Chapter 54: Ecosystems

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Chapter 54Ecosystems
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Movement of Stuff Through Ecosystems
"An ecosystem consists of all the organisms
living in a community as well as all the abiotic
factors with which they interact."
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Ecosystems

• Note that the boundaries of ecosystems are
  typically not arbitrarily defined, but instead
  are defined in some meaningful way A pond, a
  field, a forest, etc.
• Ecosystems are typically understood in terms of
• Energy flow through ecosystems
• Chemical cycling within (and through) ecosystems
• Note that both involve the movement of "stuff"
  through both biotic and abiotic components of the
  ecosystem
• Ecosystems ecologists view ecosystems as energy
  machines and matter processors. By grouping the
  species in a community into trophic levels of
  feeding relationships, we can follow the
  transformation of energy in the whole ecosystem
  and map the movements of chemical elements as
  they are used by the biotic community. (p. 1199,
  Campbell Reece, 2002)

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Energy Nutrient Movement
detrivores
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Energy Flow

• Energy does not cycle through ecosystems but
  instead enters ecosystems and is used up within
  ecosystems
• Ultimately energy is lost from ecosystems
  primarily as waste heat, the most
  thermodynamically unavailable form of energy
• "Energy enters most ecosystems in the form of
  sunlight. It is then converted to chemical energy
  by autotrophic organisms, passed to heterotrophs
  in the organic compounds of food, and dissipated
  in the form of heat . . .

Decomposers move energy out of dead things
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Energy Flow

• . . .The movements of energy and matter through
  ecosystems are related because both occur by the
  transfer of substances through feeding
  relationships. However, because energy, unlike
  matter, cannot be recycled, an ecosystem must be
  powered by a continuous influx of new energy from
  an external source (the sun). Thus, energy flows
  through ecosystems, while matter cycles within
  them.
• Note that energy flows through ecosystems mostly
  as bonds between carbon atoms and bonds between
  carbon and hydrogen atoms, e.g., as one finds in
  carbohydrates and lipids consequently, within
  and between organisms the carbon cycle and the
  flow of energy are quite similar, at least until
  the two are decoupled in the course of cellular
  respiration (i.e., the separation of carbon atoms
  from their energy)

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Net Primary Productivity
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Net Primary Productivity
Gross primary productivity is all of the light
energy that is converted to chemical energy by
producers
Only a small fraction of the sunlight striking
the earth is converted to chemical energy by
primary producers
Net primary productivity is that which is left
over after primary producers have used their
share for their metabolic needs
That sunlight energy that is converted to
chemical energy, over a given period, is termed
primary productivity
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Standing Crop Biomass

• The ratio of net primary productivity to gross
  primary productivity gives an indication of the
  cost of keeping the organism going, with large
  ratios indicative of relatively few costs (e.g.,
  algae, 50) and smaller ratios associated with
  many costs (e.g., complex plants such as trees,
  10)
• Organisms often can gain something by being more
  complex, but being more efficient in their energy
  utilization typically isnt what is gained
• That which is left over after secondary consumers
  have had their share is termed Standing Crop
  Biomass
• Standing crop biomass is another way of saying
  accumulated net primary productivity

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Limitations on Productivity

• The productivity of an ecosystem is dependent on
  the primary productivity of the primary producers
  within that ecosystem
• Other than sunlight, primary productivity is
  limited by nutrient availability
• A limiting nutrient is that nutrient which is
  found in the lowest, relative concentrations such
  that an increase in this nutrient will increase
  primary productivity while a decrease in this
  nutrient will decrease primary productivity (this
  is equivalent to the concept of limiting reagent
  in chemistry)
• Typically, either phosphorus or nitrogen serves
  as a limiting nutrient within a given ecosystem,
  though water availability can (and often does)
  also serve to limit the primary productivity of
  an ecosystem
• Water, light, iron, and temperature can also
  limit productivity

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Limiting Nutrients
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Temperature Moisture
Note trend
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Nitrogen Phosphorus
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Secondary Productivity
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Secondary Productivity

• "The rate at which an ecosystem's consumers
  convert the chemical energy of the food they eat
  into their own new biomass is called the
  secondary productivity.
• Note that secondary productivity is dependent, in
  part, on the efficiency of transfer of chemical
  energy between trophic levels.
• The transfer between trophic levels, however, is
  typically not highly efficient because of
  inefficiencies involved in energy transfers in
  general, and the fact that the consumer must use
  acquired energy to respire (i.e., keep their
  metabolism going, reproduce, repair themselves,
  etc.)
• The more energy required to keep the consumer
  going (e.g., endotherms warm blooded more
  versus ectotherms cold blooded less), the
  less efficiently primary productivity will be
  converted to secondary productivity

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Trophic Levels
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Trophic Efficiency

• "Of course, the energy contained in the feces is
  not lost from the ecosystem it can still be
  consumed by decomposers. However, the energy used
  for respiration is lost from the ecosystem thus,
  while solar radiation is the ultimate source of
  energy for most ecosystems, respiratory heat loss
  is the ultimate sink. This is why energy is said
  to flow through, not cycle within, ecosystems.
• Trophic efficiency refers to the transfer of
  energy up trophic levels, e.g., the ratio of
  secondary productivity to primary productivity
  consumed
• Trophic inefficiencies arise not just due to the
  2nd law of thermodynamics but because of
  inefficiencies in digestion (i.e., not everything
  is assimilated but instead is pooped out)

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Energy Partitioning
Dont worry about the numbers
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Pyramids
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Pyramids of Productivity

• A common way of illustrating ecological
  efficiency is via pyramids of productivity
• In these, productivity consumed is compared to
  productivity acquired, going up trophic levels,
  e.g., each level represents a drop of net
  productivity of approximately 90 (95 to 80)
• Note that this is the reason that eating "lower
  on the food chain" is more consistent with being
  a good world citizen than eating higher on the
  food chain, i.e., vegetarians make a
  substantially smaller per capita impact on our
  planet than do meat eaters
• Similar to the pyramid of productivity, pyramids
  can be constructed using biomass again, the
  variable associated with the primary producer is
  placed on the bottom with blocks associated with
  trophic levels stacked one upon the other

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Energy Pyramid
Trophic Efficiency Note 10 conversion between
trophic levels
Dont worry about actual numbers
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Biomass Pyramid (terrestrial)
Dont worry about actual numbers
A consequence of such a pyramid is that top
predator numbers tend to be small in number, thus
making top predators both slow to evolve (also
because they tend to be long lived and have long
generation times) and relatively easy to drive to
extinction
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Biomass Pyramid (aquatic)
Dont worry about actual numbers
Not everything at the lower trophic levels is
eaten, i.e., there is a reason that much of the
terrestrial world is green on the other hand,
there is a reason that many aquatic environments
are not quite as green, animals do consume most
of the planktonic photosynthesizers within
aquatic systems
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Pyramid of Numbers
Dont worry about actual numbers
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Model of Nutrient Cycling
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Biogeochemical Cycling

• "Because nutrient cycles involve both biotic and
  abiotic components of ecosystems, they are also
  called biogeochemical cycles."
• "Chemical elements such as carbon and nitrogen
  are cycled between abiotic and biotic components
  of the ecosystem. Photosynthetic organisms
  acquire these elements in inorganic form from the
  air, soil, and water and assimilate them into
  organic molecules, some of which are consumed by
  animals. The elements are returned in inorganic
  form to the air, soil, and water by the
  metabolism of plants and animals and by other
  organisms, such as bacteria and fungi, that break
  down organic wastes and dead organisms.
• Chemical cycles may be divided into two broad
  categories
• Those elements that have a gaseous form
• Those elements that do not have a gaseous form

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Biogeochemical Cycling
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The Water Cycle
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The Carbon Cycle
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Limestone (CaCO3)
Not all fixed carbon is converted back to CO2
over medium-term time scales since some
ultimately is buried as oil, coal, or limestone
(the latter is calcium carbonate)
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Greenhouse Effect
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CO2 Emissions
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Atmospheric CO2
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The Nitrogen Cycle
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The Phosphate Cycle
Note no gaseous phase
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Phosphorous Cycle
Phosphorous is gained in ecosystems by eroding
out of rocks, from upstream ecosystems, and as
carried by animals which have obtained it
elsewhere
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Biological Magnification
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Ozone Layer
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Ozone Destruction
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Ozone Depletion
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streams (Tsuga)
cycling
skiing
SCIENCE!
snowboarding
sliding?
cycling
skating
shore
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