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Chapter 54: Ecosystems
Ecosystem
All organisms living in an area along with the
abiotic factors with which they interact.
Most inclusive level of biological organization
Dynamics of an ecosystem:
Energy
flow
Chemical cycling
Trophic levels
 Primary producer
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Autotrophs; usually photosynthetic; includes plants, algae, &
many species of bacteria
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Primary consumers
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Herbivores
Secondary consumers
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Streams=Debris falling from terrestrial plants are major source of
organic materials
Limnetic zone of lakes & open oceans= Phytoplankton
Shallow freshwater & marine ecosystems= Multi-cellular algae &
plants
Aphotic zones receive energy & nutrients from overlying photic zones
Deep ocean vents= chemoautotrophs oxidize H2S for energy needs
Carnivore that eats herbivores
Tertiary consumers
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Carnivores that eat other carnivores
**many primary & higher order consumers are opportunistic
feeders (supplement diet of autotrophs with heterotrophs if
available)
 Detrivores (decomposers)
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Consumers that derive energy from detritus (organic waste) & dead
organisms from all trophic levels
Link between all organisms in an ecosystem
*Trophic relationships determine an ecosystem’s routes of energy
& chemical cycling
 Food chain
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Pathway along which energy flows from one trophic level to the next
Food web
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Feeding relationships in an ecosystem
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Production
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Consumption
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Incorporation of energy & materials into bodies of organisms
Metabolic use of assimilated molecules for growth & reproduction
Decomposition
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Breakdown of organic molecules into inorganic molecules
Energy flow in ecosystems
 Global energy budget
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Earth receives 1022 joules of solar radiation/day (1J= 0.239
cal)
Most intense radiation at equator
~1% of total solar energy is converted by photosynthesis
into organic molecules (170 billion tons of biomass/year)
Primary productivity
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Amount of light energy converted by ecosystem’s autotrophs
into organic compounds in a given time period
Gross primary productivity (GPP): total primary productivity
Net primary productivity (NPP)
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NPP=GPP-R
R=energy for respiration of the producer
Key measurement to ecologists
Represents the stored chemical energy available to consumers in the
ecosystem
*varies with the ecosystem
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Lack of nutrients (usually N or P) limits primary
productivity in aquatic ecosystems & terrestrial
ecosystems
Temperature & moisture limit primary productivity
in terrestrial & wetland ecosystems
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Measured through evapotranspiration
Secondary productivity
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Rate at which consumers convert chemical energy
in food into biomass
Energy is lost in feces
 Respiration & body heat results in energy lost
 Some energy is used to generate growth & reproduction
(adds to biomass)
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Ecological efficiency & ecological pyramids
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Trophic efficiency
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% of production transferred from one trophic level to the
next
~5-20% of energy at each trophic level is passed
on to the next level (average 10%)
Loss of energy can be represented diagrammatically
through:
Pyramid of productivity
 Pyramid of biomass
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Can be inverted if turnover time for producers is short
Pyramid of numbers
Cycling of Chemical elements in ecosytems
 Inexhaustible influx of solar energy but continuation
of life depends on recycling of essential chemical
compounds
 Biogeochemical cycles
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Move nutrients among organic & inorganic, biotic & abiotic
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Water cycle
Carbon cycle
Phosphorus cycle
Nitrogen cycle
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Nitrification
 Aerobic soil bacteria oxidize NH4+NO2-NO3Denitrification
 Anaerobic bacteria obtain oxygen by converting NO3- N2
Nitrogen fixation bacteria
 Legumes… N2 NH3+
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Decomposition rates largely determine the
rates of nutrient cycling
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Availability of water, oxygen, & temperature
influence the rate of decomposition & recycling
Tropical rain forest recycle rapidly resulting in soil with
very little nutrients
 Soils in temperate deciduous forest may contain 50% of
all of the organic materials in the ecosystem
 Decomposition in tundra can take up to 50 years
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Field experiments reveal how vegetation
regulates chemical cycling
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Hubbard Brook experiment (NH)
Long term ecological research since 1963
 Plants control amount of nutrients leaving the ecosystem
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Human impacts on ecosystems
 Human population is disrupting chemical cycles
throughout the biosphere
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Agricultural effects on nutrient cycling
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Accelerated eutrophication of lakes algal blooms
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Nutrients in food crops removed from geographical area
Soil nutrient depletion leads to use of fertilizers
Introduction of toxic materials runoff &/or ground water
contamination
Result of fertilizer runoff, sewage, & factory waste
Decrease in aerobic respiration as debris levels increase
Acid precipitation
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Burning of fossil fuels adds sulfur & nitrogen oxides that
react with water in the atmosphere
Falls as acid rain pH<5.6
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Affects soil chemistry
Leaching nutrients from soil & plants
Kills keystone species in aquatic ecosystem
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Toxins can become concentrated in successive trophic levels of
food webs= biological magnification
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Examples: DDT used to kill insect pests but found in high levels in
osprey, eagles, & other birds
Human activities are causing fundamental changes in the
composition of the atmosphere
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CO2 emissions & greenhouse effect
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Depletion of atmospheric ozone
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17% increase since 1958
best documented in Antarctica
Caused by CFC’s & refrigerants
Exploding human population is altering habitats & reducing
biodiversity worldwide
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Increasing population & related activities continue to disrupt trophic
structures, energy flow, & chemical cycling
Human encroachment has resulted in:
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Only 15% of original primary USA forest & 1% of original tall grass prairie
remaining
Tropical rainforest being cut at a rate of 500,000km2/year
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Eliminated by 2020 at current rate
Logging, war, oil spills continue to breakup & destroy habitats