Spatial scale and ecological experiments

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Spatial scale and ecological experiments
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Assumptions of experimental ecology (after John
Wiens)

• Assumptions
• In equilibrium.
• Spatially
• homogeneous
• 3. Closed

• Consequence
• Do not distinguish between short term dynamics
  and long term dynamics
• Limited replication in space.
• Ignore regional dispersal effects.

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To generalize from such experiments one must
either ignore scale or assume it doesnt
matterYet we know that scale does matter.
Wiens, 2001
How does scale matter?
Not at all
Unpredictably
Predictably
process
process
process
scale
scale
scale
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Predictably
Kunin, W.E. 1998. Extrapolating species abundance
across spatial scales. Science 2811513-1515.
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Unpredictably
Ludwig, J.A et al. 2000. Ecosystems 384-97.
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Scale discontinuities can give rise to
transmutation error
Process
Scale
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If scale matters, what type of experiments should
we do? Big or small?
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The case for whole-system experiments
Kluane (snowshoe hare) project
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Because whole-system experiments are at the same
scale as applied problems
I believe that managers and politicians in
todays conservative, economically-orientated,
resource-hungry society are even harder to
convince to act in an ecologically responsible
manner than those of 30 years ago, making it more
necessary than ever to perform experiments at
convincing scales -David Schindler, 1998
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..and contain realistic complexity
Most of the crucial questions of applied ecology
are not open to attack by microcosms It is
irresponsible for academic ecology to produce
larval microcosmologists by canalizing graduate
students into careers of small-scale
experimentation -Steve Carpenter, 1996
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Mesocosms can sometimes mimic larger systems very
well
mesocosm
Summary of ocean data
From Pace, M.L. 2001. Getting it right and wrong
Extrapolations across experimental scales. In
Scaling relations in experimental ecology.
Columbia Univ.
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but sometimes very poorly.
Increase in response Decline in response to
nutrients to Daphnia Enclosure Lake
Enclosure Lake Bacteria 2/2 0/3 0/2 0/3 abundanc
e Bacterial 2/2 3/3 0/2 1/3 production
From Pace, M.L. 2001. Getting it right and wrong
Extrapolations across experimental scales. In
Scaling relations in experimental ecology.
Columbia Univ.
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Microcosm whodunits
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The case for small scale experiments
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Large scale experiments may be realistic, but
they have very low replication

Replicates
Plot size
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The scale trade-off
Realism vs. replication (Schindler) Accuracy
vs. precision External vs. internal
validity (Naeem)
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Naeem, S. 2001. Experimental validity and
ecological scale as criteria for evaluating
research programs. In Scaling relations in
Experimental Ecology. Columbia Univ.
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Simpsons paradox

• Pooling observational data across systems can
  yield proportions that are inverse to those
  obtained within systems.
• Example Link between water fluoridation and
  cancer rates.

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Biodiversity-ecosystem function debate another
example of Simpsons paradox?
Naeem, 2001
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BioDEPTH experiment Across and within sites
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Is there a resolution to the scale debate?

• Michael Huston (1999, Ecology 80 1088-1089)
• Microcosm experiments model creation and
  parameterization
• Whole-ecosystem experiments model validation