Biotic Disturbance

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Biotic Disturbance
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White and Pickett (1985) Definition(from White
and Jentsch 2001, Italics mine)

• Disturbance is a relatively discrete event in
  time that disrupts the ecosystem, community or
  population structure and changes the resources,
  substrate availability or physical environment.
• A disturbance, in this sense, changes the state
  of structural and physical variables in the
  ecosystem, although these changes also influence
  ecosystem functions and processes.

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Azayus and Canuel 2004

• 2 sites on York R.
• Similar sediment and OM delivery
• POD
• High energy
• High physical disturbance
• LY
• Lower energy
• Biotic mixing dominates

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Box Corer
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Kasten Corer
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Fig 2

• Spikes in TOC mixing events
• Greater C/N less yummy OM (could be terrestrial
  or more degraded)
• Positive Eh at POD more aerobic conditions

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Fig 3

• Depleted 13C signature different OM sources or
  local OM processing
• Mixing model suggests difference due to local
  processing

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OM Degradation Rates1954 - 1964

• Degradation of recalcitrant compounds
  significantly greater in POC (physical mixing)
  than LY (biotic mixing)
• Degradation of labile compounds similar between
  sites
• Difference my be due to increased contact between
  OM and O2 and other electron acceptors in more
  mixed site

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Conclusions

• POD has a more severe sediment disturbance
  regime based on physical mixing
• POD has chemical evidence of more aerobic
  conditions and more degraded OM
• POD and LY are similar in the breakdown rate of
  labile C
• Physical mixing more important for the breakdown
  of recalcitrant OM

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Flecker and Taylor 2004

• Biotic disturbance in the form of Bulldozer
  grazing.
• Study Site
• Rio Las Marias
• Venezuela

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Predictions

1. Fish disturbance would create resource
   heterogeneity
2. Greatest heterogeneity would be at intermediate
   fish density
3. Benthic macroinvertebrate diversity would be
   greatest at highest resource heterogeneity

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Fish did influence resource heterogeneity
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Fish also affected expected richness but
according to predictions
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Some Questions

• Is the lack of a strong biotic effect due to
  biotic and physical having same mechanism?
• What is the importance of spatial and temporal
  scale? (see Sun et al. 2002)
• Is the big black box between disturbance and OM
  mineralization a problem?
• Is diversity the correct response variable for
  resource heterogeneity?
• Are aquatic systems unique with respect to biotic
  disturbance?

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Sun et al. 2002