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Chapter 55
Ecosystems
An ecosystem consists of all the organisms living in a community– As well as all the abiotic factors with which
they interact– They can be very small or very large
Aquarium Coniferous Forest
Energy Flow and Chemical Cycling
Ecosystem ecology emphasizes energy flow and chemical cycling
Ecosystem ecologists view ecosystems– As transformers of energy and processors of
matter
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 (energy transformations are inefficient…some energy is always lost as heat)
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
Microorganismsand other
detritivores
Detritus
Primary producers
Primary consumers
Secondaryconsumers
Tertiary consumers
Heat
Sun
Key
Chemical cycling
Energy flow
Detritus is dead organic material
Nutrient cycling
Nutrients cycle within an ecosystem
Decomposition– Connects all trophic levels
– Detritivores, mainly bacteria and fungi, recycle essential chemical elements
– By decomposing organic material and returning elements to inorganic reservoirs
Decomposition
Primary Production
Primary production in an ecosystem– Is the amount of light energy converted
to chemical energy by autotrophs during a given time period
Physical and chemical factors limit primary production in ecosystems
Ecosystem Energy Budgets
The extent of photosynthetic production– Sets the “spending limit” for the energy budget
of the entire ecosystem
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– And only ~1% of that is converted to chemical
energy by photosynthesis– …still that’s a lot of energy
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 (The plants use some of the energy for the fuel of day-to-day living)
Net primary production (NPP)– Is equal to GPP minus the energy used by the primary
producers for respiration– It is the amount of new biomass added in a given time
period– Only NPP is available to consumers
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 forestTemperate deciduous forest
Temperate evergreen forest
Swamp and marsh
Woodland and shrubland
0 10 20 30 40 50 60 0 500 1,000 1,500 2,000 2,500 0 5 10 15 20 25
Percentage of Earth’s netprimary production
Key
Marine
Freshwater (on continents)
Terrestrial
5.2
0.3
0.1
0.1
4.7
3.53.3
2.9
2.7
2.41.8
1.7
1.6
1.5
1.3
1.0
0.4
0.4
125
360
1,500
2,500
500
3.0
90
2,200
900
600
800
600
700
140
1,600
1,2001,300
2,000
250
5.6
1.2
0.9
0.1
0.040.9
22
7.99.1
9.6
5.4
3.50.6
7.1
4.9
3.8
2.3
0.3
65.0 24.4
Percentage of Earth’ssurface area
(a) Average net primaryproduction (g/m2/yr)(b) (c)
The open ocean contributes relatively little per unit of area…but there is an awful lot of area.
Forested areas contribute a great deal per unit area
180 120W 60W 0 60E 120E 180
North Pole
60N
30N
Equator
30S
60S
South Pole
Regional annual NPP (light violet lowestred highest)
Primary Production in Marine and Freshwater EcosystemsIn marine and freshwater ecosystems– Both light and nutrients are important in
controlling primary production
Light Limitation
The depth of light penetration– Affects primary production throughout the
photic zone of an ocean or lake
Nutrient Limitation
More than light, nutrients limit primary productionA 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
The addition of large amounts of nutrients to lakes has a wide range of ecological impacts
In some areas, sewage runoff has caused eutrophication (overnourishment?) of lakes, which can lead to the eventual loss of most fish species from the lakes
The overnourished upper lake has a tremendous cyanobacterial bloom
Phosphorus is often the limiting nutrient for cyanobacterial growth. Oversupply leads to blooms
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
Actual EvapotranspirationThe contrast between wet and dry climates can be represented by a measure called actual evapotranspiration
Actual evapotranspiration
– Is the amount of water
annually transpired by
plants and evaporated from
a landscape
– Is related to net primary
production (warm and moist
is better than cold and dry)Actual evapotranspiration (mm H2O/yr)
Tropical forest
Temperate forest
Mountain coniferous forest
Temperate grassland
Arctic tundra
Desertshrubland
Net
prim
ary
prod
uctio
n (g
/m2 /
yr)
1,000
2,000
3,000
0500 1,000 1,5000
On a local scale– A soil nutrient is often the limiting factor in
primary production
EXPERIMENT Over the summer of 1980, researchers added phosphorus to some experimental plots in the salt marsh, nitrogento 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
, ab
ove-
grou
nd b
iom
ass
(g d
ry w
t/m
2)
Adding nitrogen (N) boosts net primaryproduction.
300
250
200
150
100
50
0June 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.
Secondary Production
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
Production EfficiencyWhen a caterpillar feeds on a plant leaf– Only about one-sixth
of the energy in the leaf is used for secondary production
The production efficiency of an organism– Is the fraction of
energy stored in food that is not used for respiration
Plant materialeaten by caterpillar
Cellularrespiration
Growth (new biomass)
Feces100 J
33 J
200 J
67 J
67J of the 100J of assimilated energy is used for respiration.
33J of 100J is new biomass…so the PE is 33%Nondigested materials aren’t figured into PE
Production efficiency
Endotherms (birds and mammals) have low production efficiencies. In the range of 1-3%– A lot of energy is used to maintain that constant body temp.
Ectotherms have higher production efficiencies– Fish at ~10%– Insects at around 40%
Trophic Efficiency and Ecological Pyramids
Trophic efficiency– Is the percentage of production transferred from
one trophic level to the next– Usually ranges from 5% to 20%
Pyramids of ProductionThe loss of energy with each transfer in a food chain can be represented by a pyramid of net production
Tertiaryconsumers
Secondaryconsumers
Primaryconsumers
Primaryproducers
1,000,000 J of sunlight
10 J
100 J
1,000 J
10,000 J
Pyramids of BiomassMost biomass pyramids– Show a sharp decrease at successively higher
trophic levels
Trophic level Dry weight(g/m2)
Primary producers
Tertiary consumers
Secondary consumers
Primary consumers
1.5
11
37809
Certain aquatic ecosystems can have inverted biomass pyramids– Here the phytoplankton (algae) grow very rapidly and are very
productive but their consumption by zooplankton holds the population size down
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
Pyramids of Numbers
A pyramid of numbers– Represents the number of individual organisms
in each trophic levelTrophic level Number of
individual organisms
Primary producers
Tertiary consumers
Secondary consumers
Primary consumers
3
354,904
708,624
5,842,424
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
Worldwide agriculture could successfully feed many more people– If humans all fed more efficiently, eating only
plant material (processing the plant material through another food animal decreases efficiency of energy transfer)
Trophic level
Secondaryconsumers
Primaryconsumers
Primaryproducers
Relative food energy available to the human population at different trophic levels
The Green World HypothesisMost terrestrial ecosystems have large standing crops despite the large numbers of herbivores
According to the green world hypothesis
– Terrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factors
The green world hypothesis proposes several factors that keep herbivores in check
– Plants have defenses against herbivores
– Nutrients, not energy supply, usually limit herbivores (paucity of essential nutrients in their diet)
– Abiotic factors limit herbivores (temperature, water availability)
– Intraspecific competition can limit herbivore numbers (territoriality, competition, etc.)
– Interspecific interactions check herbivore densities (predators, parasites, disease, etc)
Chemical CyclingBiological and geochemical processes move nutrients between organic and inorganic parts of the ecosystemLife 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
Biogeochemical Cycles
The water cycle and the carbon cycleTransportover land
Solar energy
Net movement ofwater vapor by wind
Precipitationover ocean
Evaporationfrom ocean
Evapotranspirationfrom land
Precipitationover land
Percolationthroughsoil
Runoff andgroundwater
CO2 in atmosphere
Photosynthesis
Cellularrespiration
Burning offossil fuelsand wood Higher-level
consumersPrimaryconsumers
DetritusCarbon compounds in water
Decomposition
THE WATER CYCLE THE CARBON CYCLE
Water moves in a global cycle driven by solar energyThe carbon cycle reflects the reciprocal processes of photosynthesis and cellular respiration
More Biogeochemical Cycles
N2 in atmosphere
Denitrifyingbacteria
Nitrifyingbacteria
Nitrifyingbacteria
Nitrification
Nitrogen-fixingsoil bacteria
Nitrogen-fixingbacteria in rootnodules of legumes
Decomposers
Ammonification
Assimilation
NH3 NH4+
NO3
NO2
Rain
Plants
Consumption
Decomposition
Geologicuplift
Weatheringof rocks
Runoff
SedimentationPlant uptakeof PO4
3
Soil
Leaching
THE NITROGEN CYCLE THE PHOSPHORUS CYCLE
The nitrogen cycle and the phosphorous cycle
Decomposition and Nutrient Cycling Rates
Decomposers (detritivores) play a key role in the general pattern of chemical cycling
The rates at which nutrients cycle in different ecosystems are extremely variable, mostly as a result of differences in rates of decomposition
Consumers
Producers
Nutrientsavailable
to producers
Abioticreservoir
Geologicprocesses
Decomposers
Human activities
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
Agriculture and Nitrogen CyclingAgriculture constantly removes nutrients from ecosystems that would ordinarily be cycled back into the soil
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
Nutrient EnrichmentIn addition to transporting nutrients from one location to another…Humans have added entirely new materials, some of them toxins, to ecosystems
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
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
Sewage runoff contaminates freshwater ecosystems– Causing eutrophication, excessive algal growth,
which can cause significant harm to these 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
4.6
4.64.3
4.14.3
4.6
4.64.3
Europe
North America
Numbers indicate the average pH of precipitation in that area
By the year 2000 the entire contiguous United States was affected by acid precipitation
(defined as precipitation less than pH 5.6)
Field pH5.35.2–5.35.1–5.25.0–5.14.9–5.04.8–4.94.7–4.84.6–4.74.5–4.64.4–4.54.3–4.44.3
Toxins in the EnvironmentHumans release an immense variety of toxic chemicals– Including thousands of synthetics previously
unknown to nature (Xenobiotic)
One of the reasons such toxins are so harmful– Is that they become more concentrated in
successive trophic levels of a food web
In some cases, harmful substances– Persist for long periods of time in an ecosystem and
continue to cause harm
In biological magnification– Toxins concentrate at higher trophic levels because
at these levels biomass tends to be lower
Con
cent
ratio
n of
PC
Bs
Herringgull eggs124 ppm
Zooplankton 0.123 ppm
Phytoplankton 0.025 ppm
Lake trout 4.83 ppm
Smelt 1.04 ppm
Small concentrations of toxin spread among many individuals
Fewer individuals each with high concentration of toxin
Atmospheric Carbon Dioxide
The rising level of atmospheric carbon dioxide is human in origin
Rising Atmospheric CO2Due to the increased burning of fossil fuels and other human activities the concentration of atmospheric CO2 has been steadily increasing
CO
2 c
onc
en
trat
ion
(pp
m)
390
380
370
360
350
340
330
320
310
3001960 1965 1970 1975 1980 1985 1990 1995 2000 2005
1.05
0.90
0.75
0.60
0.45
0.30
0.15
0
0.15
0.30
0.45
Te
mp
era
ture
va
ria
tion
(C
)
Temperature
CO2
Year
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
Increased levels of atmospheric CO2 are magnifying the greenhouse effect– Which could cause global warming and
significant climatic change
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
Satellite studies of the atmosphere suggest that the ozone layer has been gradually thinning since 1975
Ozo
ne la
yer
thic
knes
s (D
obso
n un
its)
Year (Average for the month of October)
350
300
250
200
150
100
50
01955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005
The destruction of atmospheric ozone– Probably results from chlorine-releasing
pollutants produced by human activity
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
ClO
Cl2O2
O2
Chlorine atoms
Scientists first described an “ozone hole” over Antarctica in 1985– it has increased in size as ozone depletion has
increased