BIOL 3500Ecology

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Lecture 13

• BIOL 3500--Ecology
• Chapter 14 Interspecific Competition

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Learning Objectives

• Define
• Allelopathy
• Zero growth isocline
• Competitive exclusion principle
• Contrast
• Character displacement vs. competitive release
• Discuss
• Lotka-Volterra graphs
• Gauses Paramecium experiments
• Tilmans diatom experiment

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Interspecific Competition

• Occurs between different species
• Both species are negatively affected

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Remember Logistic Growth

• dN/dt rN (K-N)/K
• Now, imagine doing this for two species

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Lotka-Volterra Model

• Describes competition between two species
• Begins with the logistic growth equation
• Includes a term which has
• Other species number (N1 or N2)
• and strength of its effect (a or ß)
• Equation dN1/dt r1N1 (K1-N1-aN2)/K1

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Lotka-Volterra Graphs

• First, focus on the two axes
• Then, the diagonal line (zero growth isocline).
  What does it represent?
• Lastly, the two zones

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Lotka-Volterra Graphs

• Now compare species 1 2
• Note arrows!

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Predictable Wins

• Combine graphs
• Why are arrows curved?
• Which one of these matches previous slide?

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Either Could Win

• Divide graph into 4 zones
• Focus on each species separately

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Coexistence

• Divide graph into 4 zones
• Focus on each species separately

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7th Inning Stretch
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G.F. Gauses Experiment

• Russian biologist, in lab
• Both Paramecia feed on bacteria
• P. aurelia consistantly outcompetes P. caudatum

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Competitive Exclusion Principle

• Two complete competitors cannot coexist
• Complete competitors need exactly same set of
  conditions resources
• In other words, they cannot coexist in exactly
  the same niche
• Supported by some of Gauses experiments on
  previous slide
• But, Gause also found that some competitors can
  coexist! (see book for 3rd species of Paramecium)

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Tilmans Diatom Study

• Famous American community ecologist,
• This done in lab
• Famous for work in prairies
• Asterionella vs. Synedra
• Results also support competitive exclusion
  principle

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The Role of Abiotic Factors

• Bazzaz discovered that ideal temp for seed
  germination varies per species
• How could this affect competition?
• How would this affect competitive exclusion
  principle?

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Grasses in Zimbabwe

• Again, abiotic factors can affect competition

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Resource Partitioning

• Within a community, different species might
  coexist by using exploiting resources unavailable
  to others
• Example
• Birds and bats both eat insects. How can they
  coexist?

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Resource Partitioning in Plants

• Water is a limited resource among plants
• Annual plants in old field habitat use water at
  different soil depths

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Resource Partitioning Among Cats

• Why is canine size critical to cat species?

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Question

• Certainly, species can evolve to outcompete
  competitors.
• Can competing species evolve to coexist
  sympatrically?
• Answer yes
• Can reduce niche overlap (and hence competition)
  through character displacement

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For Discussion
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Character Displacement

• A shift in niche, associated with a physical
  change in species

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Sympatric Populations

• Where species occur together, there is evolution
  for reduction in overlap (character displacement)

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Allopatric Populations

• But, when species occur alone, beak depths are
  intermediate in value, and similar to each other
  (competitive release)

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In Your Book
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Niches in 2 Dimensions

• Can actually be 1 to several
• Compare both 1 dimensional figures to the 2
  dimensional