Community Assembly Rules

Outline 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?

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 Rules based on species co-occurences/interactions NOT on species responses to environmental variation--can determine diversity and composition

Defining “assembly rules” Rules based on species co-occurences/interactions NOT on species responses to environmental variation--can determine diversity and composition Zobel 1997 How communities are assembled out of pools after passing through abiotic environmental filters.

Outline Defining “assembly rules” The niche concept Types of assembly rules Obstacles to testing assembly rules So what?

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

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

Outline Defining “assembly rules” The niche concept Types of assembly rules Obstacles to testing assembly rules So what?

Types of assembly rules Rules based on particular species Diamond 1975 Connor and Simberloff 1979 (reply) After Wilson 1999

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

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

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

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

Outline Defining “assembly rules” The niche concept Types of assembly rules Obstacles to testing assembly rules So what?

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

Obstacles to testing assembly rules Appropriate null models Environmental heterogeneity (different spp. in different environments) Spatial autocorrelation (non-independence)

Outline Defining “assembly rules” The niche concept Types of assembly rules Obstacles to testing assembly rules So what?

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

Do assembly rules exist? How important are they?

Why are more diverse communities more resistant to invasion? 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