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Chapter 46
Ecosystems
Albia Dugger • Miami Dade College
46.1 Too Much of a Good Thing
• Phosphorus is an essential element for building ATP,
phospholipids, nucleic acids, and other biological compounds
• A lack of available phosphorus is often the main factor limiting
growth of producers at the base of the food web
• Phosphate-rich fertilizers spread on lawns and agricultural
fields can run off and pollute rivers or lakes
• The addition of nutrients to an aquatic ecosystem
(eutrophication) disrupts nutrient cycles
Lawn Fertilizer and Water Pollution
Effect of Phosphate Pollution
nitrogen,
carbon
added
nitrogen, carbon,
phosphorus added
46.2 The Nature of Ecosystems
• Ecosystem
• An array of organisms and a physical environment, all
interacting through a one-way flow of energy and a cycling
of nutrients
• Sustained by ongoing inputs of energy and nutrients (open
system)
Overview of Participants
• Primary producers (autotrophs)
• Obtain energy from nonliving sources (sunlight)
• Build organic compounds from CO2 and water
• Consumers (heterotrophs)
• Get energy and carbon from organic sources
• Carnivores, herbivores, parasites, omnivores
Overview of Participants
• Detritivores, such as earthworms and crabs, eat small
particles of organic matter (detritus)
• Decomposers, such as bacteria and fungi, feed on organic
wastes and remains and break them down into inorganic
building blocks
Energy and Nutrients
• Energy flows one way
• Producers capture light energy and convert it to bond
energy in organic molecules (photosynthesis)
• Metabolic reactions break bonds (aerobic respiration) and
give off heat, which is not recycled
• Nutrients are cycled
• Producers take up inorganic compounds from the
environment; decomposers return them
light
energy
Producers
plants; photosynthetic
protists and bacteria
energy in
chemical
bonds
materials
cycling
Consumers
animals; fungi; heterotrophic
protists, bacteria, and archaeans
heat energy
Stepped Art
Figure 46-3 p830
ANIMATED FIGURE: One-way energy flow
and materials cycling
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Trophic Structure of Ecosystems
• Trophic levels
• Hierarchy of feeding relationships in which energy is
transferred when one organism eats another
• Each trophic level is a number of transfers away from the
system’s original energy input
Food Chain
• Food chain
• A sequence of steps by which some energy captured by
primary producers is transferred to organisms at
successively higher trophic levels
• Omnivores feed at several levels
Tallgrass Prairie:
Food Chain and Trophic Levels
ANIMATED FIGURE: Food chain
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Take-Home Message: What is the trophic
structure of an ecosystem?
• An ecosystem includes a community of organisms that
interact with their physical environment by a one-way energy
flow and a cycling of materials.
• Autotrophs tap into an environmental energy source and
make their own organic compounds from inorganic raw
materials. They are the ecosystem’s primary producers.
• Autotrophs are at the first trophic level of a food chain, a
linear sequence of feeding relationships that proceeds
through one or more levels of heterotrophs or consumers.
ANIMATION: The role of organisms in an
ecosystem
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46.3 The Nature of Food Webs
• Food webs
• Multiple interconnecting food chains, including grazing and
detrital food chains
• Grazing food web
• Energy stored in producers flows to herbivores, which tend
to be large animals
• Detrital food web
• Energy in producers flows to decomposers and
detritivores, which tend to be small
Land Versus Aquatic Food Chains
• In land ecosystems, most of the energy stored in producers
moves through detrital food chains
• In aquatic ecosystems, most of the energy in producers flows
to grazers rather than detritivores
Higher
Trophic
Levels
human (Inuk)
arctic wolf
arctic fox
A sampling
of carnivores
that feed on
herbivores
and one
another
gyrfalcon
snowy owl
mosquito
Second
Trophic
Level
A sampling
of primary
consumers
(herbivores)
that eat
plants
ermine
Parasitic
consumers
feed at more than
one
trophic level.
vole
arctic hare
lemming
Detritivores and
decomposers
(nematodes,
annelids,
saprobic insects,
protists, fungi,
bacteria)
First Trophic
Level
Examples of
primary
producers
(plants)
flea
grasses, sedges
purple saxifrage
arctic willow
Stepped Art
Figure 46-5 p832
ANIMATED FIGURE: Food webs
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How Many Transfers?
• Cumulative energy losses from energy transfers between
trophic levels limits the length of food chains to four or five
trophic levels
• Food chains tend to be shortest in variable habitats, longer in
stable habitats
• Food webs with more carnivores have fewer connections;
herbivores have more connections
East River Valley, Colorado
Model for East River Valley Food Web
Take-Home Message: How does energy flow
affect food chains and food webs?
• Tissues of living plants and other producers are the basis for
grazing food webs. Remains of producers are the basis for
detrital food webs.
• Nearly all ecosystems include both grazing food webs and
detrital food webs that interconnect as the system’s food
web.
• The cumulative energy losses from energy transfers between
trophic levels limits the length of food chains.
• Even when an ecosystem has many species, trophic
interactions link each species with many others.
46.4 Energy Flow
• Primary producers capture energy and take up nutrients,
which move to other trophic levels
• Primary production
• Rate at which producers capture and store energy
• Gross primary production – amount captured
• Net primary production – amount used in growth
Net Primary Productivity
Winter Productivity
North
America
Atlantic Ocean in Winter
Africa
Spring Productivity
North
America
Atlantic Ocean in Spring
Africa
Ecological Pyramids
• A biomass pyramid depicts dry weight of organisms at each
trophic level in an ecosystem
• Largest tier is usually producers
• For some aquatic systems, pyramid is inverted
• An energy pyramid depicts the energy that enters each
trophic level in an ecosystem
• Largest tier is always producers
Biomass Pyramid: Silver Springs
top carnivores
(gar and bass)
1.5
carnivores (smaller
fishes, invertebrates)
11
herbivores
(plant-eating fishes,
invertebrates, turtles)
37
producers (algae
and aquatic plants)
5
809
detritivores
(crayfish) and
decomposers
(bacteria)
Biomass (in grams per square meter)
Stepped Art
Energy Pyramid: Silver Springs
top carnivores
carnivores
herbivores
21
detritivores + decomposers = 5,060
383
3,368
producers
20,810
Stepped Art
Annual Energy Flow: Silver Springs
ANIMATED FIGURE: Energy flow at Silver
Springs
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Ecological Efficiency
• 5 to 30% of energy in tissues of organisms at one trophic
level ends up in tissues of those at the next trophic level
• Some energy is lost as heat
• Some biomass is not digested
• Efficiency of transfers tends to be greatest in aquatic systems
(less lignin, more ectotherms)
Take-Home Message:
How does energy flow through ecosystems?
• Primary producers capture energy and convert it into
biomass. We measure this process as primary production.
• A biomass pyramid depicts dry weight of organisms at each
trophic level. Its largest tier is usually producers, but the
pyramid for some aquatic systems is inverted.
• An energy pyramid depicts the amount of energy that enters
each level. Its largest tier is always at the bottom (producers).
• The efficiency of energy transfers tends to be greatest in
aquatic systems, where primary producers usually lack lignin
and consumers tend to be ectotherms.
46.5 Biogeochemical Cycles
• In a biogeochemical cycle, an essential element moves from
nonliving environmental reservoirs, into living organisms, then
back to the reservoirs
• Elements essential to life (nutrients) include oxygen,
hydrogen, carbon, nitrogen, phosphorus
Biogeochemical Cycles
• Chemical and geologic processes move elements to, from,
and among environmental reservoirs
• Nutrients move from inorganic reservoirs (rocks, sediments,
water, atmosphere) to living systems through primary
producers
• Photosynthetic organisms take up dissolved ions and carbon
dioxide; bacteria fix nitrogen gas
Biogeochemical Cycles
Atmosphere
Living
organisms
Rocks
and
sediments
Seawater
and
fresh water
Nonliving
environmental
reservoirs
Take-Home Message:
What is a biogeochemical cycle?
• A biogeochemical cycle is the slow movement of a nutrient
among environmental reservoirs and into, through, and out of
food webs.
46.6 The Water Cycle
• The water cycle moves water moves from the world ocean
(main reservoir) through the atmosphere (by evaporation and
transpiration), onto land (by condensation and precipitation),
then back to the ocean
• Oceans cover about 70% of Earth’s surface, so most rainfall
returns water directly to the oceans
Table 46-1 p837
Where Water Moves
• A watershed is an area from which all precipitation drains
into a specific waterway
• Groundwater includes soil water and water in aquifers
(permeable rock layers that hold water)
• Runoff is water that flows over saturated ground into streams
• Flowing water carries nutrients from place to place
Atmosphere
Evaporation
from ocean
Precipitation
into ocean
Windborne water vapor
Evaporation
from land plants
(transporation)
Precipitation
onto the land
Surface and
groundwater
flow
Land
Ocean
Stepped Art
Figure 46-11 p837
Limited Fresh Water
• Most of Earth’s water is too salty to drink or irrigate crops
• Overdrafts from aquifers are common; water is drawn from
aquifers faster than natural processes replenish it
• When too much fresh water is withdrawn from a coastal
aquifer, saltwater moves in and replaces it
• In the US, about 80% of the water used by humans is for
irrigating agricultural fields
Groundwater Troubles
Take-Home Message: What is the water cycle
and how do human activities affect it?
• Water moves slowly from the world ocean—the main
reservoir—through the atmosphere, onto land, then back to
the ocean.
• Fresh water makes up only a tiny portion of the global water
supply. Excessive water withdrawals threaten many sources
of drinking water.
46.7 Carbon Cycle
• In the carbon cycle, carbon moves among Earth’s
atmosphere, oceans, rocks, and soils, and into and out of
food webs
• It is an atmospheric cycle – the atmosphere holds about 760
gigatons (billion tons) of carbon, mainly in the form of carbon
dioxide (CO2)
Terrestrial Carbon Cycle
• Land plants incorporate CO2 from the atmosphere into their
tissues through photosynthesis
• Plants and other organisms release CO2 into the atmosphere
through aerobic respiration
• Soil contains more than twice as much carbon as the
atmosphere – consisting of humus and living soil organisms
Carbon Stored in Plants and Soils
Marine Carbon Cycle
• Most of the annual cycling of carbon occurs between the
ocean and the atmosphere
• Bicarbonate (HCO3–) is the main inorganic carbon in seawater
• Photosynthesis, aerobic respiration, decomposition, and
sediments contribute to the marine carbon cycle
• Marine sediments and sedimentary rocks formed from
calcium carbonate–rich shells are Earth’s largest carbon
reservoir, with more than 65 gigatons
Carbon in Fossil Fuels
• Fossil fuels such as coal, oil, and natural gas hold an
estimated 5,000 gigatons of carbon
• Deposits of fossil fuels formed over hundreds of millions of
years from carbon-rich remains
• Currently, burning of fossil fuel adds billions of tons of CO2 to
the atmosphere every year
The Carbon Cycle
Atmospheric CO2
1 photosynthesis
6 burning fossil fuels
2
aerobic respiration
3
Land food webs
diffusion between
atmosphere and
ocean
Dissolved carbon
in ocean
4
Fossil fuels
death, burial,
compaction over
millions of years
Earth’s crust
sedimentation
5
Marine
organisms
Stepped Art
ANIMATED FIGURE: Carbon cycle
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Take-Home Message: How does carbon cycle
between its main reservoirs?
• The largest reservoir is sedimentary rock. Carbon moves into
and out of this reservoir over very long time spans.
• Seawater is the largest reservoir of biologically available
carbon. Marine producers take up bicarbonate and convert it
to CO2 for use in photosynthesis.
• On land, large amounts of carbon are stored in soil, especially
in arctic regions and in peat bogs.
• The atmosphere is the source of CO2 for land producers.
Burning of fossil fuels adds carbon to this reservoir.
46.8 Greenhouse Gases
and Climate Change
• Greenhouse gases include carbon dioxide, water, nitrous
oxide, methane, chlorofluorocarbons (CFCs)
• Greenhouse effect
• Radiant energy from the sun is absorbed by Earth’s
surface and radiated back as heat
• Gases in the upper atmosphere trap heat like a
greenhouse, and radiate it back to Earth
The Greenhouse Effect
light energy
3
1
2
heat energy
Changing Carbon Dioxide Concentrations
• Atmospheric CO2 fluctuates seasonally with patterns of
photosynthesis
• Average concentrations of CO2 and other greenhouse gases
are increasing
• Data from glacial ice, foraminiferan fossils, and other sources
show that atmospheric CO2 has risen and fallen many times
• Data also show that the current CO2 concentration is the
highest in at least 15 million years
Atmospheric CO2 Concentration
Changing Climate
• Global climate change is a long-term alteration of Earth’s
climate – global warming is one aspect of this change
• Human-induced increase in atmospheric greenhouse gases is
likely the cause of the current warming trend
• Scientists expect far-reaching effects
• Melting glaciers and rising sea levels
• Altered global precipitation patterns, droughts and
flooding, more intense hurricanes
Changes in Global Mean Temperatures
Take-Home Message:
How does carbon dioxide affect climate?
• Carbon dioxide is one of the atmospheric gases that absorb
heat and emit it toward Earth’s surface, thus keeping the
planet warm enough for life.
• The CO2 level of the atmosphere is currently rising, most
likely as a result of human activity, and global mean
temperature is rising with it. Changes in temperature affect
other climate factors such as patterns of rainfall.
Video: Does Clean Coal Exist?
Video: Finding Alternatives to Oil
ANIMATION: Matter recycling and energy
flow
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46.9 Nitrogen Cycle
• Gaseous nitrogen (N2) makes up about 80 percent of the
lower atmosphere
• Most organisms can’t use gaseous nitrogen
• The nitrogen cycle starts with nitrogen fixation
• Nitrogen-fixing bacteria convert N2 in the air to ammonia
(NH3), then to ammonium (NH4+) and nitrate (NO3-), which
plants easily take up
Other Nitrogen Inputs
• Ammonification
• Bacteria and fungi make additional ammonium available to
plants when they break down nitrogen-rich wastes and
remains
• Nitrification
• Bacteria convert ammonium to nitrite (NO2-), and then to
nitrate, which plants easily take up
Removing Nitrogen
• Denitrification
• Denitrifying bacteria convert nitrate or nitrite to gaseous
nitrogen (N2) or nitrogen oxide (NO2)
• Ammonium, nitrite, and nitrate are also lost from land
ecosystems in runoff and by leaching
Nitrogen Cycle in a Land Ecosystem
Land food webs
1 nitrogen fixation
6 denitrification
Waste and
remains
by bacteria
2 uptake
by producers
Soil ammonium
(NH4+)
3 decomposition by
bacteria and fungi
4 nitrification
by bacteria
by bacteria
5 uptake
by producers
Soil nitrates
(NO3–)
Stepped Art
ANIMATED FIGURE: Nitrogen cycle
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Take-Home Message:
How does nitrogen cycle in ecosystems?
• The atmosphere is the main reservoir for nitrogen, but only
nitrogen-fixing bacteria can access it.
• Plants take up ammonium and nitrates from the soil.
Ammonium is released by nitrogen-fixing bacteria and by
fungal and bacterial decomposers. Bacteria and archaea
produce nitrates.
• Nitrogen returns to the atmosphere when denitrifying bacteria
convert soluble forms of nitrogen to nitrogen gas.
Video: New Ideas for Dealing with Climate
Change
46.10 Disruption of the Nitrogen Cycle
• In the early 1900s, German scientists discovered a method of
fixing atmospheric nitrogen and producing ammonium on an
industrial scale
• The use of fertilizers and burning of fossil fuels has added
large amounts of nitrogen-containing compounds to our air
and water and altered the nitrogen cycle
Nitrous Oxide—A Double Threat
• Chemical fertilizers and manure from livestock increase
bacterial production of N2O
• Burning fossil fuel also releases N2O into the air
• N2O is a highly persistent and effective greenhouse gas, with
a warming potential 300 times that of CO2
• N2O contributes to destruction of the ozone layer which
protects life at Earth’s surface from damaging UV radiation
Bacteria Convert Ammonia to N2O
Nitrate Pollution
• Nitrate (NO3–) from fertilizers and septic tanks leaches into
groundwater and contaminates sources of drinking water
• Ingested nitrate inhibits iodine uptake by the thyroid gland and
may increase the risk of thyroid cancer
• The EPA mandates that public drinking water contain less
than 10 ppm nitrate and requires periodic testing
Take-Home Message: How do human activities
disrupt the nitrogen cycle?
• Burning fossil fuels releases nitrous oxide (N2O) into the air.
Nitrous oxide is a greenhouse gas and it contributes to
destruction of the ozone layer.
• Use of synthetic fertilizer encourages the production of nitrous
oxide by bacteria. It is also a source of nitrates, which pollute
drinking water.
• Wastewater that escapes from septic systems is another
source of nitrate.
46.11 The Phosphorus Cycle
• Most of Earth’s phosphorus is bonded to oxygen as
phosphate (PO43– ), an ion in rocks and sediments
• The phosphorus cycle is a sedimentary cycle that moves
phosphorus from its main reservoir (Earth’s crust) through
soils and sediments, to food webs
• Weathering and erosion move phosphates from rocks into
soil, lakes, and rivers
Phosphate in Living Organisms
• Phosphorus is a component of all nucleic acids and
phospholipids
• Land plants take up dissolved phosphate from the soil water
• Land animals get phosphates by eating the plants or one
another
• In the seas, phosphorus enters food webs when producers
take up phosphate dissolved in seawater
The Phosphorus Cycle
Land food webs
Rocks
on land
1
weathering,
excretion, death,
5
uptake
erosion
decomposition
by producers
6
2 leaching,
runoff
Phosphates in soil, lakes, rivers
7
Phosphates
in seawater
Marine
food web
8
3
4 uplifting over geologic time
Marine sediments
Stepped Art
ANIMATED FIGURE: Phosphorus cycle
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Take-Home Message:
How does phosphorus cycle in ecosystems?
• Rocks are the main phosphorus reservoir. Weathering puts
phosphates into water. Producers take up dissolved
phosphates.
• Phosphate-rich wastes are a natural fertilizer, and phosphate
from rocks can be used to produce fertilizer on an industrial
scale.