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Chapter 51
Ecosystems
Chapter 51
Ecosystems
Many global environmental problems have emerged recently.
Ecosystems consist of all the organisms that live in an area along with the nonbiological components.
Energy and nutrient flows link the biotic and abiotic environments.
Energy Flow and Trophic Structure
All ecosystems consist of four components that are linkedby the flow of energy:
• Primary producers
• Consumers
• Decomposers
• Abiotic environment (Fig. 51.1)
External energy source
PRIMARYPRODUCERS
CONSUMERS DECOMPOSERS
ABIOTIC ENVIRONMENT
Figure 51.1
External energy source
PRIMARYPRODUCERS
CONSUMERS DECOMPOSERS
ABIOTIC ENVIRONMENT
Figure 51.1
Energy Flow and Trophic Structure
Key points about energy flow through ecosystems.
• Energy enters ecosystems in the form of sunlight that is usedin photosynthesis by producers.
• Plants use only a tiny fraction of the total radiation that isavailable to them.
• Only a tiny fraction of fixed energy actually becomes availableto consumers.
Energy Flow and Trophic Structure
Key points about energy flow through ecosystems.
• Most net primary production that is consumed enters the decomposer food web.
• From there, only a small fraction is used for secondaryproduction by herbivores and carnivores.
• Most energy fixed during photosynthesis is used for respiration, not synthesis of new tissues. (Fig. 51.2)
Energy source:1,254,000kcal/m2/year
0.8% energy captured by photosynthesis. Of this...
…45% supports growth(Net primary production)
…11% entersgrazing food web
…34% entersdecomposer food webas dead material
…55% lostto respiration
Figure 51.2
Energy source:1,254,000kcal/m2/year
…11% enters grazing food web
…34% entersdecomposer food webas dead material
0.8% energy captured by photosynthesis. Of this...
…45% supports growth(Net primary production)
…55% lost to respiration
Figure 51.2
0–100100–200200–400400–600600–800>800
Productivity ranges (g/m2/yr)
Figure 51.3a
Terrestrial productivity
<3535–5555–90>90
Productivity ranges (g/m2/yr)
Figure 51.3b
Marine productivity
80.7% respiration
17.7% excretion1.6% growth and reproduction
Energy derived from plants
Figure 51.4
Predators of decomposers:
Spider
Centipede
MushroomMushroom
EarthwormEarthworm
Primary Primary decomposers:decomposers:
Bacteria and archaeaBacteria and archaeaMillipedeMillipede
NematodesNematodesPillbugsPillbugs
Salamander
305 nm 49.4 µm
Figure 51.5
PuffballPuffball
Energy Flow and Trophic Structure
Trophic structure
• Organisms that obtain their energy from the same type ofsource occupy the same trophic level.
• Each feeding level within an ecosystem represents a trophiclevel.
Energy Flow and Trophic Structure
Trophic structure
• Organisms at the top trophic level are not eaten by any other organisms.
• Productivity is highest at the lowest trophic level. (Fig. 51.6a,b)
Trophic
level4
3
2
1
Feeding strategySecondary carnivore
Carnivore
Herbivore
Autotroph
Grazing food chain Decomposer food chain
Cricket
Maple tree leaves
Owl
Shrew
Earthworm
Dead maple leaves
Cooper’shawk
Robin
Figure 51.6a
Trophic levels
4Secondary carnivore
3
Carnivore
2
Herbivore
1
Autotroph
Productivity
Figure 51.6b
Pyramid of productivity
Energy Flow and Trophic Structure
Food chains and food webs
• Food chains are typically embedded in more complexfood webs. (Fig. 51.7a,b)
Pisaster(a sea star)
Thais(a snail)
Bivalves(clams, mussels)
Figure 51.7a
Food chain
Pisaster
Thais
ChitonsLimpets
BivalvesAcornbarnacles
Gooseneckbarnacles
Figure 51.7b
Food web
Energy Flow and Trophic Structure
Food chains and food webs
• The maximum number of links in any food chain or web ranges from 1 to 6. (Fig. 51.7c)
• Hypotheses offered to explain this:
Energy transfer may limit food-chain length.
Long food chains may be more fragile.
Food-chain length may depend on environmental complexity.
Nu
mb
er o
f o
bse
rvat
ion
s
Number of links in food chain
10
8
6
4
2
01 2 3 4 5 6
Streams
Lakes
Terrestrial
Figure 51.7c
Food chains tend to have few links.
Average number of links = 3.5
Biogeochemical Cycles
The path an element takes as it moves from abiotic systems through living organisms and back again is referred to asits biogeochemical cycle. (Fig. 51.8)
Ass
imila
tio
n
Loss to erosion or leaching into groundwater
Soil nutrient pool
Decomposerfood web
Detritus
Death
Herbivore
Uptake
Plants
Feces or urine
Figure 51.8
Boreal forest
Figure 51.9 upper
Tropical rain forest
Figure 51.9 lower
Biogeochemical Cycles
A key feature in all cycles is that nutrients are recycledand reused.
The overall rate of nutrient movement is limited most by decomposition of detritus.
The rate of nutrient loss is a very important characteristic inany ecosystem. (Fig. 51.10a,b)
Devegetation experiment
Choose two similar watersheds.Document nutrient levels in soil organic matter, plants, and streams.
Figure 51.10a upper
Figure 51.10a lower
Clearcut Control
Devegetate one watershed and leave the other intact.Monitor the amount of dissolved substances in streams.
Devegetated
Net
dis
solv
ed s
ub
stan
ce (
kg/h
a)
1965–66 1966–67 1967–68 1968–69 1969–70
Control
1000
800
600
400
200
0
Year
Figure 51.10b
Nutrient runoff results
Biogeochemical Cycles
Nutrient flow among ecosystems links local cycles into one massive global biogeochemical cycle.
• The carbon cycle and the nitrogen cycle are examples of major, global biogeochemical cycles. (Fig. 51.11, 51.13a)
• Humans are now disrupting almost all biogeochemical cycles. This can have very harmful effects. (Fig. 51.12a,b; 51.13b)
THE GLOBAL CARBON CYCLEAll values in gigatons of carbon per year
Physicaland chemical processes: 92
2Ocean: 40,000 Rivers: 1
Land, biota, soil, litter, peat: 2000
Decomposition:50
Respiration:50
Photosynthesis:102
Physicaland chemical processes: 90
Deforestation:1.5
Fossilfuel use:
6.0
Atmosphere: 750 (in 1990)+3.5 per year
Aquatic ecosystems Terrestrial ecosystems Human–inducedchanges
Figure 51.11
THE GLOBAL NITROGEN CYCLE
Nitrogenfixing cyanobacteria
MudDecomposition of detritus into ammonia
Nitrogen-fixing bacteria in roots and soil
Industrial fixation
Protein andnucleic acid synthesis
Atmospheric nitrogen (N2)
Bacteria in muduse N-containing molecules as energy sources, excrete (N2)
Run–off
Lightning and rain
Figure 51.13a
Land use
Fossil fuel use
Year
An
nu
al f
lux
of
carb
on
(10
15g
)
6
5
4
3
2
1
01860 1880 1900 1920 1940 1960 1980
Figure 51.12a
Human-induced increases in CO2 flux over time
Year
CO
2 co
nce
ntr
atio
n (
pp
m)
360
350
340
330
320
3101960 1970 1980 1990
Figure 51.12b
Atmospheric CO2
Natural sources Human sources
Am
ou
nt
of
nit
rog
en (
gig
ato
ns/
year
)
160
140
120
100
80
60
40
20
0
Sources of nitrogen fixation
Lightning
Biologicalfixation
Fossil fuels
Nitrogenfertilizer
Nitrogen-fixing crops
Figure 51.13b