Binary matrices and checkerboard distributions of birds in the Bismarck Archipelago

Aim We examine a presenceabsence matrix of the avifauna of the Bismarck Archipelago, for which the concept of competitively driven community assembly rules was formulated, to determine whether data support widespread competitive determination of geographical distributions. Location Bismarck Archipelago. Methods We obtained occurrences of 154 land and freshwater bird species on 31 islands. We calculated the observed number of checkerboards for all species pairs, for congeneric species pairs and for pairs of species within guilds, and employed randomization techniques to detect unusual co-occurrence patterns. Results Compared with random expectations, there are excesses of checkerboard pairs within both genera and defined guilds, but a detailed examination shows that competition is a cogent possible explanation in few instances. For many checkerboard pairs, species are not widely interspersed but are regionally allopatric, which probably reflects historical biogeography and dispersal limitation. Most congeneric and intraguild checkerboards include a species classified as a supertramp; when supertramps are omitted, there are 11 congeneric checkerboards and four intraguild but heterogeneric checkerboards. Main conclusions In isolation, presence-absence matrices provide limited insight into the role of competition in structuring bird communities of the Bismarcks. A major problem is disentangling historical geography and colonization history of the archipelago from the present-day ecology of the species. Examination of observed checkerboards from a geographically explicit perspective and with knowledge of colonization routes suggests that many checkerboards are likely to result, at least in part, from historical biogeography and supertramps. Although species may be forced into supertramp status by competition, other factors (e.g. habitat preference) may be causal, and biogeographical distributions alone cannot distinguish between causes.