1. Chapter 54
Ecosystems

2. Overview: Ecosystems, Energy, and Matter
An ecosystem consists of all the organisms living in a community
As well as all the abiotic factors with which they interact

3. Ecosystems can range from a microcosm, such as an aquarium
To a large area such as a lake or forest
Figure 54.1

4. 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

5. Ecosystem ecologists view ecosystems
Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling
Ecosystem ecologists view ecosystems
As transformers of energy and processors of matter

6. Ecosystems and Physical Laws
The laws of physics and chemistry apply to ecosystems
Particularly in regard to the flow of energy
Energy is conserved
But degraded to heat during ecosystem processes

7. Trophic Relationships
Energy and nutrients pass from primary producers (autotrophs)
To primary consumers (herbivores) and then to secondary consumers (carnivores)

8. Energy flows through an ecosystem
Entering as light and exiting as heat
Figure 54.2
Microorganisms
and other
detritivores
Detritus
Primary producers
Primary consumers
Secondary
consumers
Tertiary consumers
Heat
Sun
Key
Chemical cycling
Energy flow

9. Nutrients cycle within an ecosystem

10. Decomposition
Decomposition
Connects all trophic levels

11. Detritivores, mainly bacteria and fungi, recycle essential chemical elements
By decomposing organic material and returning elements to inorganic reservoirs
Figure 54.3

12. Primary production in an ecosystem
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

13. Ecosystem Energy Budgets
The extent of photosynthetic production
Sets the spending limit for the energy budget of the entire ecosystem

14. The Global Energy Budget
The amount of solar radiation reaching the surface of the Earth
Limits the photosynthetic output of ecosystems
Only a small fraction of solar energy
Actually strikes photosynthetic organisms

15. Gross and Net Primary Production
Total primary production in an ecosystem
Is known as that ecosystem’s gross primary production (GPP)
Not all of this production
Is stored as organic material in the growing plants

16. Net primary production (NPP)
Is equal to GPP minus the energy used by the primary producers for respiration
Only NPP
Is available to consumers

17. Different ecosystems vary considerably in their net primary production
And in their contribution to the total NPP on Earth
Lake and stream
Open ocean
Continental shelf
Estuary
Algal beds and reefs
Upwelling zones
Extreme desert, rock, sand, ice
Desert and semidesert scrub
Tropical rain forest
Savanna
Cultivated land
Boreal forest (taiga)
Temperate grassland
Tundra
Tropical seasonal forest
Temperate deciduous forest
Temperate evergreen forest
Swamp and marsh
Woodland and shrubland 10 20 30 40 50 60
500
1,000
1,500
2,000
2,500 5 15 25
Percentage of Earth’s net
primary production
Key
Marine
Freshwater (on continents)
Terrestrial
5.2
0.3
0.1
4.7
3.5
3.3
2.9
2.7
2.4
1.8
1.7
1.6
1.5
1.3
1.0
0.4
125
360
3.0 90
2,200
900
600
800
700
140
1,600
1,200
1,300
250
5.6
1.2
0.9
0.04 22
7.9
9.1
9.6
5.4
0.6
7.1
4.9
3.8
2.3
65.0
24.4
Figure 54.4a–c
Percentage of Earth’s
surface area
(a)
Average net primary
production (g/m2/yr)
(b)
(c)

18. Overall, terrestrial ecosystems
Contribute about two-thirds of global NPP and marine ecosystems about one-third
Figure 54.5
180
120W
60W 0
60E
120E
North Pole
60N
30N
Equator
30S
60S
South Pole

19. Primary Production in Marine and Freshwater Ecosystems
Both light and nutrients are important in controlling primary production

20. The depth of light penetration
Light Limitation
The depth of light penetration
Affects primary production throughout the photic zone of an ocean or lake

21. More than light, nutrients limit primary production
Nutrient Limitation
More than light, nutrients limit primary production
Both in different geographic regions of the ocean and in lakes

22. A limiting nutrient is the element that must be added
In order for production to increase in a particular area
Nitrogen and phosphorous
Are typically the nutrients that most often limit marine production

23. Nutrient enrichment experiments
Confirmed that nitrogen was limiting phytoplankton growth in an area of the ocean
EXPERIMENT
Pollution from duck farms concentrated near Moriches Bay adds both nitrogen and phosphorus to the coastal water off Long Island. Researchers cultured the phytoplankton Nannochloris atomus with water collected from several bays.
Figure 54.6
Coast of Long Island, New York. The numbers on the map indicate
the data collection stations.
Long Island
Great South Bay
Shinnecock Bay
Moriches Bay
Atlantic Ocean 30 21 19 15 11 5 4 2

24. Figure 54.6 (a) Phytoplankton biomass and phosphorus concentration
(b) Phytoplankton response to nutrient enrichment
Great
South Bay
Moriches
Bay
Shinnecock
Starting
algal
density 2 4 5 11 30 15 19 21 24 18 12 6
Unenriched control
Ammonium enriched
Phosphate enriched
Station number
(millions of cells per mL)
Phytoplankton 8 7 3 1
Inorganic phosphorus
(g atoms/L)
(millions of cells/mL)
CONCLUSION
Since adding phosphorus, which was already in rich supply, had no effect on Nannochloris growth, whereas adding nitrogen increased algal density dramatically, researchers concluded that nitrogen was the nutrient limiting phytoplankton growth in this ecosystem.
Inorganic
phosphorus
RESULTS
Phytoplankton abundance parallels the abundance of phosphorus in the water (a). Nitrogen, however, is immediately taken up by algae, and no free nitrogen is measured in the coastal waters. The addition of ammonium (NH4) caused heavy phytoplankton growth in bay water, but the addition of phosphate (PO43) did not induce algal growth (b).

25. Experiments in another ocean region
Showed that iron limited primary production
Table 54.1

26. The addition of large amounts of nutrients to lakes
Has a wide range of ecological impacts

27. In some areas, sewage runoff
Has caused eutrophication of lakes, which can lead to the eventual loss of most fish species from the lakes
Figure 54.7

28. 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 geographic scale

29. The contrast between wet and dry climates
Can be represented by a measure called actual evapotranspiration

30. Actual evapotranspiration
Is the amount of water annually transpired by plants and evaporated from a landscape
Is related to net primary production
Figure 54.8
Actual evapotranspiration (mm H2O/yr)
Tropical forest
Temperate forest
Mountain coniferous forest
Temperate grassland
Arctic tundra
Desert
shrubland
Net primary production (g/m2/yr)
1,000
2,000
3,000
500
1,500

31. Live, above-ground biomass
On a more local scale
A soil nutrient is often the limiting factor in primary production
Figure 54.9
EXPERIMENT
Over the summer of 1980, researchers added phosphorus to some experimental plots in the salt marsh, nitrogen to other plots, and both phosphorus and nitrogen to others. Some plots were left unfertilized as controls.
RESULTS
Experimental plots receiving just phosphorus (P) do not outproduce the unfertilized control plots.
CONCLUSION
Live, above-ground biomass
(g dry wt/m2)
Adding nitrogen (N) boosts net primary
production.
300
250
200
150
100 50
June
July
August 1980
N  P
N only
Control
P only
These nutrient enrichment experiments confirmed that nitrogen was the nutrient limiting plant growth in this salt marsh.

32. The secondary production of an ecosystem
Concept 54.3: Energy transfer between trophic levels is usually less than 20% efficient
The secondary production of an ecosystem
Is the amount of chemical energy in consumers’ food that is converted to their own new biomass during a given period of time

33. Production Efficiency
When a caterpillar feeds on a plant leaf
Only about one-sixth of the energy in the leaf is used for secondary production
Figure 54.10
Plant material
eaten by caterpillar
Cellular
respiration
Growth (new biomass)
Feces
100 J
33 J
200 J
67 J

34. The production efficiency of an organism
Is the fraction of energy stored in food that is not used for respiration

35. Trophic Efficiency and Ecological Pyramids
Is the percentage of production transferred from one trophic level to the next
Usually ranges from 5% to 20%

36. Pyramids of Production
This loss of energy with each transfer in a food chain
Can be represented by a pyramid of net production
Figure 54.11
Tertiary
consumers
Secondary
Primary
producers
1,000,000 J of sunlight
10 J
100 J
1,000 J
10,000 J

37. One important ecological consequence of low trophic efficiencies
Pyramids of Biomass
One important ecological consequence of low trophic efficiencies
Can be represented in a biomass pyramid

38. Most biomass pyramids
Show a sharp decrease at successively higher trophic levels
Figure 54.12a
(a) 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.
Trophic level
Dry weight
(g/m2)
Primary producers
Tertiary consumers
Secondary consumers
Primary consumers
1.5 11 37
809

39. Certain aquatic ecosystems
Have inverted biomass pyramids
Figire 54.12b
Trophic level
Primary producers (phytoplankton)
Primary consumers (zooplankton)
(b) 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).
Dry weight
(g/m2) 21 4

40. Number of individual organisms
Pyramids of Numbers
A pyramid of numbers
Represents the number of individual organisms in each trophic level
Figure 54.13
Trophic level
Number of individual organisms
Primary producers
Tertiary consumers
Secondary consumers
Primary consumers 3
354,904
708,624
5,842,424

41. The dynamics of energy flow through ecosystems
Have important implications for the human population
Eating meat
Is a relatively inefficient way of tapping photosynthetic production

42. Worldwide agriculture could successfully feed many more people
If humans all fed more efficiently, eating only plant material
Trophic level
Secondary
consumers
Primary
producers
Figure 54.14

43. The Green World Hypothesis
According to the green world hypothesis
Terrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factors

44. Most terrestrial ecosystems
Have large standing crops despite the large numbers of herbivores
Figure 54.15

45. The green world hypothesis proposes several factors that keep herbivores in check
Plants have defenses against herbivores
Nutrients, not energy supply, usually limit herbivores
Abiotic factors limit herbivores
Intraspecific competition can limit herbivore numbers
Interspecific interactions check herbivore densities

46. Nutrient circuits that cycle matter through an ecosystem
Concept 54.4: Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem
Life on Earth
Depends on the recycling of essential chemical elements
Nutrient circuits that cycle matter through an ecosystem
Involve both biotic and abiotic components and are often called biogeochemical cycles

47. A General Model of Chemical Cycling
Gaseous forms of carbon, oxygen, sulfur, and nitrogen
Occur in the atmosphere and cycle globally
Less mobile elements, including phosphorous, potassium, and calcium
Cycle on a more local level

48. A general model of nutrient cycling
Includes the main reservoirs of elements and the processes that transfer elements between reservoirs
Figure 54.16
Organic
materials
available
as nutrients
Living
organisms,
detritus
unavailable
Coal, oil,
peat
Inorganic
Atmosphere,
soil, water
Minerals
in rocks
Formation of
sedimentary rock
Weathering,
erosion
Respiration,
decomposition,
excretion
Burning
of fossil fuels
Fossilization
Reservoir a
Reservoir b
Reservoir c
Reservoir d
Assimilation, photosynthesis

49. All elements
Cycle between organic and inorganic reservoirs

50. Biogeochemical Cycles
The water cycle and the carbon cycle
Figure 54.17
Transport
over land
Solar energy
Net movement of
water vapor by wind
Precipitation
over ocean
Evaporation
from ocean
Evapotranspiration
from land
Percolation
through
soil
Runoff and
groundwater
CO2 in atmosphere
Photosynthesis
Cellular
respiration
Burning of
fossil fuels
and wood
Higher-level
consumers
Primary
Detritus
Carbon compounds
in water
Decomposition
THE WATER CYCLE
THE CARBON CYCLE

51. Water moves in a global cycle
Driven by solar energy
The carbon cycle
Reflects the reciprocal processes of photosynthesis and cellular respiration

52. The nitrogen cycle and the phosphorous cycle
Figure 54.17
N2 in atmosphere
Denitrifying
bacteria
Nitrifying
Nitrification
Nitrogen-fixing
soil bacteria
bacteria in root
nodules of legumes
Decomposers
Ammonification
Assimilation
NH3
NH4+
NO3
NO2 
Rain
Plants
Consumption
Decomposition
Geologic
uplift
Weathering
of rocks
Runoff
Sedimentation
Plant uptake
of PO43
Soil
Leaching
THE NITROGEN CYCLE
THE PHOSPHORUS CYCLE

53. Most of the nitrogen cycling in natural ecosystems
Involves local cycles between organisms and soil or water
The phosphorus cycle
Is relatively localized

54. Decomposition and Nutrient Cycling Rates
Decomposers (detritivores) play a key role
In the general pattern of chemical cycling
Figure 54.18
Consumers
Producers
Nutrients
available
to producers
Abiotic
reservoir
Geologic
processes
Decomposers

55. The rates at which nutrients cycle in different ecosystems
Are extremely variable, mostly as a result of differences in rates of decomposition

56. Vegetation and Nutrient Cycling: The Hubbard Brook Experimental Forest
Is strongly regulated by vegetation

57. Long-term ecological research projects
Monitor ecosystem dynamics over relatively long periods of time
The Hubbard Brook Experimental Forest
Has been used to study nutrient cycling in a forest ecosystem since 1963

58. The research team constructed a dam on the site
To monitor water and mineral loss
Figure 54.19a
(a) 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.

59. In one experiment, the trees in one valley were cut down
And the valley was sprayed with herbicides
Figure 54.19b
(b) One watershed was clear cut to study the effects of the loss of vegetation on drainage and nutrient cycling.

60. Nitrate concentration in runoff
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
Figure 54.19c
(c) The concentration of nitrate in runoff from the deforested watershed was 60 times greater than in a control (unlogged) watershed.
Nitrate concentration in runoff
(mg/L)
Deforested
Control
Completion of
tree cutting
1965
1966
1967
1968
80.0
60.0
40.0
20.0
4.0
3.0
2.0
1.0

61. As the human population has grown in size
Concept 54.5: The human population is disrupting chemical cycles throughout the biosphere
As the human population has grown in size
Our activities have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems in most parts of the world

62. In addition to transporting nutrients from one location to another
Nutrient Enrichment
In addition to transporting nutrients from one location to another
Humans have added entirely new materials, some of them toxins, to ecosystems

63. Agriculture and Nitrogen Cycling
Agriculture constantly removes nutrients from ecosystems
That would ordinarily be cycled back into the soil
Figure 54.20

64. Nitrogen is the main nutrient lost through agriculture
Thus, agriculture has a great impact on the nitrogen cycle
Industrially produced fertilizer is typically used to replace lost nitrogen
But the effects on an ecosystem can be harmful

65. Contamination of Aquatic Ecosystems
The critical load for a nutrient
Is the amount of that nutrient that can be absorbed by plants in an ecosystem without damaging it

66. When excess nutrients are added to an ecosystem, the critical load is exceeded
And the remaining nutrients can contaminate groundwater and freshwater and marine ecosystems

67. Sewage runoff contaminates freshwater ecosystems
Causing cultural eutrophication, excessive algal growth, which can cause significant harm to these ecosystems

68. Combustion of fossil fuels
Acid Precipitation
Combustion of fossil fuels
Is the main cause of acid precipitation

69. North American and European ecosystems downwind from industrial regions
Have been damaged by rain and snow containing nitric and sulfuric acid
Figure 54.21
4.6
4.3
4.1
Europe
North America

70. By the year 2000
The entire contiguous United States was affected by acid precipitation
Figure 54.22
Field pH
5.3
5.2–5.3
5.1–5.2
5.0–5.1
4.9–5.0
4.8–4.9
4.7–4.8
4.6–4.7
4.5–4.6
4.4–4.5
4.3–4.4
4.3

71. Environmental regulations and new industrial technologies
Have allowed many developed countries to reduce sulfur dioxide emissions in the past 30 years

72. Toxins in the Environment
Humans release an immense variety of toxic chemicals
Including thousands of synthetics previously unknown to nature
One of the reasons such toxins are so harmful
Is that they become more concentrated in successive trophic levels of a food web

73. In biological magnification
Toxins concentrate at higher trophic levels because at these levels biomass tends to be lower
Figure 54.23
Concentration of PCBs
Herring
gull eggs
124 ppm
Zooplankton
0.123 ppm
Phytoplankton
0.025 ppm
Lake trout ppm
Smelt
1.04 ppm

74. In some cases, harmful substances
Persist for long periods of time in an ecosystem and continue to cause harm

75. Atmospheric Carbon Dioxide
One pressing problem caused by human activities
Is the rising level of atmospheric carbon dioxide

76. Rising Atmospheric CO2
Due to the increased burning of fossil fuels and other human activities
The concentration of atmospheric CO2 has been steadily increasing
Figure 54.24
CO2 concentration (ppm)
390
380
370
360
350
340
330
320
310
300
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005
1.05
0.90
0.75
0.60
0.45
0.30
0.15
0.15
 0.30
 0.45
Temperature variation (C)
Temperature
CO2
Year

77. 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
Figure 54.25

78. The Greenhouse Effect and Global Warming
The greenhouse effect is caused by atmospheric CO2
But is necessary to keep the surface of the Earth at a habitable temperature

79. Increased levels of atmospheric CO2 are magnifying the greenhouse effect
Which could cause global warming and significant climatic change

80. Depletion of Atmospheric Ozone
Life on Earth is protected from the damaging effects of UV radiation
By a protective layer or ozone molecules present in the atmosphere

81. Satellite studies of the atmosphere
Suggest that the ozone layer has been gradually thinning since 1975
Figure 54.26
Ozone layer thickness (Dobson units)
Year (Average for the month of October)
350
300
250
200
150
100 50
1955
1960
1965
1970
1975
1980
1985
1990
1995
2000
2005

82. The destruction of atmospheric ozone
Probably results from chlorine-releasing pollutants produced by human activity
Figure 54.27 1 2 3
Chlorine from CFCs interacts with ozone (O3), forming chlorine monoxide (ClO) and
oxygen (O2).
Two ClO molecules react, forming chlorine peroxide (Cl2O2).
Sunlight causes Cl2O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again.
Sunlight
Chlorine O3 O2
ClO
Cl2O2
Chlorine atoms

83. Scientists first described an “ozone hole”
Over Antarctica in 1985; it has increased in size as ozone depletion has increased
(a) October 1979
(b) October 2000
Figure 54.28a, b