Competition experiments

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Competition experiments
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What is competition?
What are possible response variables for a
competition experiment?
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• Some possible types of competition questions
• Does associate species X affect the
  growth/fitness of focal species Y?
• How does interspecific competition (effect of X
  on Y) compare to intraspecific competition
  (effect of Y on Y)?
• Is competition asymetrical? That is, does either
  X affect Y more than Y affect X, or vice versa?
• Is the strength of competition dependent on the
  density of the associate species (X)? Density of
  the focal species (Y)?

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Focal species
Additive design
Associate
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N associate
Monoculture of focal
N focal species
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Substitutive design (replacement series)
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Monoculture of associate
N associate
Monoculture of focal
N focal species
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Response surface
Monocultures of associate
N associate
Monocultures of focal
N focal species
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Groups
Additive
Substitutive
Gina Rebecca Heather Aerin
Annie David Riann
Liz Hillary Marie Nick
Aliice Katsky James
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• What two factors/processes are confounded in your
  experimental design?
• Which type of competition question can your
  design answer? Which can it not?
• How would you analyze results from your
  experimental design?

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Cultural exchange
Additive
Substitutive
Gina Rebecca Heather Aerin
Annie David Riann
Liz Hillary Marie Nick
Aliice Katsky James
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• What two factors/processes are confounded in your
  experimental design?
• Which type of competition question can your
  design answer? Which can it not?
• How would you analyze results from your
  experimental design?

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• Some possible types of competition questions
• Does associate species X affect the
  growth/fitness of focal species Y?
• How does interspecific competition (effect of X
  on Y) compare to intraspecific competition
  (effect of Y on Y)?
• Is competition asymetrical? That is, does either
  X affect Y more than Y affect X, or vice versa?
• Is the strength of competition dependent on the
  density of the associate species (X)? Density of
  the focal species (Y)?

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• Some possible types of competition questions
• Does associate species X affect the
  growth/fitness of focal species Y? Additive
• How does interspecific competition (effect of X
  on Y) compare to intraspecific competition
  (effect of Y on Y)?
• Is competition asymetrical? That is, does either
  X affect Y more than Y affect X, or vice versa?
• Is the strength of competition dependent on the
  density of the associate species (X)? Density of
  the focal species (Y)?

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• Some possible types of competition questions
• Does associate species X affect the
  growth/fitness of focal species Y? Additive
• How does interspecific competition (effect of X
  on Y) compare to intraspecific competition
  (effect of Y on Y)? Substitutive
• Is competition asymetrical? That is, does either
  X affect Y more than Y affect X, or vice versa?
• Is the strength of competition dependent on the
  density of the associate species (X)? Density of
  the focal species (Y)?

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• Some possible types of competition questions
• Does associate species X affect the
  growth/fitness of focal species Y? Additive
• How does interspecific competition (effect of X
  on Y) compare to intraspecific competition
  (effect of Y on Y)? Substitutive
• Is competition asymetrical? That is, does either
  X affect Y more than Y affect X, or vice versa?
  L-shaped additive or substitutive
• Is the strength of competition dependent on the
  density of the associate species (X)? Density of
  the focal species (Y)?

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• Some possible types of competition questions
• Does associate species X affect the
  growth/fitness of focal species Y? Additive
• How does interspecific competition (effect of X
  on Y) compare to intraspecific competition
  (effect of Y on Y)? Substitutive
• Is competition asymetrical? That is, does either
  X affect Y more than Y affect X, or vice versa?
  L-shaped additive or substitutive
• Is the strength of competition dependent on the
  density of the associate species (X)? Density of
  the focal species (Y)? Additive, response surface

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Competition models (two examples) Example 1
Additive series design for container mosquito
larvae
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Aedes albopictus invades N America from SE Asia
in 1985, via used tire trade at Houston
1985
1996
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Breeds in containers habitats (old tires,
treeholes), like native species, Aedes aegypti
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Ranges overlap, potential for interspecific
competition

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Experimental design (Additive series)
Aedes albopictus
Aedes aegypti
Juliano, S. 1998. Species introduction and
replacement amongst mosquitoes interspecific
resource competition or apparent competition?
Ecology 79 255-268.
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Experimental design (Additive series)
Aedes albopictus
Aedes aegypti
Juliano, S. 1998. Species introduction and
replacement amongst mosquitoes interspecific
resource competition or apparent competition?
Ecology 79 255-268.
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Experimental design
Can asses Aedes aegypti performance in these
treatments
Aedes albopictus
Aedes aegypti
Juliano, S. 1998. Species introduction and
replacement amongst mosquitoes interspecific
resource competition or apparent competition?
Ecology 79 255-268.
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Hypotheses
Albo has an effect on Aeg, and effect of 1 Albo
1Aeg
Aedes aegypti response
aeg 20 20 40 60 albo 0 40
20 0
Aedes albopictus
Albo has no effect on Aeg, so effect of 1 Albo
0 Aeg
Aedes aegypti response
Aedes aegypti
aeg 20 20 40 60 albo 0 40
20 0
Albo has an effect on Aeg, but effect of 1 Albo lt
1Aeg
Aedes aegypti response
aeg 20 20 40 60 albo 0 40
20 0
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Hypotheses
Albo has an effect on Aeg, and effect of 1 Albo
1Aeg
Aedes aegypti response
aeg 20 20 40 60 albo 0 40
20 0
Aedes albopictus
Albo has no effect on Aeg, so effect of 1 Albo
0 Aeg
Aedes aegypti response
Aedes aegypti
aeg 20 20 40 60 albo 0 40
20 0
Albo has an effect on Aeg, but effect of 1 Albo lt
1Aeg
Aedes aegypti response
aeg 20 20 40 60 albo 0 40
20 0
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Results
Aedes aegypti survivorship
aeg 20 20 40 60 albo 0 40
20 0
ANOVA treatment x food x tire(block) Followed
by t-tests (posthoc)
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Competition example 2 Tidepool sculpins
Oligocottus maculosus
Clinocottus globiceps
Pfister, C.A. 1995. Estimating competition
coefficients from census data a test with field
manipulations of tidepool fishes. Am. Nat. 146
271-291
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• Types of response variables
• Static Within one generation (most studies)
• - Difficult to related to population dynamics
  (ßij)
• Dynamic Differences between generations
• - Can calculate ßij directly

Lotka-Volterra dNi/dt ri Ni 1-Ni/Ki (ßij
Nj)/Ki
Impact of one individual of species i relative to
one individual species j
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Lotka-Volterra dNi/dt ri Ni 1-Ni/Ki (ßij
Nj)/Ki or LnNi(t1)/Ni(t) ri (r/ Ki)
Ni(t) (ßij/Ki) Nj(t)
Change in species i density over time
Densities of two species
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Lotka-Volterra dNi/dt ri Ni 1-Ni/Ki (ßij
Nj)/Ki or LnNi(t1)/Ni(t) ri (ri/ Ki)
Ni(t) (ßij/Ki) Nj



Change in species i density over time
slopes
intercept
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Tidepool sculpins observed 5 tidepools over 2
years
Lotka-Volterra LnNi(t1)/Ni(t) ri (ri/
Ki) Ni(t) (ßij/Ki) Nj



Change in density over time of Oligocottus
0 (nonsig)
-0.718 (sig)
intercept
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• What Biol 404 students learnt last year
• Two weeks is long enough to get a measure of
  growth
• Lambda is a useful measure of population growth.
• Duckweed growth was not strongly impacted by
  nutrient supply or starting density over 2 weeks.
• Duckweed takes a long time to count! But digital
  images arent the answer in the short term
• Algae invaded after 1 week, may have impacted
  growth?

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Does algal cover reduce duckweed growth (lambda)?
Correlations (not experimental manipulations) for
2 different species
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1. Set-up of pilot experiment. 2. Take down
pilot experiment 2 weeks later. 3. Use power
analysis to decide on sample sizes for your
proposed experiment, using pilot data. 4. Hand
in research proposal (based on pilot expt and
analysis, but outlines a much more ambitious
experiment)