Chapter 51 Ecosystems
Mar 27, 2015
Chapter 51
Ecosystems
Ecosystems
Population: all the individuals of a certain species that live in a particular area
Community: all the different species that interact together within a particular area
Ecosystems consist of all the organisms that live in an area along with the nonbiological (abiotic) components.
Ecosystems
Many global environmental problems have emerged recently.
Ecosystem ecology follows the flow of energy and nutrients through ecosystems
Humans have artificially affected the flow of these components
Energy Flow within Ecosystems
Energy enters an ecosystem primarily though sunlight:
Energy Flow and Trophic Structure
Species within an ecosystems are classified into different trophic levels:
• Primary producers: autotrophs, photosynthetic- plants, algae, some bacteria
• Consumers
• Primary consumers: herbivores that eat producers (plants)- deer, rabbits, etc.
• Secondary consumers: carnivores that eat herbivores: wolf eating a deer
• Tertiary consumers: carnivores that eat carnivores: a hawk eating a snake that ate a mouse
• Decomposers: fungi, bacteria that break down organic material (dead plants and animals)
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
Different Trophic Levels in an Ecosystem
External energy source
PRIMARYPRODUCERS
CONSUMERS DECOMPOSERS
ABIOTIC ENVIRONMENT
Energy Flow in an ecosystem
Predators of decomposers:
Spider
Centipede
MushroomMushroom
EarthwormEarthworm
Primary Primary decomposers:decomposers:
Bacteria and archaeaBacteria and archaeaMillipedeMillipede
NematodesNematodesPillbugsPillbugs
Salamander
305 nm 49.4 µm
PuffballPuffball
Decomposers
Energy Flow and Trophic Structure
Key points about energy flow through ecosystems.
• Plants use only a tiny fraction of the total radiation that isavailable to them.
• Most energy fixed during photosynthesis is used for respiration, not synthesis of new tissues.
• Only a tiny fraction of fixed energy actually becomes availableto consumers.
• Most net primary production that is consumed enters the decomposer food web.
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
Ecological Efficiency: percent of energy transferred from one trophic level to the next
Ecosystem Processes
Production: rate at which energy/nutrients are converted into growth
• Includes Primary Production: growth by autotrophs
• Includes Secondary Production - growth by heterotrophs
Consumption - the intake and use of organic material by heterotrophs
Decomposition - the chemical breakdown of organic material
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
Very little of the energy consumed by primary consumers are used for secondary production
4Secondary carnivore
3
Carnivore
2
Herbivore
1
Autotroph
Productivity
Example: 100g of plant becomes 5-20g of
grasshopper then 0.25-1g of mouse
Pyramid of productivity
Pisaster(a sea star)
Thais(a snail)
Bivalves(clams, mussels)
The Different Trophic levels in an ecosystem is often pictured as a Food chain
Energy Flow and Trophic Structure
Food chains and food webs
• Food chains are typically embedded in more complexfood webs.
• Many organisms feed at more than one trophic level
Pisaster
Thais
ChitonsLimpets
BivalvesAcornbarnacles
Gooseneckbarnacles
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.
• 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
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.
Examples: nitrogen cycle, carbon cycle, phosphorus cycle
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
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.
Boreal forest: nutrients are put back into the soil slowly, so organic material builds up
Tropical rain forest: decomposition is rapid so there is very little organic build up
Result: if living material is removed from tropical rain forests, the soil is nutrient poor to support new growth
Devegetation experiment
Choose two similar watersheds.Document nutrient levels in soil organic matter, plants, and streams.
The rate of nutrient loss is a very important characteristic inany ecosystem.
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
Nutrient runoff results
Nutrient export increases dramatically in devegetated plot
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.
• Humans are now disrupting almost all biogeochemical cycles. This can have very harmful effects.
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
Humans are adding significant amounts of carbon into the atmosphere
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
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
Industrial fixationNitrogen
fixing cyanobacteria
MudDecomposition of detritus into ammonia
Nitrogen-fixing bacteria in roots and soil
Protein andnucleic acid synthesis
Atmospheric nitrogen (N2) =78%
Bacteria in muduse N-containing molecules as energy sources, excrete (N2)Run–off
Lightning and rain
Only nitrogen-fixing bacteria can use N2
make ammonia (NH3) or nitrate (NO3) limiting nutrient (demand exceeds supply) for plants
All organisms require nitrogen to make protein Animals get nitrogen from their diets, not the air
THE GLOBAL NITROGEN CYCLE
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
Human activities now fix almost as much nitrogen each year as natural sources