Ecosystems
Chapter 31
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31.1 Energy Flows
Through Ecosystems
Ecology – Study of interactions of living organisms
with one another and their physical environment
Community – Collection of all organisms living
together in an area
Habitat – The place where an organism lives
Ecosystem – A largely self-sustaining collection of
organisms and their physical environment
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The Path of Energy in Ecosystems
Energy flows into the biological world from the sun
Producers capture sunlight and transform it into
chemical energy by photosynthesis
Also called autotrophs
Consumers obtain their energy from consuming
plants or other animals
Also called heterotrophs
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A trophic, or feeding,
level consists of all
organisms feeding at
the same energy level
Food chain
Passage of
food energy
through
ecosystem
trophic levels
in a linear path
Fig. 31.1
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Fig. 31.2
Because animals
eat at different
trophic levels,
most ecosystems
have paths of
energy that are
not linear
A complicated
path of energy
flow is called a
food web
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Producers
Green plants and algae
Use solar energy to build
energy-rich carbohydrates
Herbivores
Animals that eat plants
The primary consumers
of ecosystems
Carnivores
Animals that eat herbivores
The secondary consumers of ecosystems
Omnivores are animals that eat both plants and animals
Tertiary consumers are animals that eat other carnivores
Detritivores
Organisms
that eat dead
organisms
Decomposers
Organisms that
break down organic
substances
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Energy Flows Through Trophic Levels
Primary productivity
Total amount of light energy converted to
organic compounds in a given area per unit time
Net primary productivity
Primary productivity minus energy expended by
the photosynthetic organisms to fuel metabolism
Biomass
Total weight of all organisms in an ecosystem
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Biomass available at
About one order of
the next trophic level
magnitude of available
energy is lost from one
trophic level to the next
Reason why food
chains generally consist
of only 3 or 4 steps
Fig. 31.4 How heterotrophs
use food energy
Cayuga Lake
In NY
Fig. 31.6 Energy loss in an ecosystem
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31.2 Ecological Pyramids
A plant fixes about 1% of the sun’s energy that falls
on its green parts
Successive members of a food chain incorporate ~
10% of energy available in organisms they consume
Therefore, there are far more individuals at the
lower tropic levels
The biomass of the lower trophic levels also
tends to be greater
These relationships appear in a diagram as
pyramids
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Fig. 31.6 Ecological pyramids
Fairly large
animals
Found in larger numbers, but
still contain 90% less energy
Inverted pyramid
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31.3 The Water Cycle
Unlike energy, the physical components of
ecosystems are passed around and reused
Recycling or cycling
In each cycle the chemical resides for a time in an
organism, then returns to the non-living environment
A biogeochemical cycle
Water cycles within ecosystems in two ways
Environmental water cycle
Organismic water cycle
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Environmental Water Cycle
Water vapor condenses and falls to earth
Reenters atmosphere by evaporation
Fig. 31.7
Solar
energy
Transpiration
Evaporation
Precipitation
Oceans
Runoff
Lakes
Percolation
in soil
Aquifer
Groundwater
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31.4 The Carbon Cycle
The carbon cycle is begun by plants who use CO2
to build organic molecules
Carbon atoms are returned to the atmosphere by
Respiration
Most organisms extract energy from food and
release CO2 as a by-product
Combustion
Burning of wood or fossil fuels (coal or oil) releases
trapped carbon in the form of CO2
Erosion
When limestone (calcium carbonate in sediment)
erodes, CO2 is released
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Fig. 31.9 The carbon cycle
CO2 in
atmosphere
Combustion of fuels
Industry and home
Diffusion
Respiration
Photosynthesis
Plants
Animals
Dissolved CO2
Bicarbonates
Photosynthesis
Animals
Plants and algae
Carbonates in sediment Death
Death
and
decay
Fossil fuels
(oil, gas, coal)
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31.5 Soil Nutrients and Other
Chemical Cycles
Many other chemicals cycle through the ecosystem
Nitrogen
Phosphorus
Sulfur
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The Nitrogen Cycle
The atmosphere is 78% nitrogen gas (N2)
However, most organisms are unable to use N2
The triple covalent bond is very difficult to break
Some bacteria can break the triple bond and bind its
N atoms to hydrogen forming ammonia (NH3)
This process is termed nitrogen fixation
Animals eat plants that have taken up fixed nitrogen
Nitrogen reenters the ecosystem through
Animal excretion
Decomposition by detritivores and decomposers
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Fig. 31.10 The nitrogen cycle
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The Phosphorus Cycle
Phosphorus (P) has no atmospheric form
Exists mostly as the mineral calcium phosphate
Phosphate ions are absorbed by plant roots and
used to build organic molecules like ATP and DNA
When animals and plants die and decay, bacteria
convert organic phosphorus into ions
Low phosphorus levels in freshwater lakes limits the
overgrowth of algae
An excessive increase in phosphorus, or other
nutrients, is called eutrophication
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Fig. 31.11 The phosphorus cycle
Plants
Land
animals
Soluble soil
phosphate
Loss in
drainage
Decomposers
(bacteria and
fungi)
Animal tissue
and feces
Phosphates
in solution
Animal tissue
and feces
Urine
Decomposers
(bacteria and
fungi)
Rocks and
minerals
Aquatic
animals
Plants and
algae
Precipitates
Loss to deep sediment
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The Sulfur Cycle
Sulfur is pumped into the
atmosphere by coal-burning
power plants
Large amounts can harm an
ecosystem
Excess sulfur combines with
water vapor to produce sulfuric
acid
This acid then reenters the
ecosystem as precipitation
Acid rain
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