Invasion of pollination networks on oceanic islands: importance of invader complexes and endemic super generalists

The structure of pollination networks is described for two oceanic islands, the Azorean Flores and the Mauritian Ile aux Aigrettes. At each island site, all interactions between endemic, non-endemic native and introduced plants and pollinators were mapped. Linkage level, i.e. number of species interactions per species, was significantly higher for endemic species than for non-endemic native and introduced species. Linkage levels of the two latter categories were similar. Nine types of interaction may be recognized among endemic, non-endemic native and introduced plants and pollinators. Similar types had similar frequencies in the two networks. Specifically, we looked for the presence of 'invader complexes' of mutualists, defined as groups of introduced species interacting more with each other than expected by chance and thus facilitating each other's establishment. On both islands, observed frequencies of interactions between native (endemic and non-endemic) and introduced pollinators and plants differed from random. Introduced pollinators and plants interacted less than expected by chance. Thus, the data did not support the existence of invader complexes. Instead, our study suggested that endemic super-generalist species, i.e. pollinators or plant species with a very wide pollination niche, include new invaders in their set of food plants or pollinators and thereby improve establishment success of the invaders. Reviewing other studies, super generalists seem to be a widespread island phenomenon, i.e. island pollination networks include one or a few species with a very high generalization level compared to co-occurring species. Low density of island species may lead to low interspecific competition, high abundance and ultimately wide niches and super generalization.