Community Ecology
Community assembly and null models
Outline:
1. Definition of community assembly
2. Effects of scale (local and regional)
3. Diamond’s assembly rules, derived from checkerboard patterns of species’ incidences on islands
- Mechanism: competition, resulting in permissible and forbidden combinations
4. Connor and Simberloff’s criticism of Diamond
A. Null models
B. Let the war begin!
5. Nestedness
6. Final comments
Terms/people:
Assembly (cf. succession)assembly rulesJared Diamond
Checkerboard patternpermissible and forbidden species combinations
Null modelDan Simberloffnestedness
High-S speciessupertrampscompanions in starvation
Colespecies incidence functiondeletion rules
community assembly
cf. succession
Is community assembly predictable?(Reminiscent of Clements/Gleason argument.) If so, there should be assembly rules:
Jared Diamond 1975 - birds of New Guinea and its satellite islands
“checkerboard pattern” of species co-occurrence with “permissible” and “forbidden” species combos based on life history traits and competition
6 types of species combos: “high-S” species A-D tramps “suptertramps”
tradeoff between dispersal and competitive ability (for more info, read Cadotte et al. 2006)
species incidence functions
Connor and Simberloff 1979, 1984, 1986 - criticized Diamond’s use of competition to explain community patterns (and his tacit assumption of equilibrium)
null model
Diamond’s (1975) assembly rules for communities:
a. If one considers all the combinations that can be formed from a group of related species, only certain ones of these combinations exist in nature.
b. Permissible combinations resist invaders that would transform them into forbidden combinations.
c. A combination that is stable on a large or species-rich island may be unstable on a small or species-poor island.
d. On a small or species-poor island, a combination may resist invaders that would be incorporated on a larger or more species-rich island.
e. Some pairs of species never coexist, either by themselves or as part of a larger combination.
f. Some pairs of species that form an unstable combination by themselves may form part of a stable larger combination.
g. Conversely, some combinations that are composed entirely of stable subcombinations are themselves unstable.
Connor and Simberloff’s (1979) criticisms of these rules:
Re: Rule c: Basically a statement that species-rich islands contain more combinations than species- poor islands, which seem inevitable. Therefore, Rule c is trivial.
Re: Rule b: The evidence for “resistance” is weak; rule then states only that permissible combinations occur and forbidden combinations do not. Therefore, Rule b is a tautology.
Re: Rule d: A combination of Rules b and c; therefore, Rule d is a trivial tautology.
Re: Rule f: Because there are no islands that contain just a pair of species, pairs cannot occur by themselves. Therefore, Rule f is untestable.
Re: Rules a, e, and g: Use null model analysis to show that “there is nothing about the absence of certain species pairs or trios...that would not be expected were the birds not randomly distributed over the islands...Since there are so many possible sets of species, it is to be expected that a few sets are not found on any island; this does not imply that such sets are actively forbidden by any deterministic forces.”
Degenerated into a fierce debate. What emerged (to the benefit of ecology as a whole) was an appreciation of a benchmark for comparison, a null model. As in statistics, where a null model is used to provide a standard of comparison, an ecological null model provides a way of distinguishing true pattern from randomness (Gotelli and Graves 1996).Something to think about: a truly random assemblage (true null model) is biologically unrealistic so why use null models?
"face in the tortilla" analogy
Cole 1983 - mangrove ant communities invasion/removal experiments
Found that dispersal capabilities and chance (à la null model) may determine who gets to an island first, but once established, could repel other species from competition (à la Diamond).
Nested community structure& predictability
Bottom line: The process of community assembly begins with a species pool, from which some species are able to colonize a local area; of these, some species become established to form the community. But because this process is a continuing one and local extinctions occur, community composition is dynamic rather than fixed.
Next lecture: priority effects and the lottery hypothesis of community assembly
References:
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Cadotte, M.W., D.V. Mai, S. Jantz, M.D. Collins, M. Keele and J.A. Drake. 2006. On testing the competition-colonization tradeoff in a multispecies assemblage. Am. Nat. 168:704-709.
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