Interspecific Competition

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Interspecific Competition
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Population interactions
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Logistic Curve
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Logistic model

• Logistic equation
• dN/dt rN((K-N)/K)
• Assumptions of the logistic model
• Each individual has identical ecological
  properties
• Instantaneous response to environmental change
• Constant upper limit to population size and rate
  of gain is directly related to remaining gap
• Limited space and constant food supply
• Age distribution is stable

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Lotka/Volterra model

• dN1/dt r1N1(K1-N1- ?12N2)/K1
• ? is the competition coefficient
• ?12 is effect of species 2 on species 1
• dN2/dt r2N2(K2-N2- ?21N1)/K2
• if ?12 gt1, then individuals of sp. 2 have a
  greater inhibitory effect on individuals of sp. 1
  than does species 1 on species 1.
• if ?12 lt1, then sp. 2 individuals have less effect

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Lotka/Volterra assumptions

• Environment is homogeneous and stable
• Migration is unimportant
• Coexistence requires a stable equilibrium point
• Competition is the only important biological
  interaction

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Model with isocline
Growth is 0 along the line
Species 1 growth
Species 2 growth

• See Fig. 12.1 and 12.2

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Finding K/?

• dN1/dt r1N1(K1-N1- ?12N2)/K1
• if growth is 0, and r is gt0, then the term
  (K1-N1- ?12N2) must be 0
• therefore N1 K1 - ?12N2
• when N1 0, N2 K1/?12

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Possible outcomes
Fig. 12.3
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Possible outcomes
Fig. 12.3
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Competition - 2 species
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Competition - 3 species
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Diffuse competition

• Effect of several species

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Diffuse competition

• Effect of several species

• Exclusion may occur

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How does coexistance occur?

• Unpredictable gaps the poorer competitor is a
  better colonizer
• Unpredictable gaps the pre-emption of space
• Fluctuating environments
• Ephemeral patches with variable life-spans
• Aggregated distributions

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If we observe niche differences in the field,
what does it mean?

• Current competition
• Evolutionary avoidance through competition
• Evolutionary avoidance (without competition)

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DeWitt diagrams

• Plants sown at different densities (400, 3010,
  2020, 1030, 040)
• Seeds harvested

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