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					Ecosystem Ecology Chapter 54.
 Ecosystem ecology emphasizes energy flow
 and chemical cycling.

 An ecosystem consists of all of the
 organisms in a community as well as the
 abiotic components they interact with.
Ecosystems and physical laws
 Ecosystems follow established physical laws.

 Energy can neither be created nor destroyed it can
 only be converted from one form to another.

 The amount of entropy (or disorder) in a system
 increases (i.e at every step in energy conversion
 some energy is dissipated as heat, which is not
 available for work).
Energy from the sun underpins
global ecosystems
 The Earth is not a closed system. Energy
 comes in from outside (i.e., from the sun),
 moves through ecosystems and ultimately is
 dissipated into space as heat.
 Unlike energy, nutrients are constantly
 recycled from one form to another and pass
 through multiple trophic levels to
 decomposers to abiotic forms and back to
 living organisms again.
Ecosystem ecologists deal with large-scale
processes and so group organisms into broad
classes (primary producers [e.g. plants, primary
consumers [e.g. grazing animals], etc.).

Detritivores (or decomposers) are a group of
major interest to ecosystem ecologists as these
break down non-living organic material (e.g., dead
leaves, wood, carrion) releasing their components.
 Main decomposers are fungi and bacteria. These
 break down organic matter and release chemical
 elements into the soil, water and air where
 producers can recycle them into organic

 Without the action of decomposers life would
 cease because essential nutrients would remain
 locked up in detritus and unavailable to organisms.
Fungal decomposition
   of a tree stump
Primary Production
 About 1% of the visible light that strikes
 earth is converted by photosynthesis into
 chemical energy.

 This is enough energy to create about 170
 billion tons of organic matter annually.
Gross and net primary
 In an ecosystem gross primary
 productivity (GPP) is the amount of light
 energy converted into chemical energy per
 unit time.

 Net primary productivity (NPP) is gross
 primary productivity minus the energy used
 by the primary producers for respiration.
Net primary productivity
 Net primary productivity indicates how
 much energy is available for use by other
 trophic levels.

 It is measured as biomass of vegetation
 added to the ecosystem per unit area per
 unit time (g/m2/yr).
Net primary productivity
 NPP is influenced by light and nutrient
 availability and differs among ecosystems.

 Tropical rain forests have high NPP as do
 estuaries and coral reefs. Lakes, tundra and
 the open ocean have relatively low NPP.
Global net primary productivity
 Tropical rainforests contribute about 22% of the
 Earth’s total NPP and open ocean about 24%. The
 open ocean has a much lower rate of NPP, but
 covers a far larger area.

 Various temperate forests and grasslands and the
 continental shelf (shallow continental waters)
 contribute most of the rest.
Global net primary productivity
 Overall, terrestrial ecosystems contribute
 about 66% of NPP and marine ecosystems
 the remainder.
Limits on primary productivity
 In marine and freshwater ecosystems both light
 and nutrients are important in controlling NPP.

 The inability of light to penetrate the water limits
 photosynthesis to the upper layers. More than
 50% of solar radiation is absorbed in the first
 meter. Even in clear water, only about 5-10% of
 radiation reaches a depth of 20m.
Limits on primary productivity
 Even though there is a gradient in light
 energy from tropics to poles there is no
 corresponding gradient in NPP.

 Suggests light not only factor limiting NPP
Regional annual net primary production for Earth
Limits on primary productivity
 Nutrient limitation is a major factor
 affecting NPP in aquatic biomes.

 In marine environments the nutrients
 limiting primary productivity are usually
 nitrogen and phosphorus, which are scarce
 in the photic zone.
Limits on primary productivity
 Duck farms on Long Island add nitrogen and
 phosphorus to the ocean water.

 Phytoplankton growth parallels levels of inorganic
 phosphorus, but nitrogen is the limiting nutrient
 because adding nitrogen (in the form of
 ammonium) increases phytoplankton growth but
 adding phosphorus does not.
Limits on primary productivity
 Consistent with the hypothesis that nutrients limit
 NPP in the ocean is the observation that areas of
 upwelling, where deep nutrient-rich waters are
 brought to the surface are the most productive.

 The world’s great fisheries are all located in such
 areas: Peruvian Anchovy Fishery, Grand Banks.
Chemical Cycling
 Nutrients move between organic and inorganic
 parts of the ecosystem in biogeochemical cycles.

 Cycles may be global or local. Nutrient cycles
 with a gaseous component (carbon, sulfur,
 nitrogen) are global whereas phosphorus,
 potassium and calcium cycle more locally (at least
 on short time scales).
General Structure of Nutrient Cycles
 There are four basic reservoirs for any
 nutrient each defined by two characteristics:
 whether it is organic or inorganic and
 whether or not nutrients are directly
 available for use by living organisms.
General Structure of Nutrient Cycles
 Nutrients can move from one reservoir to another
 by a variety of processes.

 For example: available inorganic nutrients can
 become available organic nutrients by
 assimilation and photosynthesis. Conversely,
 available organic nutrients can become available
 inorganic nutrients through respiration,
 decomposition and excretion.
General Structure of Nutrient Cycles
 Unavailable nutrients can become available
 from both organic sources such as oil and
 coal by burning fossil fuels (releasing e.g.
 CO2) and inorganic sources by weathering
 and erosion of rocks.
Major Nutrient Cycles
 Major nutrient cycles include:
Carbon Cycle
 Carbon forms the basis of all organic
 molecules (organic chemistry is the study of
 carbon chemistry). Fats, sugars, DNA,
 proteins etc. all contain carbon.
 Photosynthetic organisms take in CO2 from
 the air and using energy from sunlight join
 carbon atoms to make sugars (energy is
 stored in the chemical bonds).
Carbon Cycle
 Major reservoirs of carbon include:
 atmospheric CO2, ocean (dissolved carbon
 compounds), biomass of organisms, fossil
 fuels and sedimentary rocks.
Carbon Cycle
 Photosynthesis removes large amounts of
 atmospheric CO2
 An approximately equal amount of CO2 is
 returned to atmosphere by cellular
 Burning of fossil fuels adds large amounts
 CO2 to the atmosphere.
Decomposition and nutrient cycling
 The rates at which nutrients cycle is
 strongly affected by the rates at which
 decomposers work. In the tropics, warmer
 temperatures and abundant moisture cause
 organic material to decompose 2-3 times
 faster than it does in temperate regions.
Decomposition and nutrient cycling
 High rate of decomposition means little
 organic material accumulates as leaf litter.

 In tropical forest about 75% of nutrients are
 in woody trunks of trees and only about
 10% in soil. In temperate forest about 50%
 of nutrients are in the soil because
 decomposition is slower.
Decomposition and nutrient cycling
 In aquatic ecosystems decomposition in
 anaerobic sediments can be very slow (50
 years or more).

 As a result sediments are often a nutrient
 sink and only when there is upwelling are
 marine ecosystems highly productive.
Human effects on nutrient cycles
 Agriculture and nutrient cycling. Soils differ in
 the amount of nutrients stored in organic matter
 that they contain.

 Soils with large stores (e.g. prairie soils) can be
 used for agriculture for many years before
 requiring fertilization. In tropical forest soils,
 however, there are few stored nutrients and the
 soil quickly becomes exhausted.
Human effects on nutrient cycles
 Nitrogen is main nutrient removed through
 agriculture (when the biomass is removed
 from the field).

 The removed nitrogen needs to be replaced
 and industrially produced fertilizers are
Human effects on nutrient cycles
 Recent studies suggest that human activities
 (fertilization and increased planting of
 legumes) have approximately doubled the
 supply of nitrogen available to plants.

 A major problem with intensive farming is
 that fertilizer runoff.
Human effects on nutrient cycles
 Fertilizers that are applied in amounts
 greater than plants can use or that are
 applied when plants are not in the fields
 leach into groundwater or run off into
 streams and rivers.
Human effects on nutrient cycles
 The heavy supply of nutrients causes
 blooms of algae and cyanobacteria as well
 as explosive growth of water weeds.
Human effects on nutrient cycles
 Because respiration by plants depletes the
 oxygen levels at night this process of
 eutrophication can cause fish kills.

 Eutrophication of Lake Erie, for example,
 wiped out commercially important
 populations of fish including lake trout, blue
 pike and whitefish in the 1960’s.
Acid rain
 Burning wood, coal and other fossil fuels
 releases oxides of sulfur and nitrogen that
 react with water in the atmosphere to form
 sulphuric and nitric acid.

 These acids fall to Earth as acid
 precipitation (rain, snow, sleet), which has a
 pH of less than 5.6.
Acid rain
 Pollutants produced by power plants travel large
 distances on the prevailing winds before falling to
 the Earth.

 As a result, the areas harmed by acid rain are
 usually far from the pollution’s source.

 Acid rain in Eastern U.S. caused by power plants
 in midwest.
Fig. 54.21
Acid rain
 In terrestrial ecosystems the acid rain can
 leach nutrients from the soil and stunt plant

 Freshwater lakes very vulnerable to effects
 of acid rain especially where bedrock is
 granite. Such lakes lack a buffering
 capacity because bicarbonate levels are low.
Acid rain
 Many fish are intolerant of low pH levels
 (e.g. at <pH 5.4 newly hatched trout die).

 Thus, acid rain has had major effects on fish
Acid rain
 Environmental regulation and new technology
 have reduced sulfur dioxide emissions over the
 past 30 years in the U.S. and they fell 31%
 between 1993 and 2002.

 Water chemistry in eastern U.S. is improving, but
 will require another 10-20 years to recover even if
 emissions continue to decrease.
Environmental toxins
 Huge variety of toxic chemicals are
 produced and move through food webs.

 Some are excreted, but other accumulate in
 fat and become more concentrated in upper
 levels of the food chain (a process called
Environmental toxins
 Chlorinated hydrocarbons (which include
 pesticides such as DDT and industrial
 chemicals such as PCBs) are well known to

 PCB levels in herring gulls in Great Lakes
 are 5,000 times greater than in
Environmental toxins
 At higher levels chlorinated hydrocarbons
 can be toxic or severely affect hormone

 Story of DDT is well known. Widely
 sprayed after WWII to kill mosquitoes and
 agricultural pests.
Environmental toxins
 DDT accumulated in tissues of predatory birds
 and interfered with deposition of calcium in
 eggshells so eggs were brittle and could not be

 DDT was banned in U.S. in 1971 after public
 outcry inspired by Rachel Carson’s Silent Spring.
 Still used elsewhere on the globe.
Increases in atmospheric CO2
 Since the industrial revolution carbon dioxide
 levels in atmosphere have increased due to
 burning of fossil fuels and burning of forests.

 Since 1958 CO2 levels have increased 17%. By
 2075, at current rates of increase, CO2 levels will
 be double what they were in the mid 1800’s.
Increases in atmospheric CO2
 Consequences of increased CO2 levels
   effects on growth of plants
   changes in plant distributions
   effects on global climate
Increases in atmospheric CO2
 Most plants grow better with higher levels of CO2.
 However, the growth of one group of plants called
 C3 plants is more limited by low atmospheric CO2
 Under hot dry conditions, when they must limit
 water loss by closing the air exchange pores in
 their leaves (stomata), the CO2 levels in their
 leaves falls so much that photosynthesis almost
 shuts down.
Increases in atmospheric CO2
 If global CO2 levels increase, these plants may
 spread into areas where they have not previously
 occurred displacing the other group (C4 plants),
 which are more efficient at photosynthesis under
 low CO2 levels.

 Major agricultural crops include both C3 plants
 (rice, wheat and soybeans) and C4 plants (corn) so
 changes in CO2 levels may affect which plants
 farmers choose to plant.
Increases in atmospheric CO2
 Effects of increased CO2 levels on forests
 are being explored in a large-scale
 experiment at the Duke University Forest.

 In the long-term work high levels of CO2
 are being pumped into the air over forest
 plots. In the experimental plots CO2 levels
 are increased to 1.5X current CO2 levels.
Increases in atmospheric CO2
 Comparison of tree growth in experimental
 and control plots has shown higher rates of
 photosynthesis in experimental plots, higher
 soil respiration, and that pine seeds are
 heavier in experimental plots.
Increases in atmospheric CO2
 Concerns have been raised about effects of
 increased CO2 levels on global climate.

 CO2 and water vapor in atmosphere trap
 some heat radiated from Earth and reflect it
 back. This greenhouse effect keeps the
 planet warm.
Increases in atmospheric CO2
 There is widespread concern that increased
 CO2 levels is causing global warming.

 Temperature data show a great deal of
 variation and there is debate about how
 much warming has occurred and how fast it
 it occurring. However, there is a general
 consensus that warming has occurred.
Increases in atmospheric CO2
 An increase of one or two degrees Celsius
 in global average temperature could have
 major consequences.

 For example, melting of the polar icecaps
 could raise sea levels dramatically.
Increases in atmospheric CO2
 Models of climate change are complex and
 include a lot of assumptions, but global
 climate changes might be dramatic.

 For example a shift in the direction of the
 Gulf Stream caused by changes in sea
 temperatures would cause the western
 European climate to become much colder.
Increases in atmospheric CO2
 Many models also predict an increase in the
 frequency and strength of hurricanes as warmer
 sea temperatures provide energy that feeds these

 Increases in atmospheric CO2 levels may also
 increase the acidity of oceans as some CO2 is
 converted to acids such as carbonic acid.
 Increased acidity may have drastic effects on coral
 reefs, which may dissolve.
Increases in atmospheric CO2
 Efforts to reduce CO2 emissions are
 underway, but there is considerable political
 wrangling because of economic concerns.

 Whether the current trends will be reversed
 is thus unclear.
Depletion of ozone levels
 A protective layer of ozone (O3) shields the
 earth from harmful levels of UV radiation.

 However, the ozone layer has been thinning
 since the mid 1970’s especially over the
 southern hemisphere.
Depletion of ozone levels
 Cause of the depletion appears to be
 accumulation of chlorfluocarbons (CFCs,
 which are used as refrigerants and aerosol

 When CFC breakdown products rise into
 the stratosphere, chlorine contained in them
 breaks down ozone and produces oxygen.
Depletion of ozone levels
 Cold temperatures over Antarctic facilitate these
 atmospheric reactions. Ozone hole over Australia
 has resulted in higher incidences of skin cancer.

 CFCs have now been widely banned and the rate
 of ozone depletion has slowed, but the chlorine
 already in the atmosphere will continue to exert an
 effect for at least a century.

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