Interspecific competition: Paramecium

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Interspecific competition Paramecium

• George Gause
• P. caudatum goes extinct
• Strong competitors, use the same resource (yeast)
• Competitve asymmetry
• Competitive exclusion

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End 23rd lecture
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Interspecific competition Paramecium

• P. caudatum P. burseria coexist
• What is different?
• P. burseria is photosynthetic
• Competitive coexistence
• Apparently stable

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Mechanism of coexistence

• Paramecium caudatum
• nonphotosynthetic feeds on yeasts only
• must be near surface (O2)
• Paramecium burseria
• photosynthetic also feeds on yeasts
• endosymbiotic algae photosynthesis produce O2
• can feed in the bottom of the test tube
• Two species used different resources
• weak interspecific competition coexistence

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Resources

• component of the environment
• availability increases population growth
• can be depleted or used up by organisms
• A resource is limiting if it determines the
  growth rate of the population
• Liebigs law resource in shortest supply
  determines growth

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Resources for 0 growth
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Essential resourcesboth required
Soil nutrients for plants
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Competition for 1 resource
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Dynamics of competition for 1 resource
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Prediction for 2 species competing for 1 resource

• The species with the lower R will eliminate the
  other in competition
• Independent of initial numbers
• Coexistence not possible
• R rule

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Competitive exclusion principle

• Two species in continued, direct competition for
  1 limiting resource cannot coexist
• Focus on mechanism
• Coexistence requires 2 independently renewed
  resources
• Text pp. 182 -185

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Interspecific competition in nature

• Interspecific competiton may affect
• distribution and abundance
• species resource use
• morphology and behavior (evolutionary time)
• community composition, species co-occurrence
• community set of species living in one place at
  one time and potentially affecting each other

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Competition among barnaclesCompetitive exclusion
affects distribution abundance

• Rocky intertidal zone
• adult barnacles immobile on rocks
• larvae settle on rocks from plankton
• Joseph Connell (1961) Ecology 42710-723

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Distributions of Balanus Chthamalus
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End 24th lecture
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Chthamalus Balanus

• Larvae settle throughout much of the intertidal
• Chthamalus adults only in the high intertidal
• Balanus adults only in the mid low intertidal
• Hypothesis Balanus excludes Chthamalus
• Hypothesis Chthamalus cannot tolerate
  submergence
• Hypothesis Balanus cannot tolerate desiccation

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Experiments

• Rocks with larvae and young adults
• remove Balanus
• control count, no removal
• Rocks with young adults of one species
• transplant Balanus to high low intertidal
• transplant Chthamalus to high low intertidal
• Follow fates of marked individuals over years

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Experimental result 1
Undercut
Crushed

• Balanus individuals grow rapidly
• Shell undercuts or crushes adjacent Chthamalus
• Competition for space Balanus wins

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Experimental result 2

• Chthamalus survives well in the low intertidal
  only if Balanus is removed
• With Balanus present, Chthamalus is completely
  eliminated
• Distribution of Chthamalus is limited by
  interspecific competition with Balanus
• Local competitive exclusion

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Experimental results 3

• Balanus does not survive in the high intertidal,
  regardless of Chthamalus
• Desiccation
• Chthamalus tolerates dry conditions
• Balanus upper limit set by physical environment
• Chthamalus has a refuge from competition, a place
  where it escapes effects of its competitor

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Barnacles one example of the role of
interspecific competition

• Is interspecific competition common in nature?
• Is it often severe enough to cause competitive
  exclusion?
• How is exclusion avoided? Does competition cause
  natural selection?

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Role of interspecific competition

• Competition experiments
• Remove a species ? predict competitor ?
• Add a species ? predict competitor ?
• control (no manipulation)
• Reviews
• Schoener 1983 Am. Naturalist 122661-696
• Connell 1983 Am. Naturalist 122240-285

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Prevalence of competition

• Schoener 164 studies -- 90 find interspecific
  competition
• Connell 69 studies -- 86 find interspecific
  competition
• Does NOT mean 90 of all species compete
• Conclusion When observations lead to the
  hypothesis of competition, that hypothesis is
  usually correct

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Likelihood of exclusion

• Competitive asymmetry - Competitive exclusion
• Schoener 85 studies
• 60 asymmetrical
• 12 symmetrical
• 28 unclear
• Connell 54 experiments
• 61 asymmetrical
• 39 symmetrical
• Conclusion Exclusion should be very common

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Avoiding competitive exclusion

• Differences in resource use
• habitats, food, behavior
• Consider seed eating birds
• Morphology and resource use related
• Big bill ? big seeds
• Small bill? small seeds

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Quantitative traits Resource use
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Selection and competition
TIME
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Differences in resource use

• Low overlap can originate in 2 ways
• 1) Evolution in response to selection by
  competition
• 2) Independent of competition, pre-existing
  differences enable 2 species to coexist when they
  meet
• Resource partitioning use of different
  resources by potential competitors facilitates
  coexistence
• Includes both 1) and 2)
• Character displacement evolution of
  morphological differences where two species
  co-occurr
• Includes only 1)

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End 25th lecture
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Morphology Resource use

• What evidence exists to show that species with
  different morphology
• use different resources?
• compete less intensely?
• Example Anolis lizards
• Insectivorous, arboreal, Caribbean Islands
• Evidence for resource partitioning
• Probably not character displacement

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Caribbean Anolis

• St. Maarten
• A. gingivinus SVL41 mm
• A. wattsi SVL38 mm
• Competition experiment
• A. gingivinus A. wattsi
• less food in stomach
• lower growth rate
• compared to A. gingivinus alone

• St. Eustatius
• A. bimaculatus SVL53 mm
• A. wattsi SVL40 mm
• Competition experiment
• A. bimaculatus A. wattsi
• same amount in stomach
• same growth rate
• compared to A. bimaculatus alone

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Character displacement

• Birds
• Large Bill Size crack large seeds
• Small Bill Size crack small seeds
• Selection for resource partitioning
• examine 2 species where they are
• together (sympatry)
• separate (allopatry)
• Predict species DIFFER more in sympatry

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Darwins Finches

• Galapagos Islands
• Different seed-eating finches on different
  islands
• Recently evolved from a common South American
  ancestor

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Bill sizes of Darwins Finches
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Character displacement

• Evolution of morphological divergence in places
  where two otherwise similar species occur
  together
• Usual hypothesis is that selection occurs due to
  competition
• For finches, presumably competition for seeds

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Species interactions

• Interspecific competition
• interspecific competition is mutually negative
  (-,-)
• dN/N dt ? by competition
• Exploitation (predation, parasitism, herbivory)
• One species benefits, one harmed (,-)
• dN/N dt of consumer ?, dN/N dt of victim ?
• Mutualism
• Both species benefit (,)
• dN/N dt ? by mutualism

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Exploitation - specifically predation

• Predator kills and eats victim
• snake, wolf, fish, lion, spider, seed weevil,
  etc.
• Parasite lives intimately with victim and
  usually does not necessarily kill victim
• tapeworm, flea, louse, aphid, malaria, etc.
• Herbivore/Carnivore distinction not that
  important for dynamics

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Predation and population dynamics

• Assigned reading
• Gotelli Ch. 6, pp. 140-149 (problem 1)
• Note different symbols
• Note Krebs does not give these models
• Lotka-Volterra predator-prey model
• Symbols
• N number in prey population
• P number in predator population
• note symbols as in lab Populus

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End 26th lecture