Dynamics of Ecosystems
Chapter 57
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Biogeochemical Cycles
•  Ecosystem: includes all the organisms that
live in a particular place, plus the abiotic
environment in which they live and interact
•  Biogeochemical cycles: chemicals moving
through ecosystems; biotic and abiotic
•  Biogeochemical cycles usually cross the
boundaries of ecosystem
–  One ecosystem might import or export
chemicals to another
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Biogeochemical Cycles
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Biogeochemical Cycles
•  Carbon fixation: metabolic reactions
that make nongaseous compounds from
gaseous ones
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Biogeochemical Cycles
•  Methane producers
– Microbes that break down organic
compounds by anaerobic cellular
respiration provide an additional
dimension to the carbon cycle
– Methanogens: produce methane (CH4)
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Biogeochemical Cycles
•  Over time, globally, the carbon cycle may
proceed faster in one direction
•  This can cause large consequences if
continued for many years
•  Earth’s present preserves of coal, and other
fossil fuels were built up over geological
time
•  Human burning of fossil fuels is creating
large imbalances in the carbon cycle
•  The concentration of CO2 in the atmosphere
is going up year by year
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Biogeochemical Cycles
•  Water Cycle
– All life depends on the presence of water
– 60% of the adult human body weight is
water
– Amount of water available determines the
nature and abundance of organisms
present
– It can be synthesized and broken down
• Synthesized during cellular respiration
• Broken down during photosynthesis
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Biogeochemical Cycles
•  Basic water cycle
– Liquid water from the Earth’s surface
evaporates into the atmosphere
– Occurs directly from the surfaces of
oceans, lakes, and rivers
– Terrestrial ecosystems: 90% of
evaporation is through plants
– Water in the atmosphere is a gas
– Cools and falls to the surface as
precipitation
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Biogeochemical Cycles
•  Groundwater: under ground water
– Aquifers: permeable, underground
layers of rock, sand, and gravel
saturated with water
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Biogeochemical Cycles
•  Water cycle
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Biogeochemical Cycles
•  Changes in the supply of water to an
ecosystem can radically alter the nature
of the ecosystem
•  Deforestation disrupts the local water
cycle
• Water that falls
as rain drains
away
•  Tropical rain forest à semiarid desert
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Biogeochemical Cycles
•  Nitrogen Cycle
– Nitrogen is a component of all proteins
and nucleic acids
– Usually the element in shortest supply
– Atmosphere is 78% nitrogen
– Availability
• Most plants and animals can not use N2
(gas)
• Use instead NH3, and NO312
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Biogeochemical Cycles
•  Nitrogen fixation: synthesis of nitrogen
containing compounds from N2
– Nitrification: N2 --> NH3 --> NO3– Denitrification: NO3- --> N2
– Both processes are carried out by
microbes: free or living on plant roots
– Nitrogenous wastes and fertilizer use
radically alter the global nitrogen cycle
– Humans have doubled the rate of transfer
of N2 in usable forms into soils and water
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Biogeochemical Cycles
•  Nitrogen Cycle
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Biogeochemical Cycles
•  Phosphorus cycle
– Phosphorus is required by all organisms
• Occurs in nucleic acids, membranes,
ATP
– No significant gas form
– Exists as PO43- in ecosystems
– Plants and algae use free inorganic
phosphorus, animals eat plants to obtain
their phosphorus
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Biogeochemical Cycles
•  Phosphorus cycle
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Biogeochemical Cycles
•  Limiting nutrient: weak link in an
ecosystem; shortest supply relative to
the needs of organisms
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Biogeochemical Cycles
Every year millions of metric tons of ironrich dust is carried by the trade winds,
from the Sahara Desert, across the globe
to as far as the Pacific Ocean
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Biogeochemical Cycles
•  Biogeochemical cycling in a forest
ecosystem-- Hubbard Brook Experiment
•  Undisturbed forests are efficient at
retaining nutrients
•  Disturbed (cut trees down) amount of
water runoff increased by 40%
– Loss of Ca; increased nine fold
– Loss of Phosphorus did not increase
– Loss of NO3-; 53kg/hectare/yr
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Biogeochemical Cycles
The Hubbard Brook Experiment
38-acre watershed. Orange curve shows
nitrate concentration in the runoff water from
the deforested watershed. Green curve shows
the nitrate concentration in runoff from an
undisturbed watershed
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Flow of Energy in Ecosystems
•  Energy is never recycled
•  Energy exists as;
– Light
– Chemical-bond energy
– Motion
– Heat
•  First Law of Thermodynamics: energy
is neither created nor destroyed; it
changes forms
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Flow of Energy in Ecosystems
•  Organisms cannot convert heat to any
of the other forms of energy
•  Second Law of Thermodynamics:
whenever organisms use chemicalbond or light energy some is converted
to heat (entropy)
•  Earth functions as an open system for
energy
•  Sun our major source of energy
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Flow of Energy in Ecosystems
•  Earth’s incoming and outgoing flows of
radiant energy must be equal for global
temperatures to stay constant
•  Human activities are changing the
composition of the atmosphere
•  Greenhouse effect: heat accumulating
on Earth, causing global warming
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Flow of Energy in Ecosystems
•  Trophic levels: which level an organism
“feeds” at
•  Autotrophs: “self-feeders” synthesize the
organic compounds of their bodies from
inorganic precursors
– Photoautotrophs: light as energy source
– Chemoautotrophs: energy from
inorganic oxidation reactions
• prokaryotic
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Flow of Energy in Ecosystems
•  Heterotrophs: cannot synthesize
organic compounds from inorganic
precursors;
– animals that eat plants and other
animals;
–  fungi that use dead and decaying
matter (detritivores)
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Flow of Energy in Ecosystems
•  Trophic levels
– Primary producers: autotrophs
– Consumers: heterotrophs
• Herbivores: first consumer level
• Primary carnivores: eat herbivores
• Secondary carnivores: eat primary
carnivores or herbivores
• Detritivores: eat decaying matter
– Decomposers: microbes that break
up dead matter
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Trophic levels within an ecosystem
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Flow of Energy in Ecosystems
•  Productivity: the rate at which the
organisms in the trophic level
collectively synthesize new organic
matter
•  Primary productivity: productivity of
the primary producers
•  Respiration: rate at which primary
producers break down organic
compounds
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Flow of Energy in Ecosystems
•  Gross primary productivity (GPP):
raw rate at which primary producers
synthesize new organic matter
•  Net primary productivity (NPP): is the
GPP less the respiration of the primary
producers
•  Secondary productivity: productivity
of a heterotroph trophic level
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Flow of Energy in Ecosystems
•  Standing crop biomass: chief static
property of a population or trophic level;
the amount of organic matter present at
a particular time
•  Fraction of incoming solar radiant
energy captured by producers is ~ 1%/
year
•  Used to make chemical-bond energy
•  Break bonds in ATP for metabolic
processes
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Ecosystem productivity per year
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Flow of Energy in Ecosystems
•  Limits on top carnivores: exponential
decline of chemical-bond energy limits the
lengths of trophic chains and the numbers
of top carnivores an ecosystem can support
– Little energy
– Large carnivores
– Longest chains occur in the oceans
– Top carnivore populations are small
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Flow of Energy in Ecosystems
Flow of energy through the trophic levels
of Cayuga Lake
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Flow of Energy in Ecosystems
•  Trophic level interactions
– Trophic cascade: process by which
effects exerted at an upper level flow
down to influence two or more lower
levels
– Top-down effects: when effects flow
down
– Bottom-up effects: when effect
flows up through a trophic chain
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Biodiversity and Stability
David Tilman: species richness may increase
stability of an ecosystem
•  Plots with more species showed less yearto-year variation in biomass
•  Drought: decline in biomass negatively
related to species richness
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Biodiversity and Stability
•  Tilman’s conclusion not accepted by all
ecologists
•  Critics question the validity and
relevance:
– When more species are added to a
plot the greater the probability that
one species will be highly productive
– Plots would have to exhibit “over
yielding”
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Biodiversity and Stability
•  Species richness is influenced by
ecosystem characteristics
– Primary productivity
– Habitat heterogeneity
• Accommodate more species
– Climatic factors
• More species might be expected to
coexist in seasonal environment
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Biodiversity and Stability
Factors that affect species richness
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Biodiversity and Stability
•  Tropical regions have the highest diversity
– Species diversity cline: biogeographic
gradient in number of species correlated
with latitude
• Reported for plants and animals
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Biodiversity and Stability
Latitudinal cline in species richness
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Island Biogeography
•  Robert MacArthur and Edward O. Wilson
proposed that species-area relationship
was a result of the effect of geographic area
and isolation
– Islands have a tendency to accumulate
more and more species through
dispersion
– Rate of colonization must decrease as the
pool of potential colonizing species
becomes depleted
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Island Biogeography
•  The rate of extinction should increase--the
more species on an island
•  At some point extinctions and colonizations
should be equal
•  MacArthur and Wilson equilibrium
model: island species richness is a
dynamic equilibrium between colonization
and extinction
Island size and distance from the mainland
would affect colonization and extinction
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Equilibrium model
•  The equilibrium model is still being tested by
Wilson and Simberloff
•  Long-term experimental field studies are
suggesting that the situation is more
complicated than first believed
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