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PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Chapter 54Chapter 54
Ecosystems
Overview: Ecosystems, Energy, and Matter
• An ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they interact
• Ecosystems range from a microcosm, such as an aquarium, to a large area such as a lake or forest
• Regardless of an ecosystem’s size, its dynamics involve two main processes: energy flow and chemical cycling
• Energy flows through ecosystems while matter cycles within them
Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling
• Ecologists view ecosystems as transformers of energy and processors of matter
Ecosystems and Physical Laws
• Laws of physics and chemistry apply to ecosystems, particularly energy flow
• Energy is conserved but degraded to heat during ecosystem processes
Trophic Relationships
• Energy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) and then to secondary consumers (carnivores)
• Energy flows through an ecosystem, entering as light and exiting as heat
• Nutrients cycle within an ecosystem
LE 54-2
Microorganismsand other
detritivores
Tertiaryconsumers
Secondaryconsumers
Detritus Primary consumers
Sun
Primary producers
Heat
Key
Chemical cycling
Energy flow
Decomposition
• Decomposition connects all trophic levels
• Detritivores, mainly bacteria and fungi, recycle essential chemical elements by decomposing organic material and returning elements to inorganic reservoirs
Concept 54.2: Physical and chemical factors limit primary production in ecosystems
• Primary production in an ecosystem is the amount of light energy converted to chemical energy by autotrophs during a given time period
Ecosystem Energy Budgets
• The extent of photosynthetic production sets the spending limit for an ecosystem’s energy budget
The Global Energy Budget
• The amount of solar radiation reaching the Earth’s surface limits photosynthetic output of ecosystems
• Only a small fraction of solar energy actually strikes photosynthetic organisms
Gross and Net Primary Production
• Total primary production is known as the ecosystem’s gross primary production (GPP)
• Net primary production (NPP) is GPP minus energy used by primary producers for respiration
• Only NPP is available to consumers
• Ecosystems vary greatly in net primary production and contribution to the total NPP on Earth
LE 54-4
Open oceanContinental shelf
Upwelling zonesExtreme desert, rock, sand, ice
Swamp and marshLake and stream
Desert and semidesert scrubTropical rain forest
Temperate deciduous forestTemperate evergreen forest
Tropical seasonal forest
SavannaCultivated land
EstuaryAlgal beds and reefs
Boreal forest (taiga)Temperate grassland
Woodland and shrublandTundra
0.40.4
1.01.31.51.61.71.82.42.72.93.33.54.7
0.30.10.1
5.265.0
Freshwater (on continents)TerrestrialMarine
Key Percentage of Earth’ssurface area
Average net primaryproduction (g/m2/yr)
6050403020100 2,5002,0001,5001,0005000Percentage of Earth’s netprimary production
2520151050
125
2,500
3601,500
5003.090
900600
800
2,200
600
250
1,6001,2001,300
2,000
700140
0.3
7.99.19.6
5.43.5
0.67.1
4.93.8
2.3
24.45.6
1.20.9
0.10.040.9
22
• Overall, terrestrial ecosystems contribute about two-thirds of global NPP
• Marine ecosystems contribute about one-third
LE 54-5
North Pole
60°N
30°N
South Pole
Equator
60°S
30°S
60°W 60°E 120°E120°W180° 0° 180°
Primary Production in Marine and Freshwater Ecosystems
• In marine and freshwater ecosystems, both light and nutrients control primary production
Light Limitation
• Depth of light penetration affects primary production in the photic zone of an ocean or lake
Nutrient Limitation
• More than light, nutrients limit primary production in geographic regions of the ocean and in lakes
• A limiting nutrient is the element that must be added for production to increase in an area
• Nitrogen and phosphorous are typically the nutrients that most often limit marine production
• Nutrient enrichment experiments confirmed that nitrogen was limiting phytoplankton growth in an area of the ocean
LE 54-6
Atlantic Ocean
ShinnecockBay
Moriches Bay
Long Island
2
4 5
30
1115
19
21
Coast of Long Island, New York
Great South Bay
Phytoplankton
Inorganicphosphorus
GreatSouth Bay
MorichesBay
ShinnecockBay
Station number2119153011542
8
543210
67
8
543210
67
Phytoplankton biomass and phosphorus concentration
Phyt
opla
nkto
n(m
illio
ns o
f cel
ls/m
L)
Inor
gani
c ph
osph
orus
(µm
ato
ms/
L)
Ammonium enriched
Station number2119153011542
30
Phyt
opla
nkto
n(m
illio
ns o
f cel
ls p
er m
L)
Startingalgal
density
Phytoplankton response to nutrient enrichment
24
18
12
6
0
Phosphate enrichedUnenriched control
• Experiments in another ocean region showed that iron limited primary production
• The addition of large amounts of nutrients to lakes has a wide range of ecological impacts
• In some areas, sewage runoff has caused eutrophication of lakes, which can lead to loss of most fish species
Primary Production in Terrestrial and Wetland Ecosystems
• In terrestrial and wetland ecosystems, climatic factors such as temperature and moisture affect primary production on a large scale
• Actual evapotranspiration can represent the contrast between wet and dry climates
• Actual evapotranspiration is the water annually transpired by plants and evaporated from a landscape
• It is related to net primary production
LE 54-8
Mountain coniferous forest
Temperate forest
Tropical forest
Temperate grassland
Arctic tundra
Desertshrubland
1,5001,00050000
1,000
2,000
3,000
Actual evapotranspiration (mm/yr)
Net
prim
ary
prod
uctio
n (g
/m2 /y
r)
• On a more local scale, a soil nutrient is often the limiting factor in primary production
LE 54-9
Control
August 1980JulyJune00
100
200
300Li
ve, a
bove
-gro
und
biom
ass
(g d
ry w
t/m2 )
50
150
250 N + P
N only
P only
Concept 54.3: Energy transfer between trophic levels is usually less than 20% efficient
• Secondary production of an ecosystem is the amount of chemical energy in food converted to new biomass during a given period of time
Production Efficiency
• When a caterpillar feeds on a leaf, only about one- sixth of the leaf’s energy is used for secondary production
• An organism’s production efficiency is the fraction of energy stored in food that is not used for respiration
LE 54-10
Growth (new biomass)
Cellularrespiration
Feces100 J
33 J
67 J
200 J
Plant materialeaten by caterpillar
Trophic Efficiency and Ecological Pyramids
• Trophic efficiency is the percentage of production transferred from one trophic level to the next
• It usually ranges from 5% to 20%
Pyramids of Production
• A pyramid of net production represents the loss of energy with each transfer in a food chain
LE 54-11
1,000,000 J of sunlight
10,000 J
1,000 J
100 J
10 JTertiaryconsumers
Secondaryconsumers
Primaryconsumers
Primaryproducers
Pyramids of Biomass
• In a biomass pyramid, each tier represents the dry weight of all organisms in one trophic level
• Most biomass pyramids show a sharp decrease at successively higher trophic levels
LE 54-12a
Trophic level Dry weight(g/m2)
Tertiary consumersSecondary consumers
Primary consumersPrimary producers
1.51137809
Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data from a bog at Silver Springs, Florida.
• Certain aquatic ecosystems have inverted biomass pyramids: Primary consumers outweigh the producers
LE 54-12b
Trophic level Dry weight(g/m2)
Primary consumers (zooplankton)Primary producers (phytoplankton)
214
In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton) supports a larger standing crop of primary consumers (zooplankton).
Pyramids of Numbers
• A pyramid of numbers represents the number of individual organisms in each trophic level
LE 54-13
Trophic level Number ofindividual organisms
Tertiary consumersSecondary consumers
Primary consumersPrimary producers
3354,904708,6245,842,424
• Dynamics of energy flow in ecosystems have important implications for the human population
• Eating meat is a relatively inefficient way of tapping photosynthetic production
• Worldwide agriculture could feed many more people if humans ate only plant material
LE 54-14
Trophic level
Secondary consumers
Primary consumers
Primary producers
The Green World Hypothesis
• Most terrestrial ecosystems have large standing crops despite the large numbers of herbivores
• The green world hypothesis proposes several factors that keep herbivores in check:
– Plant defenses
– Limited availability of essential nutrients
– Abiotic factors
– Intraspecific competition
– Interspecific interactions
Concept 54.4: Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem
• Life depends on recycling chemical elements
• Nutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles
A General Model of Chemical Cycling
• Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally
• Less mobile elements such as phosphorus, potassium, and calcium cycle on a more local level
• A model of nutrient cycling includes main reservoirs of elements and processes that transfer elements between reservoirs
• All elements cycle between organic and inorganic reservoirs
LE 54-16
Fossilization
Reservoir a Reservoir b
Reservoir c Reservoir d
Organicmaterialsavailable
as nutrients
Organicmaterials
unavailableas nutrients
Inorganicmaterialsavailable
as nutrients
Inorganicmaterials
unavailableas nutrients
Livingorganisms,detritus
Coal, oil,peat
Atmosphere,soil, water
Mineralsin rocks
Assimilation,photosynthesis Burning
of fossil fuels
Weathering,erosion
Formation ofsedimentary rock
Respiration,decomposition,excretion
Biogeochemical Cycles
• In studying cycling of water, carbon, nitrogen, and phosphorus, ecologists focus on four factors:
1. Each chemical’s biological importance
2. Forms in which each chemical is available or used by organisms
3. Major reservoirs for each chemical
4. Key processes driving movement of each chemical through its cycle
LE 54-17a
Transportover land
Precipitationover landEvaporation
from oceanPrecipitationover ocean
Net movement ofwater vapor by wind
Solar energy
Evapotranspirationfrom land
Runoff andgroundwater
Percolationthroughsoil
LE 54-17b
Cellularrespiration
Burning offossil fuelsand wood
Carbon compoundsin water
Photosynthesis
Primaryconsumers
Higher-levelconsumers
Detritus
Decomposition
CO2 in atmosphere
LE 54-17c
Assimilation
N2 in atmosphere
DecomposersNitrifyingbacteria
Nitrifyingbacteria
Nitrogen-fixingsoil bacteria
Denitrifyingbacteria
NitrificationAmmonification
Nitrogen-fixingbacteria in rootnodules of legumes
NO3–
NO2–NH4
+NH3
LE 54-17d
Sedimentation
Plants
Rain
Runoff
Weatheringof rocks
Geologicuplift
SoilLeaching
Decomposition
Plant uptakeof PO4
3–
Consumption
Decomposition and Nutrient Cycling Rates
• Decomposers (detritivores) play a key role in the general pattern of chemical cycling
• Rates at which nutrients cycle in different ecosystems vary greatly, mostly as a result of differing rates of decomposition
LE 54-18
Nutrientsavailable
to producers
Decomposers
Geologicprocesses
Abioticreservoir
Consumers
Producers
Vegetation and Nutrient Cycling: The Hubbard Brook Experimental Forest
• Vegetation strongly regulates nutrient cycling
• Research projects monitor ecosystem dynamics over long periods
• The Hubbard Brook Experimental Forest has been used to study nutrient cycling in a forest ecosystem since 1963
• The research team constructed a dam on the site to monitor loss of water and minerals
LE 54-19
Concrete dams and weirs built across streams at the bottom of watersheds enabled researchers to monitor the outflow of water and nutrients from the ecosystem.
One watershed was clear cut to study the effects of the loss of vegetation on drainage and nutrient cycling.
The concentration of nitrate in runoff from the deforested watershed was 60 times greater than in a control (unlogged) watershed.
Deforested
Control
Completion oftree cutting
Nitr
ate
conc
entr
atio
n in
runo
ff(m
g/L)
1965 19681966 1967
80.060.040.020.0
4.03.02.01.0
0
• In one experiment, the trees in one valley were cut down, and the valley was sprayed with herbicides
• Net losses of water and minerals were studied and found to be greater than in an undisturbed area
• These results showed how human activity can affect ecosystems
Concept 54.5: The human population is disrupting chemical cycles throughout the biosphere
• As the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystems
Nutrient Enrichment
• In addition to transporting nutrients from one location to another, humans have added new materials, some of them toxins, to ecosystems
Agriculture and Nitrogen Cycling
• Agriculture removes nutrients from ecosystems that would ordinarily be cycled back into the soil
• Nitrogen is the main nutrient lost through agriculture; thus, agriculture greatly impacts the nitrogen cycle
• Industrially produced fertilizer is typically used to replace lost nitrogen, but effects on an ecosystem can be harmful
Contamination of Aquatic Ecosystems
• Critical load for a nutrient is the amount that plants can absorb without damaging the ecosystem
• When excess nutrients are added to an ecosystem, the critical load is exceeded
• Remaining nutrients can contaminate groundwater and freshwater and marine ecosystems
• Sewage runoff causes cultural eutrophication, excessive algal growth that can greatly harm freshwater ecosystems
Acid Precipitation
• Combustion of fossil fuels is the main cause of acid precipitation
• North American and European ecosystems downwind from industrial regions have been damaged by rain and snow containing nitric and sulfuric acid
LE 54-21
North America
Europe
4.3 4.6
4.14.34.6
4.34.6
4.6
• By the year 2000, acid precipitation affected the entire contiguous United States
• Environmental regulations and new technologies have allowed many developed countries to reduce sulfur dioxide emissions
LE 54-22
Field pH≥5.35.2–5.35.1–5.25.0–5.14.9–5.04.8–4.94.7–4.84.6–4.74.5–4.64.4–4.54.3–4.4<4.3
5.3
5.3
5.35.3
5.3 5.3
5.3
5.2
5.3
5.6
5.9
5.4
5.2
5.2
5.2
5.2
5.4
5.56.0
5.0
5.4
6.3
5.3
5.3
6.15.5
5.4
5.4
5.4
5.4
5.6
5.5
5.5
5.6
5.65.2
5.1
5.15.74.9
5.7
5.0
5.0
5.0
5.04.9
4.9
4.9
4.9
4.14.3
4.3
4.34.4
4.44.44.4
4.4
4.5
4.54.5
4.54.54.54.5
4.54.5
4.54.5
4.5
4.5
4.54.54.5
4.5
4.54.54.5
4.5
4.64.6
4.64.6
4.6
4.6
4.64.6
4.64.64.6
4.64.6
4.6
4.7
4.7
4.74.74.7
4.7
4.7
4.74.7
4.7
4.74.7
4.7
4.7
4.7
4.84.8
4.8
4.8
4.8
4.8
4.8
4.8
4.8
5.0
5.0
5.3
5.2
5.1
5.1
5.1
5.1
5.25.2
5.2
5.3
5.4
5.4
5.5
5.5
4.74.7
4.7
4.7
4.7
4.7
4.7
4.9
4.84.8
4.64.7
4.7
4.7
4.84.8
4.8
4.8
4.9
4.9
4.9
5.0
5.0
5.0
5.1
5.1
5.0
5.0 5.05.1
5.2
5.3
5.45.4
5.7
Toxins in the Environment
• Humans release many toxic chemicals, including synthetics previously unknown to nature
• In some cases, harmful substances persist for long periods in an ecosystem
• One reason toxins are harmful is that they become more concentrated in successive trophic levels
• In biological magnification, toxins concentrate at higher trophic levels, where biomass is lower
LE 54-23
Zooplankton0.123 ppm
Phytoplankton0.025 ppm
Lake trout4.83 ppm
Smelt1.04 ppm
Herringgull eggs124 ppm
Con
cent
ratio
n of
PC
Bs
Atmospheric Carbon Dioxide
• One pressing problem caused by human activities is the rising level of atmospheric carbon dioxide
Rising Atmospheric CO2
• Due to the burning of fossil fuels and other human activities, the concentration of atmospheric CO2has been steadily increasing
How Elevated CO2 Affects Forest Ecology: The FACTS-I Experiment
• The FACTS-I experiment is testing how elevated CO2 influences tree growth, carbon concentration in soils, and other factors over a ten-year period
The Greenhouse Effect and Global Warming
• The greenhouse effect caused by atmospheric CO2 keeps Earth’s surface at a habitable temperature
• Increased levels of atmospheric CO2 are magnifying the greenhouse effect, which could cause global warming and climatic change
Depletion of Atmospheric Ozone
• Life on Earth is protected from damaging effects of UV radiation by a protective layer or ozone molecules in the atmosphere
• Satellite studies suggest that the ozone layer has been gradually thinning since 1975
LE 54-26
Ozo
ne la
yer t
hick
ness
(Dob
son
units
)
350
300
250
200
150
100
50
01960 1965 1970 1975 1980 1985 1990 1995 2000 2005
Year (Average for the month of October)1955
• Destruction of atmospheric ozone probably results from chlorine-releasing pollutants produced by human activity
LE 54-27
Chlorine atoms
O3Chlorine
Cl2 O2
CIO
O2
O2
CIO
Chlorine from CFCs interacts with ozone (O3 ), forming chlorine monoxide (CIO) and oxygen (O2 ).
Sunlight causes Cl2 O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again.
Two CIO molecules react, forming chlorine peroxide (Cl2 O2 ).
Sunlight
• Scientists first described an “ozone hole” over Antarctica in 1985; it has increased in size as ozone depletion has increased
LE 54-28
October 1979 October 2000