Outline:

1.  Definition of community assembly

A. Species sorting

2.  Diamond’s assembly rules, derived from checkerboard patterns of species’ incidences on islands

A.     Mechanism: competition-dispersal tradeoffs, resulting in permissible and forbidden combinations

3.  Simberloff’s criticism of Diamond

         A. Let the war begin!

                     i. Null models: a game-changer for community ecology

                     ii. “Face in the tortilla” analogy

         B. Peace reigns

4. More recent work

There is no more fundamental question in community ecology than what determines the number, abundances, and kinds of species in an area. Why should only a limited subset of the regional species pool be found in a given area? This question is one of community assembly.

community assembly

         (contrast with succession)

Traditionally, the assembly process has been viewed as leading to a community that is in equilibrium (stable). Such a community resists invasion. The sorting out of species that produces this high level of integration and organization is viewed as a result in large part from interspecific competition. Species sorting - 

If this is true, then we should be able to detect assembly rules for communities:

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 à A-D tramps à “suptertramps”

note the tradeoff between dispersal and competitive ability (for more info, see Levins and Culver 1971, Cadotte et al. 2006)

species incidence functions – graphs illustrating occurrence of species à allows identification of high-S and supertramp species

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.

Dan Simberloff (1984), Connor & Simberloff (1979, 1984, 1986) - criticized Diamond’s use of competition to explain community patterns (and his tacit assumption of equilibrium)

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: Null model analysis shows 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: “The line between scientific discourse and name calling became blurred” (Chase and Leibold 2003, p. 13).

This debate changed ecology. 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             

Caused a paradigm shift in community ecology with respect to use of null models

Cole 1983a,b - 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).

More recent work:

Numerous recent studies and meta-analyses have documented significant checkerboard patterns compared to null models (some negative, like Diamond’s, where species avoid each other, but also some positive; these differences can be attributed to different spatial scales [more negative relationships being found at local scales and more positive ones at larger spatial scales]) – see e.g. Gotelli and McCabe (2002), Veech (2006), and Gotelli and Ulrich (2012).

The presence of such patterns indicates that there must be some deterministic mechanisms (i.e., assembly rules) that dictate which species from a regional pool can arrive at, become established at, and persist at a given site. Since the foundational work I presented today, the study of community assembly has expanded more broadly to focus on trait-based community assembly, based on functional traits that are phenotypically defined. (I recommend Mittelbach and McGill 2019 for more information.)

To summarize: 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. One of the mechanisms that works to filter species from the regional pool down to the local assemblage is competition, which will be our focus for the next few weeks. But other factors, including abiotic limitations and dispersal limitation, can also determine the membership in a community.

References:

Abbott, I. 1980. Theories dealing with the ecology of landbirds on islands. Advances in Ecological Research 11:329-371.

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.

Chase, J.M., and M.A. Leibold. 2003. Ecological Niches: Linking Classical and Contemporary Approaches. University of Chicago Press, Chicago, IL.

Cole, B.J. 1983a. Assembly of mangrove ant communities: patterns of geographical distribution. J. Anim. Ecol. 52:339-347.

Cole, B.J. 1983b. Assembly of mangrove ant communities: colonization abilities. J. Anim. Ecol. 52:349-355.

Connor, E.F., and D. Simberloff. 1979. The assembly of species communities: chance or competition? Ecology 60:1132-1140.

Connor, E.F., and D. Simberloff. 1984. Neutral models of species’ co-occurrence patterns. Pp. 316-331 in: Ecological Communities: Conceptual Issues and the Evidence (D.R. Strong, Jr., D. Simberloff, L.G. Abele, and A.B. Thistle, eds.). Princeton University Press, Princeton, NJ.

Connor, E.F., and D. Simberloff. 1986. Competition, scientific method, and null models in ecology. Amer. Sci. 74:155-162.

Diamond, J.M. 1975. Assembly of species communities. Pp. 342-444 in: Ecology and Evolution of Communities (M.L. Cody and J.M. Diamond, eds.). Belknap Press, Cambridge, MA.

Diamond, J.M. 1982. Effects of species pool size on species occurrence frequencies: musical chairs on islands. Proc. Natl. Acad. Sci. USA 52:2420-2424.

Diamond, J.M., and M.E. Gilpin. 1982. Examination of the “null” model of Connor and Simberloff for species co-occurrences on islands. Oecologia 52:64-74. Drake, J.A. 1990. Communities as assembled structures: do rules govern patterns? Trends Ecol. Evol. 5:159-164.

Gilpin, M.E., M.P. Carpenter, and M.J. Pomerantz. 1986. The assembly of a laboratory community: multispecies competition in Drosophila. Pp. 23-40 in: Community Ecology (J. Diamond and T.J. Case, eds.). Harper & Row, New York, NY.

Gilpin, M.E., and J.M. Diamond. 1982. Factors contributing to non-randomness in species co- occurrences on islands. Oecologia 52:75-84.

Gilpin, M.E., and J.M. Diamond. 1984. Are species co-occurrences on islands non-random, and are null hypotheses useful in community ecology? Pp. 297-315 in: Ecological Communities: Conceptual Issues and the Evidence (D.R. Strong, Jr., D. Simberloff, L.G. Abele, and A.B. Thistle, eds.). Princeton University Press, Princeton, NJ.

Gotelli, N.J., and D.J. McCabe.  2002.  Species co-occurrence: a meta-analysis of J.M. Diamond’s assembly rules model.  Ecology 83:2091-2096.

Gotelli, N.J., and W. Ulrich. 2012. Statistical challenges in null model analysis. Oikos 121:171-180.

Levins, R., and D. Culver. 1971. Regional coexistence of species and competition between rare species. Proc. Natl. Acad. Sci. USA 68:1246-1248.

Mittelbach, G.G., and B.J. McGill. 2019. Community Ecology, 2nd ed. Oxford University Press, Oxford, UK.

Simberloff, D. 1984. Properties of coexisting bird species in two archipelagoes. Pp. 234-253 in: Ecological Communities: Conceptual Issues and the Evidence (D.R. Strong, Jr., D. Simberloff, L.G. Abele, and A.B. Thistle, eds.). Princeton University Press, Princeton, NJ.

Veech, J.A. 2006. A probability-based analysis of temporal and spatial co-occurrence in grassland birds. J. Biogeogr. 33:2145-2153.

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