Community Assembly Rules

• Defining “assembly rules”

• The niche concept

• Types of assembly rules

• Obstacles to testing assembly rules

• So what?

Outline

• Defining “assembly rules”

• The niche concept

• Types of assembly rules

• Obstacles to testing assembly rules

• So what?

Outline

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

Defining “assembly rules”

How communities are assembled out of pools after passing through abiotic environmental filters.

Zobel 1997

• Defining “assembly rules”

• The niche concept

• Types of assembly rules

• Obstacles to testing assembly rules

• So what?

Outline

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

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)

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.

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

MacArthur and Levins 1967

Limiting similarity: “There is a limit to the similarity (and hence to the number) of competing species which can coexist.” MacArthur and Levins 1967

esource gradient

Limiting similarity

• Defining “assembly rules”

• The niche concept

• Types of assembly rules

• Obstacles to testing assembly rules

• So what?

Outline

Types of assembly rules1. Rules based on particular species

Diamond 1975

Connor and Simberloff 1979 (reply)

After Wilson 1999

Types of assembly rules1. Rules based on particular species

2. Rules based on presence/absence-variance in richness

-local vs. regional richnessCornell and Lawton 1992

-large-scale distributions

After Wilson 1999

Types of assembly rules1. Rules based on particular species

2. Rules based on presence/absence

3. Rules based on species’ characters-texture convergence

Watkins and Wilson 2003

-limiting similarityWatkins and Wilson 1992, Kelt and Brown 1999

-guild/functional group basedFox and Brown 1993, Wilson and Roxburgh 1994, Fox 1999

After Wilson 1999

Types of assembly rules1. Rules based on particular species

2. Rules based on presence/absence

3. Rules based on species’ characters-intrinsic guilds

Wilson and Roxburgh 1994, Cody 1999

After Wilson 1999

Types of assembly rules1. Rules based on particular species

2. Rules based on presence/absence

3. Rules based on species’ characters

4. Rules based on species abundance-biomass constancy

-abundance-based guild proportionality

-Relative Abundance Distributions (RAD)

After Wilson 1999

• Defining “assembly rules”

• The niche concept

• Types of assembly rules

• Obstacles to testing assembly rules

• So what?

Outline

Obstacles to testing assembly rules

• Appropriate null modelsNarcissus 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 errorI.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

Obstacles to testing assembly rules

• Appropriate null models• Environmental heterogeneity (different spp. in

different environments)• Spatial autocorrelation (non-independence)

• Defining “assembly rules”

• The niche concept

• Types of assembly rules

• Obstacles to testing assembly rules

• So what?

Outline

So what?

•Invasibility

•Community dis-assembly

•Does limiting similarity exist?

•Do local processes constrain richness?

Do assembly rules exist?

How important are they?

1-Species

4-species

.

Time Time

Per

cen

t co

ver

1 - Species system

2 - Species system

3 - Species system

native species abundance total unoccupied spaceWhy are more diversecommunities moreresistant to invasion?

More diverse communities should have less spaceavailable