Ecosystems

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Chapter 54

• Ecosystems

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What is an Ecosystem?

• All organisms living in an area plus the abiotic
  factors
• Energy machines and matter processors
• Energy flows through and matter cycles

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Trophic Relationships

• Primary producers
• Ultimately support all trophic levels
• Autotrophic organisms
• Organisms above producers
• Heterotrophic
• Depend on photosynthetic output of producers
• Decomposers
• Get energy from detritus (nonliving organic
  material)
• Play a central role in material cycling

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Decompositions Role

• Decomposers link all trophic levels
• Re-supply ecosystems with organic material
• Liberate otherwise unusable material
• Earthworms eat soil and extract nutrients and
  birds eat worms
• Worms cant eat dirt

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Laws of Physics and Chemistry

• First law of thermodynamics
• Energy is neither created nor destroyed, only
  transformed
• Second law of thermodynamics
• Energy is lost when it changes states
• Chemistry laws
• Energy moves through an ecosystem
• Nutrients cycle through an ecosystem

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Primary Producers

• Organisms that convert energy from the sun into
  chemical energy (stored in bonds of sugars)
• Primary producers provide all the energy for a
  given ecosystem
• Lots of plants means lots of energy

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Global Energy Budget

• Each day Earth receives 1022joules of solar
  radiation
• Equivalent of 100 million atomic bombs
• Most of that energy is reflected by the
  atmosphere, or absorbed by water or the ground
• Plants only use the part of solar radiation we
  call visible light
• Of visible light only 1 is converted to organic
  matter
• But producers generate 170 billion tons of
  organic matter a year

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Gross and Net Primary Production

• Gross Primary Production
• Amount of light energy converted to chemical
  energy by photosynthesis per unit of time
• Not all of this production is stored as organic
  material because the plants use some molecules as
  fuel for respiration
• Net Primary Production
• Amount of chemical energy available to the next
  trophic level
• NPP GPP R (respiration)

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Net Primary Production

• Measurement of most interest to ecologists
• Primary production can be expressed as biomass
  (weight of vegetation added to ecosystem per area
  per time or g/m2/yr) or energy per area per unit
  time or (J/m2/yr)

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Earths Ecosystems and Biomass
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Production in Marine Ecosystems

• Limiting factors in Marine Ecosystems
• Light more than half the light is absorbed in
  the first meter of water
• Even clear water at 20 m only 5-10 of the light
  is available
• We should then see an increase in production from
  poles toward the equator but we dont
• Its actually nutrients that are more limiting in
  marine ecosystems than light

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Production in Marine Ecosystems

• Limiting nutrient
• Nutrients that must be added for production to
  increase
• In marine ecosystems nitrogen and phosphorus are
  limiting
• Iron is also limiting in parts of the ocean
• Iron is supplied by wind blown dust
• Where iron is abundant so is nitrogen
• Cyanobacteria need iron to fix atmospheric
  nitrogen into a useable form

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Production in Freshwater Ecosystems

• Limiting factors
• Light
• Temperature
• Nutrients unlike marine ecosystems, phosphorus
  is the main limiting factor

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Terrestrial Ecosystems

• Main factors are
• Water
• Temperature
• Nutrients

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Secondary Production in Ecosystems

• Amount of chemical energy in food converted into
  organic matter
• Cow eats grass secondary production is the
  amount of grass that is converted into cow
• The cow does not use the entire grass, some of it
  passes through the digestive system

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Production Efficiency

• Example of a caterpillar
• Eats 200 J of plant material
• 33 J is used for growth
• 100 J passes through the caterpillar as feces
• Not lost energy because decomposers will use this
  energy
• 67 J is used for cellular respiration
• This energy is lost as heat
• Efficiency measure
• Production efficiency net secondary production
  / assimilation of primary production
• Fraction of food energy that is not used for
  respiration, or energy that is not lost to the
  environment
• Our example production efficiency 33 J (growth)
  / 33 67 J (energy used for assimilation, feces
  dont count) 33
• Birds and mammals average 1-3
• Fish average 10
• Insects average 40

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Efficiency of Energy Transfer
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Trophic Efficiency and Ecological Pyramids

• Trophic efficiency percentage of production
  transferred from trophic level to the next
• Generally only 10 of the total energy at one
  level is available for use at the next level

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Energy efficiency of trophic levels
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Pyramids of numbers
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Green World Hypothesis

• With so many primary consumers (herbivores) why
  is everything still so green
• Herbivores actually consume very little plant
  biomass because they are held in check by several
  factors
• Total of 830,000,000,000 metric tons of carbon in
  plants
• 50,000,000,000 tons is added each year
• Herbivores only consume 17 of this biomass every
  year so they are just really a nuisance to plants
• Factors that keep herbivores in check
• Plant defenses
• Nutrients, not energy supply, limit herbivores
• Abiotic factors
• Intraspecific competition
• Interspecific interactions

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Cycling of Elements

• Solar energy is replenished daily
• Carbon, Nitrogen, Phosphorus, etc. are not
  replenished (at all)
• Earth relies on the cycling of these nutrients.

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Biological and Geological Processes That Drive
Nutrient Cycling

• Biogeochemical cycles nutrient pathways that
  involve both living and nonliving processes
• Two general paths for chemicals
• Global gaseous forms of chemicals that can move
  through the atmosphere (N, C, O, S)
• Atmosphere is the main reservoir
• Local less mobile in the environment (P, K, and
  Ca)
• Soil is the main reservoir

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General Model of Chemical Cycling

• Shows the main reservoirs and general processes
  that involve moving chemicals from one reservoir
  to the next
• Most nutrients accumulate in 4 reservoirs defined
  by two characteristics
• Whether it contains organic or inorganic material
• Whether it is available for use or not
• Cycling of water does not fit this model well

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Water Cycle
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Nitrogen Cycle
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Nitrogen Cycle

• Earths atmosphere is about 80 nitrogen
• It is in an unusable form (N2)
• Usable nitrogen (NH4 or NO3enters ecosystems two
  ways (depends on the ecosystem)
• Atmospheric deposition (5-10)
• Added by being dissolved in rain or settling as
  fine dust
• Nitrogen fixation
• Only certain bacteria can fix atmospheric
  nitrogen into usable form
• Bacteria are either free living or part of
  symbiotic relationship (Rhizobium)
• One unnatural way is through the use of
  fertilizers by humans
• Runoff from fields

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Carbon Cycle
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Phosphorus Cycle

• Organisms require phosphorus for DNA, ATP, and
  phospholipids (cell membranes)
• Does not include the atmosphere
• No significant phosphorus containing gases
• Usable form is PO43- which plants can absorb
• Main reservoir is rock

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Phosphorus Cycle
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Decomposition and Nutrient Cycling

• Different ecosystems cycle nutrients at different
  rates
• Depends on rate of decomposition
• Tropical rain forest few months to few years
• Temperate forests 4 to 6 years
• Tundra 50 years
• Temperature and oxygen play vital roles
• Tropical rain forests phosphorus occurs in the
  soil at levels far below that of a temperate
  forest
• why?
• Nutrients are taken up as soon as they are
  available
• Rain forest soil is very poor

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Human Impact on Ecosystems and Biosphere

• Disruption of chemical cycles
• Depletion of soil nutrients in one area, and
  excess in another
• Food crops
• Addition of new toxins to an area
• Agricultural effects on nutrient cycling
• An area is cleared for farming, and nutrients
  exist in the soil
• The crop is grown, but the nutrients the crop
  took from the soil is transported elsewhere
• Nutrients must be added back to the soil
• Nitrogen is a big limiting factor

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Critical Load and Nutrient Cycles

• Excess nitrogen minerals in the soil are leaking
  into rivers, and eventually the ocean
• This does have positive effects for some forests
• Nitrogen runoff fertilizes forests
• Critical Load amount of nutrients that begins
  to have negative affects

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Accelerated Eutrophication of Lakes

• Eutrophic lake high in nutrients
• Oligotrophic low in nutrients
• Sewage and factory runoff adds nutrients to lakes
• Results in explosive increase of phytoplankton
• Banks become choked with weeds
• Algae blooms deplete oxygen content
• Results in fish death

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Combustion of Fossil Fuels and Acid Rain

• Burning wood, coal, and oil results in release of
  oxides of sulfur and nitrogen that react with
  water to form sulfuric and nitric acid
• These fall back to the earth as acid
  precipitation
• pH less than 5.6
• Some parts of Europe receive rain with a pH of
  3.0
• Acid rain causes leaching of nitrogen and
  phosphorus from the soil
• Lakes and streams are heavily damaged due to slow
  buffering capacity
• Results in predator death and replacement with
  acid tolerant species which affects the food web

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Acid Precipitation
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Toxins

• Synthetic materials that become concentrated in
  ecosystems due to the inability of organisms to
  break it down
• Mercury in waters
• Becomes part of fish tissue and humans can die
  from eating these fish
• DDT and biological magnification
• Top level carnivores are the most severely
  affected by these toxins

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Biological Magnification
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Climate Change

• Rising CO2 levels in the atmosphere
• Caused by combustion of fossil fuels and wood
  from deforestation
• CO2 concentration before 1850 274 ppm
• 1958 316 ppm
• Today 370 ppm
• Greenhouse effect
• CO2 and water vapor absorb and reflect much of
  the solar radiation that reaches Earth
• Increase CO2 means more absorption
• Its like thickening the glass of a greenhouse, it
  will raise the temperature

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Global Warming

• Hard to understand due to all the abiotic and
  biotic factors
• Predictions and estimations
• End of 21st century CO2 concentrations double and
  average global increase of 2 C
• Increase of 1.3 C would make the world warmer
  than any time in the past 100,000 years
• Polar ice would melt and raise the sea level by
  100 m (New York, Miami, and LA would be under
  water)
• Central US would be much drier

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Ozone Depletion

• Ozone protects us from harmful UV radiation
• Ozone is O3 it absorbs UV rays
• been gradually thinning since 1975
• Due to increase of CFCs (use in refrigeration)
• May cause increase of skin cancer and cataracts

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Ozone Depletion