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Competition experiments

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1 Competition experiments

2 What are possible response variables for a competition experiment?
What is competition? What are possible response variables for a competition experiment?

3 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)?

4 Focal species Additive design Associate

5 N associate Monoculture of focal N focal species

6 Substitutive design (replacement series)

7 Monoculture of associate
N associate Monoculture of focal N focal species

8 Monocultures of associate
Response surface Monocultures of associate N associate Monocultures of focal N focal species

9 Groups Additive Substitutive Gina Rebecca Heather Aerin Annie David Riann Liz Hillary Marie Nick Aliice Katsky James

10 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?

11 Cultural exchange Additive Substitutive Gina Rebecca Heather Aerin Annie David Riann Liz Hillary Marie Nick Aliice Katsky James

12 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?

13 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)?

14 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)?

15 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)?

16 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)?

17 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

18 Competition models (two examples) Example 1:
Additive series design for container mosquito larvae

19 Aedes albopictus invades N America from SE Asia in 1985, via used tire trade at Houston
                               1985 1996

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

21 Ranges overlap, potential for interspecific competition

22 Experimental design (Additive series)
Replacement series Aedes albopictus Aedes aegypti Juliano, S Species introduction and replacement amongst mosquitoes: interspecific resource competition or apparent competition? Ecology 79:

23 Experimental design (Additive series)
Aedes albopictus Addition design Aedes aegypti Juliano, S Species introduction and replacement amongst mosquitoes: interspecific resource competition or apparent competition? Ecology 79:

24 Aedes aegypti performance in these treatments Aedes albopictus
Experimental design Can asses Aedes aegypti performance in these treatments Aedes albopictus Aedes aegypti Juliano, S Species introduction and replacement amongst mosquitoes: interspecific resource competition or apparent competition? Ecology 79:

25 Aedes aegypti response Aedes aegypti response Aedes aegypti response
Hypotheses: Albo has an effect on Aeg, and effect of 1 Albo = 1Aeg Aedes aegypti response aeg albo Aedes albopictus Albo has no effect on Aeg, so effect of 1 Albo = 0 Aeg Aedes aegypti response Aedes aegypti aeg albo Albo has an effect on Aeg, but effect of 1 Albo < 1Aeg Aedes aegypti response aeg albo

26 Aedes aegypti response Aedes aegypti response Aedes aegypti response
Hypotheses: Albo has an effect on Aeg, and effect of 1 Albo = 1Aeg Aedes aegypti response aeg albo Aedes albopictus Albo has no effect on Aeg, so effect of 1 Albo = 0 Aeg Aedes aegypti response Aedes aegypti aeg albo Albo has an effect on Aeg, but effect of 1 Albo < 1Aeg Aedes aegypti response aeg albo

27 Aedes aegypti survivorship
Results Aedes aegypti survivorship aeg albo ANOVA: treatment x food x tire(=block) Followed by t-tests (posthoc)

28 Competition example 2: Tidepool sculpins
                                      Oligocottus maculosus Clinocottus globiceps Pfister, C.A Estimating competition coefficients from census data: a test with field manipulations of tidepool fishes. Am. Nat. 146:

29 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

30 Lotka-Volterra: 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

31 } } } Lotka-Volterra: 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

32 } } } 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 -0.718 (sig) (nonsig) intercept

33 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?

34 Does algal cover reduce duckweed growth (lambda)?
Correlations (not experimental manipulations) for 2 different species

35 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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