Competition

1
Competition

• Lecture 15

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Competition

• Ecologists have used a variety of types of
  evidence to detect effects of competition
• Studies of con-generic species
• Species co-occurrence data and shifts in
  distribution after removal of a competitive
  dominant
• Character displacement studies

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Dendroica wablers

• Five closely related wood warblers
• Taxonomically-- closely related
• Similar morphologies
• Eat insects

4
Use of habitat
Cape May
Blackburnian
Black-throated green
Black-throated green
Bay Breasted
Myrtle Warbler
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Character displacement

• Members of the same species should undergo the
  most intense competition
• Sexual dimorphism in bill size
• Male and female select different prey

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Character displacement

• Comparisons of the same species in allopatry and
  sympatry
• Allopatricgeographically separated
• Sympatricoverlapping distributions

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Snails in Denmark Allopatric
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Snails in Denmark Sympatric
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Desert plants

• In arid and semi-arid environments
• Water is scarce therefore we would expect plants
  to compete for water
• Desert plants have many adaptations for
  conserving water
• Thick cuticle
• Low surface area leaves

10
Desert plants

• Distribution patters can be characterized as
• Random
• Evenly or over-dispersed
• Clumped or under-dispersed

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Randomly dispersed
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
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Clumped
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
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Over-dispersed
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
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If we start with a randomly dispersed plant
community, what would we expect under conditions
of competition?
o
o
o
o
o
o
o
o
o
o
o
o
o
o
High survival
o
o
Low survival
o
o
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Over time the population will become more evenly
dispersed
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
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Evidence that desert plants are evenly dispersed

• Several species of desert plants are evenly
  dispersed
• But, many other species are clumped or have
  random dispersion patterns
• Water availability could confound the observed
  patterns

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A more direct way to look for evidence of
competition

• Would be to examine individual plants in the
  presence and absence of potential predators
• Freidman et al. Experiments with Artemisia
  herba-alba
• Transplanted seedlings near the co-dominant
  species Zygophllum dumosum

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Size of plants
Mass
Distance to nearest
Focal plant
Neighbour
High
Distance to nearest
Focal plant
Neighbour
Low
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Survival of Artemisia
Survival
Distance between
Artemisia
Zygophllum
High
Distance between
Artemisia
Zygophllum
Low
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Conclusions

• Competition is widespread
• Evidence from a wide range of taxa
• Using a variety of types of evidence
• How widespread and important competition is
  relative to other forces that structure
  communities is less clear