical cycles. Divide students into small groups and have each group take
one of the cycles, either on land, in freshwater, or in the oceans, and research the
human effects. Depending on the size of the class, you may need to divide them
further; you might do so by industrialized countries, emerging economies, and nonindustrialized countries, or by continent. Groups could present their findings as
written reports, posters, Web pages, or oral presentations.
4. Have students make a table that compares and contrasts the major biogeochemical
cycles, their reservoirs, (indicating which the sink is and which is the source), time
frames for flux, and other pertinent information. This could be done in groups or
individually, and during a lab period or for homework.
5. Consider having students make a miniature biosphere as a lab project. They have to
think about producers, consumers, detritivores, nutrient cycling, water cycles, energy
input, and waste heat. Small aquariums covered with glass or plastic sheeting can be
used if you have space for such a long-term project. Otherwise, see the ideas listed at
www.bottlebiology.org, a website maintained by the University of WisconsinMadison through an NSF grant. Some ideas there are very simplistic, but students
can create a fairly sophisticated biosphere using the techniques. These biospheres
have the advantages of being inexpensive and taking up little space in a class or lab.
6. Community Service: Ask students to brainstorm, individually or as a group, ways in
which they might explore the issues of this chapter in their community and take
action. A specific example might be to educate consumers about the use of
phosphates in dishwashing detergents. If your course contains a community service
component, some students might want to take an idea from this section as a project.
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Additional Resources
Websites
1. Mississippi River Basin and Gulf of Mexico Hypoxia, U.S. Environmental Protection
Agency (http://www.epa.gov/owow/msbasin)
This website provides background information about the Gulf of Mexico and describes
the Gulf of Mexico Program and its accomplishments.
2. National Centers for Coastal Ocean Science Gulf of Mexico Hypoxia Assessment,
NOAA’s National Ocean Service (www.nos.noaa.gov/products/pubs_hypox.html)
This website provides an integrated assessment of hypoxia in the Gulf of Mexico.
3. Water Resources of the United States, USGS (http://water.usgs.gov)
This website is the gateway to data, reports, and educators’ resources on water
science.
4. PMEL Ocean Acidification Home Page, NOAA (http://www.pmel.noaa.gov/co2/OA/)
An extensive set of resources on ocean acidification and the problems that it creates
for marine organisms.
5. CSiTE: Carbon Sequestration in Terrestrial Ecosystems Homepage, DOE,
(http://csite.esd.ornl.gov/)
A good starting point for research on carbon sequestration.
6. Landscape Ecology, EPA (http://www.epa.gov/nerlesd1/land-sci/intro.htm)
This site contains definitions, history, projects and more.
Audiovisual Materials
1. Strange Days on Planet Earth, National Geographic
(http://shop.nationalgeographic.com/index.jsp)
This two-DVD set follows scientists as they try to solve a series of mysteries.
2. World’s Last Great Places, National Geographic
(http://shop.nationalgeographic.com/index.jsp)
This 11-part series examines specific locations to illustrate biome characteristics.
3. The Cycle Series, How Stuff Works
(http://videos.howstuffworks.com)
This website has basic videos on the water cycle, the carbon cycle, and the nitrogen cycle.
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4. Environmental Science Videos, Science Daily
(http://www.sciencedaily.com/videos/earth_climate/environmental_science/)
This website has hundreds of short video clips on all aspects of environmental
science.
5. Environmental Videos, National Geographic,
(http://video.nationalgeographic.com/video/player/environment/)
This one contains hundreds of short environmental videos.
Weighing the Issues: Facts to Consider
Ecosystems Where You Live
Facts to consider: When describing ecosystems, one needs to include the interdependent
biotic and abiotic components (temperature, precipitation, latitude, and elevation, as well
as water table, soil type, and nutrient flow). Because all ecosystems are part of a set of
contiguous ecosystems, what flows into one ecosystem has flowed out of at least one
other ecosystem. If one ecosystem is disturbed, ecosystems that are ―downstream‖ will
suffer a loss of materials (such as water) or be susceptible to damage from any artificial
additions at the disturbed site (such as petroleum or fertilizers). Flora and fauna of the
disturbed area will be greatly affected, possibly affecting those in nearby systems.
Invasive species, which are better competitors in stressful environments, may establish
themselves in the disturbed ecosystems, forcing native species to disappear through local
extinction or through migration to more favorable habitats.
Your Water
Facts to consider: It is important to consider all aspects of the hydrologic cycle when
looking at water quality and quantity issues, as well as how hydrologic cycles will differ
from region to region. A good answer will refer to precipitation, infiltration, and recharge
while discussing the relationship between surface water and groundwater. Regional
information about water quantity and quality issues can be found at the United States
Geological Survey’s website (http://water.usgs.gov).
Here are some guiding questions to help students find out about their regional water
issues: What is the water source? How many users are there? Is the population growing?
Has the depth of the water table changed? Is the economic base changing (i.e., becoming
more particular about the water quality while at the same time producing more hazardous
waste from facilities such as computer chip factories)? What happens to sewage, storm
runoff, and hazardous waste?
Nitrogen Pollution and Its Financial Impacts
Facts to consider: This is a political question with no clear right or wrong answer. When
the interests of different regions are at odds, it’s to their advantage to work out their
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differences themselves because they know the problem best and may be able to reach an
agreement that satisfies all sides. However, if the federal government intervenes, it
should take into account the problems and needs of all affected parties to attempt a fair
compromise. This is a cost-benefit analysis, where agriculture’s benefit to the nation is to
be weighed against the value of the Gulf fisheries and the massive environmental impact
on the land and water. The health of the Gulf’s waters should be taken into consideration,
as well as the impact on human health in future decades.
The Science behind the Stories:
Thinking Like a Scientist
Hypoxia and the Gulf of Mexico “Dead Zone”
Observation: Spots in the northern Gulf of Mexico appeared to ―die‖ each summer
because of low oxygen levels, or hypoxia.
Hypothesis: The hypoxia was not isolated or rare, and it represented a growing
environmental problem.
Experiments: Long-term, widespread oxygen monitoring; water sampling for nitrogen
and other substances; dives to observe sea life; measurements of current and historical
levels and sources of nitrogen in the Mississippi River.
Results: The Gulf’s dead zone covered thousands of square miles and reappeared each
summer, fed by rivers polluted from farm runoff. The hypoxia stunned, drove away, or
killed sea life. The U.S. government is instituting new policies to control river
pollution and shrink the dead zone.
FACE-ing a High CO2 Future
Observation: Today’s atmosphere contains 35% more carbon dioxide than it did two
centuries ago and this increase is warming our planet. The principal sink for carbon
dioxide is known to be plants and other autotrophs.
Hypothesis: Increased CO2 will mean more plant growth and storage of CO2, which will
stabilize the system through negative feedback mechanisms.
Experiment: At least 36 different experiments are being performed; a typical one uses
towers and pipes that ring plots of trees, bathing them in an atmosphere 50% richer in
CO2 than today’s. A variety of control plots using both normal air, or polluted air, are
also included in the various experiments.
Results: The results to date are not definitive but overall it appears that there are a
number of negative and unintended consequences that will limit the benefits from this
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approach. Moderate levels of ozone, which often appear with increased CO2 in urban
areas, seem to offset the increased growth from the CO2. Higher CO2 levels also appear
to delay leaf death into winter making the trees vulnerable to frost damage.
Answers to End-of-Chapter Questions
Testing Your Comprehension
1. Negative feedback loops are most common in nature, whereas positive loops
sometimes result from human actions. When they do, they tend to move systems
away from their homeostatic condition.
2. Excess nutrients in runoff from the land fertilize phytoplankton in the coastal marine
ecosystem. This results in rapid growth of the phytoplankton population. Dead
phytoplankton and waste products then accumulate on the bottom, where bacterial
decomposition consumes most of the available oxygen in the water column above,
causing hypoxia.
3. An ecosystem includes both the biotic community and its abiotic environment. A
community includes only the populations of living organisms in a given area.
4. Typically, energy moves through an ecosystem from its source (the sun) to plants or
other photosynthetic primary producers. These producers are then consumed by a
variety of organisms, which in turn are consumed by others. At each step, some
energy is passed along to the consumer and some is lost as waste heat. Matter is also
transferred up the food chain as organisms eat organisms on lower trophic levels.
But, matter from organisms’ waste and from dead organisms is consumed and
broken down by detritivores and decomposers, returning matter to the soil so that it
can be recycled through the food web.
5. Net primary production is the energy content available to consumers from plants
after they have used some of their assimilated energy for their metabolism. The rate
at which production occurs is net primary productivity, usually expressed as biomass
per unit area per unit time – kg/meter2/day, e.g. See the text for a listing of typically
low and high productivity ecosystems.
6. If patches in a landscape mosaic are widely spaced, an organism’s population may
not be able to travel between them. Subpopulations of a species may be at risk of
extinction because they won’t be able to mate with members of other subpopulations.
7. Evaporation is the conversion of water from its liquid to gaseous form. Transpiration
is the special case of evaporation and release of water vapor from the leaves of
plants. The water cycle impacts the other nutrient cycles because these nutrients all
have water-soluble forms: CO2 dissolved as bicarbonate ions; nitrogen in the form of
ammonia, nitrates, or nitrites; and phosphorus as phosphates. Distribution and
delivery of these nutrients is mediated by the hydrologic cycle.
8. Cars release significant amounts of CO2 into the atmosphere from the combustion of
oil. Photosynthesis is the process by which plants remove CO2 from the atmosphere
and store the carbon as sugar. The oceans are home to a large volume of
phytoplankton which can remove CO2 from the air by photosynthesis. CO2 also
dissolves in seawater, is incorporated into carbonate minerals, and collects as
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sediment on the ocean floor. The sediments eventually can become part of the
Earth’s crust. This carbon returns to the atmosphere when the crustal plates are
themselves recycled along plate boundaries, and some of the plate material is melted
and erupts volcanically.
9. Nitrogen-fixing bacteria convert N2 gas into a usable form for plants. Denitrifying
bacteria convert nitrates back into N2 gas.
10. Human activity has accelerated the release of CO2 into the atmosphere from the
combustion of fossil fuels and cement production, as well as from deforestation.
High CO2 concentrations in the atmosphere contribute to the greenhouse effect. We
mine rocks containing phosphorus for use as fertilizer, and then allow excess
phosphates to run into waterways, causing eutrophication. We also fix more nitrogen
now by the Haber-Bosch process than is fixed naturally. This also can cause
eutrophication, as well as some health effects on humans and other animals
consuming nitrate-contaminated drinking water.
Calculating Ecological Footprints
Fertilizer
application
To the typical ¼
acre lawn
To all the lawns in
your hometown
To all the lawns in
the United States
Acreage of lawn
Fertilizer used
(lbs)
Water Used
(gal)
Gasoline used
(gal)
.25
37
150,000
4.9
Answers will
vary
Answers will
vary
Answers will
vary
Answers will
vary
40,500
1,500,000
2.43 x 1010
794,000
Answers are approximations and rounded
1. Inorganic nitrogen-based fertilizer results from human production via the Haber-Bosch
process. Much of the nitrogen is taken up by the grass, but much of it runs off and ends up in
local waterways.
2. Some types of grasses require less fertilizer (and water) than others. Switching to these types
or grasses or growing plants that require less fertilizer is an obvious step. Barriers or trenches
to contain runoff might be another solution.
3. Calculation: a. 4.9 gal/.25 acres x 40,500 acres = 793,800 gallons.
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