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## Competition experiments

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What are possible response variables for a competition experiment?

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)?

Additive

Substitutive

Gina

Rebecca

Heather

Aerin

Annie

David

Riann

Liz

Hillary

Marie

Nick

Aliice

Katsky

James

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?

Additive

Substitutive

Gina

Rebecca

Heather

Aerin

Annie

David

Riann

Liz

Hillary

Marie

Nick

Aliice

Katsky

James

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?

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)?

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)?

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?

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

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

Breeds in containers habitats (old tires, treeholes), like native species, Aedes aegypti

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.

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.

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.

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 < 1Aeg

Aedes aegypti response

aeg 20 20 40 60

albo 0 40 20 0

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 < 1Aeg

Aedes aegypti response

aeg 20 20 40 60

albo 0 40 20 0

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)

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

- 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

dNi/dt= ri Ni [1-Ni/Ki – (ßij Nj)/Ki] or

Ln[Ni(t+1)/Ni(t)] = ri – (r/ Ki) Ni(t)– (ßij/Ki)Nj(t)

Change in species “i” density over time

Densities of two species

dNi/dt= ri Ni [1-Ni/Ki – (ßij Nj)/Ki] or

Ln[Ni(t+1)/Ni(t)] = ri – (ri/ Ki) Ni(t)– (ßij/Ki)Nj

}

}

}

Change in species “i” density over time

slopes

intercept

Tidepool sculpins: observed 5 tidepools over 2 years

Lotka-Volterra:

Ln[Ni(t+1)/Ni(t)] = ri – (ri/ Ki) Ni(t)– (ßij/Ki)Nj

}

}

}

Change in density over time of Oligocottus

0

(nonsig)

-0.718

(sig)

intercept

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 aren’t the answer in the short term
- Algae invaded after 1 week, may have impacted growth?

Does algal cover reduce duckweed growth (lambda)?

Correlations (not experimental manipulations) for 2 different species

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)

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