Population Ecology

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Population Ecology

• What kind of competition are we considering
  here?
• Exploitative depression of shared resources
• Examples? Food, nutrients
• Interference actions reduce the exploitation
  efficiencies of others
• Examples? Birds, ants, allelopathy

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Population Ecology

• Another category of competition is pre-emptive
  competition
• Irrespective of the type of competition, the
  result should be a lowering of all species in
  question, however, they should be modeled
  differently

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Population Ecology

• One problem with interference competition is
  modeling it..why?
• Because its very nature suggests it is a density
  dependent phenomena

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Population Ecology

• Competitive exclusion of species 1

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Population Ecology

• Competitive exclusion of species 2

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Population Ecology

• Coexistence in a stable equilibrium

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Population Ecology

• Competitive exclusion in an unstable equilibrium

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Population Ecology

• Competitive Exclusion
• The worst scenario for species 1 will be if its
  own abundance is close to 0 (N1 0) and the
  abundance of its competitor is close to carrying
  capacity (N2 K2)

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Population Ecology

• If species 1 can achieve positive growth, then it
  should be able to invade (e.g. dN1/dt1/N1 gt
  0)
• Since r1 is always positive, it must be that (K1
  aK2)/K1 gt 0 or K1 /K2 gt a
• For species 2 it is K2/K1 gt ß

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Population Ecology
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Population Ecology

• These numbers help explain the principle of
  competitive exclusion
• If 2 species are close in resource use and body
  size, then a and ß will be close to 1
• 1/ß gt K1/K2 gt a for stable coexistence
• 1/0.9 (1.1) gt K1/K2 gt 0.9
• 1/0.2 (5) K1/K2 gt 0.2

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Population Ecology

• Populations are highly variable and differences
  may be the result of different r and starting
  population size

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Population Ecology

• It is counterintuitive that a small r is more
  stable. Why?

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Population Ecology

• Relationship between pop(n) variability and risk
  of extinction

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Population Ecology

• Adding a competitor or predator generally makes
  population dynamics less stable

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Population Ecology
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Population Ecology

• It is counterintuitive that a small r is more
  stable. Why?

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Population Ecology

• If the competitive or predatory relationship is
  particularly strong, we might see an ecological
  response (change in resource use or range shift)
  or an evolutionary response (e.g. character
  displacement or habitat shift)more later

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Population Ecology

• Parasitism and herbivory rarely threaten a
  population extinction, but both can significantly
  depress a populations numbers

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Population Ecology

• Other factors that can influence population
  dynamics

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Population Ecology

• Empirical demonstrations of competitive exclusion
• Most successful examples are relatively simple
  cases in species-poor systems

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Population Ecology

• Growth of Paramecium separately and in the
  presence of one another

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Population Ecology

• Given that one species is almost always more
  competitive than others, why dont we see more
  extinctions?

Variable competitive landscape
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Population Ecology

• Park studied two species of Tribolum (flour
  beetles)

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Population Ecology

• When conditions were altered to a cooler climate
  with less humidity, Tribolium confusum was the
  winner

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Population Ecologycompetitive coexistence
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Population Ecology

• Similar results have been shown with a series of
  fruit fly experiments (Ayala 1970) where
  Drosophila serrata would outcompeted D.
  pseudoobscura at temperatures gt25o C but would
  win when temperatures were lt22o C

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Population Ecologycompetitive coexistence

• Competitive advantages vary with changing
  environmental conditions
• Coexistence can be possible even amongst strong
  competitors in a variable (spatially or
  temporally) environment
• Another option may be competitive advantages
  occur at different stages of the life cycle

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Population Ecologycompetitive coexistence

• Hutchinson (1953) pointed out
• periodicity of environmental fluctuation
• ecologically similar species
• generation time

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Population Ecologycompetitive coexistence

• Huston (1979) has shown that introducing
  occasional episodes of strong density-independent
  mortality can almost indefinitely delay
  competitive outcomes

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Population Ecologycompetitive coexistence

• Sale (1977, 1979) has examined the influence of
  chance into competitive outcomes
• Some systems (e.g. corals) occupancy is
  completely random
• Consequently, no one consistently wins and all
  species are able to coexist

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Population Ecologycompetitive coexistence

• Fig 2.2

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Population Ecology

• What about empirical demonstrations of
  predator-prey extinctions

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Population Ecology

• Huffakers (1958) classic mite study

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• simple predator-prey dynamics

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