Community Assembly Rules

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Community Assembly Rules
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Outline

• Defining assembly rules
• The niche concept
• Types of assembly rules
• Obstacles to testing assembly rules
• So what?

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Outline

• Defining assembly rules
• The niche concept
• Types of assembly rules
• Obstacles to testing assembly rules
• So what?

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Defining assembly rules
Ecological restrictions on the observed patterns
of species presence or abundance that are based
on the presence or abundance of one or more other
species or groups of species (not simply the
response of individual species to the
environment). Wilson Gitay 1995
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Defining assembly rules
Zobel 1997
How communities are assembled out of pools after
passing through abiotic environmental filters.
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Outline

• Defining assembly rules
• The niche concept
• Types of assembly rules
• Obstacles to testing assembly rules
• So what?

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The niche concept
No concept in ecology has been more variously
defined or more universally confused than the
niche. Real Levin 1991
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The niche concept

• No concept in ecology has been more variously
  defined or more universally confused than the
  niche.
• Real Levin 1991
• Origins of niche theory
• -Grinnell 1917 (habitat)
• -Elton 1927 (biological function)
• -Gause 1934 (competitive exclusion)
• -Hutchinson 1957 (fundamental and realized)

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The niche concept

• No concept in ecology has been more variously
  defined or more universally confused than the
  niche.
• Real Levin 1991
• Origins of niche theory
• One definition
• A species fundamental niche is defined by the
  combination of conditions and resources which
  allow the species to maintain a viable
  population. Its realized niche is defined by the
  conditions and resources utilized by the species
  in the presence of (negative) interspecific
  interactions.

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The niche concept

• No concept in ecology has been more variously
  defined or more universally confused than the
  niche.
• Real Levin 1991
• Origins of niche theory
• One definition
• Niche theory and community assembly
• -Limiting similarity

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Limiting similarity There is a limit to the
similarity (and hence to the number) of competing
species which can coexist. MacArthur and Levins
1967
Limiting similarity
esource gradient
MacArthur and Levins 1967
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Outline

• Defining assembly rules
• The niche concept
• Types of assembly rules
• Obstacles to testing assembly rules
• So what?

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Types of assembly rules

• Rules based on particular species
• Diamond 1975
• Connor and Simberloff 1979 (reply)

After Wilson 1999
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Types of assembly rules

• Rules based on particular species
• Rules based on presence/absence
• -variance in richness
• -local vs. regional richness
• Cornell and Lawton 1992
• -large-scale distributions

After Wilson 1999
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Types of assembly rules

• Rules based on particular species
• Rules based on presence/absence
• Rules based on species characters
• -texture convergence
• Watkins and Wilson 2003
• -limiting similarity
• Watkins and Wilson 1992, Kelt and Brown 1999
• -guild/functional group based
• Fox and Brown 1993, Wilson and Roxburgh 1994, Fox
  1999

After Wilson 1999
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Types of assembly rules

• Rules based on particular species
• Rules based on presence/absence
• Rules based on species characters
• -intrinsic guilds
• Wilson and Roxburgh 1994, Cody 1999

After Wilson 1999
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Types of assembly rules

• Rules based on particular species
• Rules based on presence/absence
• Rules based on species characters
• Rules based on species abundance
• -biomass constancy
• -abundance-based guild proportionality
• -Relative Abundance Distributions (RAD)

After Wilson 1999
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Outline

• Defining assembly rules
• The niche concept
• Types of assembly rules
• Obstacles to testing assembly rules
• So what?

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Obstacles to testing assembly rules

• Appropriate null models
• Narcissus effect-Type II error
• I.e. sampling from a post-competition pool
  underestimates the role of competition, since its
  effect is already reflected in the pool.
  Colwell and Winkler 1984
• Jack Horner effect-Type I error
• I.e. demonstration of the obvious e.g. some
  sites are richer than others, some species have
  larger ranges than others, from failure to
  build into the null model features one does not
  want to be told about. Wilson 1995

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Obstacles to testing assembly rules

• Appropriate null models
• Environmental heterogeneity (different spp. in
  different environments)
• Spatial autocorrelation (non-independence)

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Outline

• Defining assembly rules
• The niche concept
• Types of assembly rules
• Obstacles to testing assembly rules
• So what?

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So what?

• Invasibility
• Community dis-assembly
• Does limiting similarity exist?
• Do local processes constrain richness?

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Do assembly rules exist? How important are they?
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Why are more diverse communities more resistant
to invasion? More diverse
communities should have less space available
native species abundance
total unoccupied space
.
1 - Species system
2 - Species system
Percent cover
3 - Species system
Time
Time