Competition between individuals

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Competition between individuals
PB 1.2

• Lecture 4
• Nick Brown

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Objective To describe the effects of
competition on the quality and quantity of
individuals.
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Competition a definition

• An interaction brought about by a shared
  requirement for a resource in limited supply.
• Competition leads to a reduction in
• growth
• reproduction and/or
• survivorship
• of at least some of the individuals.

Begon, Harper Townsend (1996)
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The effects of intraspecific competition are
density dependent
The opportunities for a plant to capture light,
water and nutrients depend on

• Number
• Proximity
• Size

of neighbours already capturing them.
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Density dependent mortality
Number of deaths
Plant density
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Density dependence in natural plant populations
Most plants do not experience average
population density
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Density dependence in natural animal populations
In island Roe deer (Capreolus capreolus)
populations, body size strongly affects the
number of fawns produced but density explained
lt10 of the variation in body size
Andersen Linnell (2001)
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The law of constant final yield
Total yield (kg m-2)
Plant population density
Plants at high densities are unable to grow to a
large size. High density is offset by poor
individual growth.
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The reciprocal yield law
Log10 mean plant weight
Log10 population density
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Self-thinning
Log10 mean dry weight per plant
Log10 number of survivors (ha-1)
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Self-thinning rule
Log10 mean dry weight per plant
Log10 number of survivors (ha-1)
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Forestry

• Spacing determines quality and quantity
• High density high total production
• Low density high quality
• Thinning is it worth it?

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Negative competition the Allee effect
Per capita birth/death rate
Death rate
Birth rate
Density
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Intraspecific competition drives evolutionary
diversification

• Competition in a population can drive niche
  expansion onto new resources for which
  competition is less severe

Bolnick, D. (2001) Intraspecific competition
favours niche width expansion in Drosophila
melanogaster. Nature 410 (6827) 463-466