Trends Expected in Stressed Ecosystems

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Trends Expected in Stressed Ecosystems

• First of all I want to define the term stress as
  - a detrimental or disorganizing influence

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Ecosystem Stress

• - When ecosystems are not affected by strong
  external perturbations, such as human
  disturbances, we observe certain well defined
  developmental trends
• - For example, the biomass and sizes of organisms
  tend to increase and net community production
  decreases
• - In 1969, Eugene Odum published a table entitled
  Trends to be expected in ecosystem development,
  which contrasted early and late stages of
  succession in terms of 24 ecosystem properties

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Ecosystem Stress

• As our awareness of global changes has increased
  over the past 20 years a number of ecologists
  (including Odum) have adapted theories founded in
  pure ecosystem ecology (based on Odums 1969
  work), to predict how ecosystems will respond to
  anthropogenic stresses. This theoretical work
  comes from a paper published by
• Odum, E.P. 1985. Trends expected in stressed
  ecosystems. BioScience 35419-422.

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Energetic Responses

• - Item 1. Theoretically, an increase in community
  respiration should be the first early-warning
  sign of stress since repairing damage caused by
  the disturbance requires diverting energy from
  growth and reproduction to maintenance
• - Item 2. Therefore, the R/B ratio (or P/B ratio)
  or maintenance to biomass ratio increases
• - Item 3. P/R (Production/respiration) becomes
  unbalanced (usually lt1). The P/R ratio tends
  toward balance in undisturbed ecosystems
• - Item 4. Importance of auxiliary energy increases

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To summarize, community respiration per unit of
biomass tends to increase and biomass
accumulation decreases as organisms cope with the
disorder caused by some unusual disturbance

• - Item 5. Exported or unused primary production
  increases

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Nutrient Cycling

• - Item 6. Nutrient turnover increases (less put
  into production)
• - Item 7. Horizontal transport increases and
  vertical cycling of nutrients decreases (e.g.,
  tropical forest)
• - Item 8. Nutrient loss increases (system becomes
  more leaky) (again, tropical forest)

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• These three trends are of course interdependent
• - Horizontal transport (or one-way flow) instead
  of internal cycling is one of the principle ways
  that humans disturb natural ecosystems
• - Mining, soil erosion, stream pollution and
  fertilizer runoff from croplands are all familiar
  examples

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Community Structure

• - Item 9. Proportion of r-strategists increases
• - Item 10. Size of organisms decreases
• - Item 11. Lifespan of organisms or parts (leaves
  for example) decrease - e.g., chronic air
  pollution on trees
• - Item 12. Food chains shorten because of reduced
  energy flow at higher trophic levels and/or
  greater sensitivity of predators to stress
  (bioaccumulation of toxins)

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Community structure

• - Item 13. Species diversity decreases and
  dominance increases at the ecosystem level
  redundancy of parallel processes theoretically
  declines

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General System Level Trends

• - Item 14. Ecosystem becomes more open (i.e.,
  input and output environments become more
  important as internal cycling is reduced
• - Item 15. Autogenic successional trends reverse
  (succession reverts to earlier stages)
• - Item 16. Efficiency of resource use decreases
• - Item 17. Parasitism and other negative
  interactions increase, and mutualism and other
  positive interactions decrease

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General System Level Trends

• - Item 18. Functional properties (such as
  community metabolism) are more robust
  (homeostatic - resistant to stressors) than are
  species composition and other structural
  properties

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Schindlers Whole-Lake Experiments

• Schindler, D.W. 1990. Experimental perturbations
  of whole lakes as tests of hypotheses concerning
  structure and function. Oikos 5725-41.

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Expected response vs. Acidified lakesEnergetics

• 1. Community Respiration increases
• 2. P/R becomes unbalanced
• 3. P/B and R/B ratios increase
• 4. Importance of auxiliary energy increases
• 5. Exported or unused primary production increases

• 1. Periphyton community respiration increases
• 2. P/R increases
• 3. P/B No change, R/B decreases
• 4. Not tested
• 5. No change

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Expected Response vs. Acidified LakesNutrient
Cycling

• 6. Nutrient cycling increases
• 7. Horizontal transport increases, vertical
  cycling of nutrients decrease
• 8. Nutrient loss increases

• 6. Nitrogen cycle may decrease slightly
• 7. Decreased transport of N and C, both
  horizontally vertically
• 8. Losses of nitrogen sulfur

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Expected Response vs. Acidified LakesCommunity
Structure

• 9. Proportion of r-strategists increases
• 10. Size of organisms decreases
• 11. Lifespans decrease
• 12. Food chains shorten
• 13. Species diversity decreases and dominance
  increases redundancy declines

• 9.Increase in zooplankton decrease in fish
• 10. Fish increase, zooplankton decrease,
  phytoplankton increase
• 11. lifespan of fishes benthic crustaceans
  decrease
• 12. Food chain shortens due to elimination of
  mid-food chain
• 13. Species diversity declines, dominance
  increases at all levels, redundancy declines

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Expected Response vs. Acidified LakesGeneral
System-Level Trends

• 14. Ecosystems become more open
• 15. Successional trends reverse
• 16. Efficiency of resource use decreases
• 17. Negative interactions increase, positive
  interactions decrease
• 18. Functional properties are more robust than
  species composition
• 19. Addition - reproduction decreases, dominance
  increases, general impoverishment

• 14. No change in inputs, export of nitrogen
  increases slightly
• 15. Not tested
• 16. Reduced use of nitrate, ammonium,
  allochthonous organic matter
• 17. Not tested
• 18. Generally true
• 19. Extirpation of sensitive species, incresed
  dominance, decreased reproduction

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Schindlers whole-lake experiments

• When large amounts of acid or nutrients were
  introduced into lakes, primary productivity and
  other aspects of community metabolism were
  remarkably homeostatic, but species composition
  of plankton and fish were greatly altered
• In other words, species replacement and other
  adjustments keep the overall function of the
  system steady

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Schwartz, M.W., et al. 2000. Linking biodiversity
to ecosystem function implications for
conservation ecology. Oecologia 122297-305.

• Evaluate the hypothesis that a large portion of
  native species richness is required to maximize
  ecosystem stability and function

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This assessment is important for conservation
strategies because maintenance of ecosystem
functions has been used as an argument for the
conservation of species

• If ecosystem functions are sustained at
  relatively low species richness, then arguing for
  the conservation of ecosystem function, no matter
  how important in its own right, does not strongly
  argue for the conservation of species

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Type B
High
Ecosystem Function
Type A
Low
Low High
Biodiversity
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Results

• Few empirical studies demonstrate improved
  function at high levels of species richness
• Reason why high species richness may not
  contribute significantly to function or stability
  is that most communities are characterized by
  strong dominance such that a few species provide
  the vast majority of the community biomass
• Rapid turnover of species may rescue the concept
  that diversity leads to maximum function and
  stability (but has not been investigated)