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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
Autotrophs; usually photosynthetic; includes plants, algae, &
many species of bacteria
Primary consumers
Herbivores
Secondary consumers
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
Carnivores that eat other carnivores
**many primary & higher order consumers are opportunistic
feeders (supplement diet of autotrophs with heterotrophs if
available)
Detrivores (decomposers)
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
Pathway along which energy flows from one trophic level to the next
Food web
Feeding relationships in an ecosystem
Production
Consumption
Incorporation of energy & materials into bodies of organisms
Metabolic use of assimilated molecules for growth & reproduction
Decomposition
Breakdown of organic molecules into inorganic molecules
Energy flow in ecosystems
Global energy budget
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
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)
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
Lack of nutrients (usually N or P) limits primary
productivity in aquatic ecosystems & terrestrial
ecosystems
Temperature & moisture limit primary productivity
in terrestrial & wetland ecosystems
Measured through evapotranspiration
Secondary productivity
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)
Ecological efficiency & ecological pyramids
Trophic efficiency
% 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
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
Move nutrients among organic & inorganic, biotic & abiotic
Water cycle
Carbon cycle
Phosphorus cycle
Nitrogen cycle
Nitrification
Aerobic soil bacteria oxidize NH4+NO2-NO3Denitrification
Anaerobic bacteria obtain oxygen by converting NO3- N2
Nitrogen fixation bacteria
Legumes… N2 NH3+
Decomposition rates largely determine the
rates of nutrient cycling
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
Field experiments reveal how vegetation
regulates chemical cycling
Hubbard Brook experiment (NH)
Long term ecological research since 1963
Plants control amount of nutrients leaving the ecosystem
Human impacts on ecosystems
Human population is disrupting chemical cycles
throughout the biosphere
Agricultural effects on nutrient cycling
Accelerated eutrophication of lakes algal blooms
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
Burning of fossil fuels adds sulfur & nitrogen oxides that
react with water in the atmosphere
Falls as acid rain pH<5.6
Affects soil chemistry
Leaching nutrients from soil & plants
Kills keystone species in aquatic ecosystem
Toxins can become concentrated in successive trophic levels of
food webs= biological magnification
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
CO2 emissions & greenhouse effect
Depletion of atmospheric ozone
17% increase since 1958
best documented in Antarctica
Caused by CFC’s & refrigerants
Exploding human population is altering habitats & reducing
biodiversity worldwide
Increasing population & related activities continue to disrupt trophic
structures, energy flow, & chemical cycling
Human encroachment has resulted in:
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
Eliminated by 2020 at current rate
Logging, war, oil spills continue to breakup & destroy habitats