Evol Ecol Res 11: 771-786 (2009)     Full PDF if your library subscribes.

Pairwise co-existence of Bismarck and Solomon landbird species

James G. Sanderson1, Jared M. Diamond2 and Stuart L. Pimm3

1Wildlife Conservation Network, Los Altos, California, 2Geography Department, University of California, Los Angeles, California and 3Nicholas School of the Environment, Duke University, Durham, North Carolina, USA

Correspondence: S.L. Pimm, Nicholas School of the Environment, Duke University, Durham, NC 27709, USA.
e-mail: stuartpimm@me.com


Questions: Can the difference between chance and pattern be determined by the composition of species across islands in an archipelago? In particular, will one find ‘checkerboards’ – a pattern of mutual exclusivity that is the simplest pattern that might occur under competitive exclusion?

Organisms: 150 and 141 species of land birds inhabiting 41 and 142 islands of the Bismarck and the Solomon Archipelagos, respectively. (See http://evolutionary-ecology.com/data/2447_Supplement.pdf)

Analytical methods: For each pair of species within each archipelago, the observed number of co-occurrences is compared to the distribution of the number of co-occurrences derived from a collection of 106 representative unique random, or null, communities. Those species pairs actually co-occurring less often than they do in 5% of those nulls are ‘unusually negative’ pairs; those co-occurring more often than they do in 95% of those nulls are ‘unusually positive’ pairs. Islands are ranked from those with the smallest number of species to the largest. A species incidence is the span from the smallest to the largest number of species on islands on which it is found.

Results: In each archipelago, proportionately more congeneric species pairs than non-congeneric species pairs are unusually negative pairs. This holds even for species pairs that overlap in their incidences. Among congeneric species pairs found in both archipelagos, a pair that is unusual in one archipelago generally proves to be unusual in the other archipelago as well and to belong to a genus segregating ecologically by means of spatial niche differences.

Conclusions: Diamond (1975) suggested that island bird communities were structured by assembly rules that could be deduced by observation of which species did or did not co-occur on particular islands. Critics countered with analyses arguing that co-occurrence patterns in several ecological communities did not differ from random expectations. We conclude that the difference between chance and pattern can be unequivocally determined.

Keywords: competition, co-occurrence, incidence function, null model, species pairs.

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