Ecology BSC 201

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Ecology (BSC 201)

• Part 2. Steven Juliano
• Office 335 FSA
• Hours Wed 200PM,
• Thu 1000AM,
• by appointment
• Phone [phone]
• sajulia_at_mail.bio.ilstu.edu

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Lectures on the web

• http//www.bio.ilstu.edu/juliano/juliano.htm
• how to use lectures on the web

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Reading

• Review chapters 19, 18
• Read chapter 17

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Competition

• Negative effects of one individual on another
• Resource competition use and depletion of a
  shared resource
• Interference competition direct harm to other
  individuals
• direct aggression, attack
• chemical interactions
• at population level, with increased N,
  competition lowers dN / Ndt

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Competition

• intraspecific competition
• among members of 1 species
• interspecific competition
• between members of different species
• Interspecific competition mutually negative

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Assumptions of Lotka-Volterra Competition models

• r1, r2, K1, K2, a, b are constants
• do not vary over time
• Effects of competition (inter- intraspecific)
  are linear declines in dN / Ndt as Ns increase
• as opposed to nonlinear
• Either some resource(s) are limiting or there is
  interspecific interference

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Competitive Exclusion Principle

• Competitive Exclusion local extinction of one
  species through interspecific competition
• Two species in continued direct competition
  cannot coexist unless interspecific competition
  is weak relative to intraspecific competition
• Weak interspecific competition?
• low a or b
• Use different resources
• Use different physical spaces

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Laboratory tests

• Flour beetles
• Tribolium confusum Tribolium castaneum
• Text pp. 368-369
• Protozoans Paramecium aurelia, Paramecium
  caudatum, Paramecium burseria

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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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Interspecific competition Paramecium

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

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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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End 1st lecture
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2nd exam mean 69.9SD 15.2
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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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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. 366-368

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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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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
• Resource?
• Space
• 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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End 2nd lecture
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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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Morphology Resource use

• Evidence that species with different morphology
• use different resources?
• compete less intensely?
• Example Anolis lizards
• Insectivorous, arboreal
• 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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End 3rd lecture
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Character displacement

• Birds
• Large Bill Size crack large seeds
• Small Bill Size crack small seeds
• Ch. 16, pp. 311-312
• 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
• Ch. 20, pp. 389-390

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Bill sizes of Darwins Finches
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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
• Hypothesis natural selection due to competition
• For finches, presumably competition for seeds
• Evidence that seeds are a limiting resource and
  that changes in seed availability select for bill
  size (pp. 311-312)

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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
• Ch. 17, 18

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Predictions of ? logistic

• 1. Inefficient predator
• isoclines dont cross
• predicts predator extinction
• 2. Intermediate predator efficiency 1
• isoclines cross to right of peak
• predicts stable coexistence with damped
  oscillations

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Predictions of ? logistic

• 3. Intermediate predator efficiency 2
• isoclines cross near peak
• predicts stable oscillations
• 2.Highly efficient predator
• isoclines cross to left of peak
• predicts expanding oscillations extinction

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Predator (P)
Prey (N)
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Predictions of ? logistic
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Implications of improved predator-prey models

• Different patterns of dynamics are possible
• Stable cycles are only one special case
• Prey may be exterminated (efficient predators)
• Prey may be reduced to stable populations below K
• Biological control Introduce enemies to reduce
  or eliminate pests

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Gauses predator-prey experiments
Didinium Predatory ciliate
Paramecium Prey
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Didinium - Paramecium predator-prey experiment
Paramecium
Density (N or P)
Didinium
Time (t )
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Gauses Predator-Prey experiments

• No cycles (stable or otherwise)
• Predator exterminates prey
• Predator dies out shortly after
• Inconsistent with Lotka-Volterra predator-prey
  models
• Consistent with more realistic models (e.g., ?
  logistic)

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End 4th lecture