Competitive assembly of South Pacific invasive ant communities

First, traditional null model tests showed support for negative species co-occurrence patterns among plots within islands, but not among islands. A plausible explanation for this result is that analyses at larger spatial scales merge heterogeneous habitats that have considerable effects on species occurrences. Second, analyses of ant abundances showed that samples with high ant abundances had fewer species than expected by chance, both within and among islands. One ant species, the invasive yellow crazy ant Anoplolepis gracilipes, appeared to have a particularly strong effect on community structure correlated with its abundance. Third, abundances of most ant species were inversely correlated with the abundances of all other ants at both spatial scales. This result is consistent with competition theory, which predicts species distributions are affected by diffuse competition with suites of co-occurring species.Our results support a pluralistic explanation for ant species abundances and assembly. Both stochastic and deterministic processes interact to determine ant community assembly, though abundance patterns clearly drive the deterministic patterns in this community. These deterministic patterns were observed at two spatial scales. Results indicate that abundance-based null models may be more sensitive in detecting non-random patterns in community assembly than species co-occurrences analyses.Opinions on the processes governing community assembly are polarized [1-3]. One school of thought maintains that ecological communities are assembled deterministically according to 'assembly rules', generated by biotic interactions [4-6]. An opposing view asserts that communities are assembled stochastically by chance dispersal of species with life history characteristics suited to local environmental conditions [7-9]. The relative importance of chance and determinism in structuring ecological communities has been fiercely debated for nearly a century and we are far from a reso