Abstract

Numerous organisms have been relocated by human agency among remote regions of the world. One of the consequences of alien invasions (and associated native extinctions) is that the taxonomic similarity between areas trends to increase, a process called biotic homogenization. This process is studied in plants using naturalized species, while the role of non-established (but nevertheless present) plants is not investigated. Here we evaluate if adding the non-established component to the pool of alien plants modifies similarity patterns recorded in a preceding study of biotic homogenization in six Oceanic Islands of the South-eastern Pacific. Although our analyses confirm previous findings of floristic homogenization for these Islands, the effect of adding the non-established component results in several changes. By consideration of only naturalized plants we detected two cases of increased similarity (i.e. floristic homogenization), while the other 13 comparisons yielded non-significant changes. By adding non-established plants, four pairs of Islands show increased similarity, while in one case there is decreased similarity (i.e. floristic differentiation). These results support the hypothesis that non-established species can modify qualitative and quantitative trends of floristic change, revealing the complexity of biotic change. Although discrimination between only native and naturalized species appears reasonable for some groups, among plants it becomes a critical decision because of the importance of non-established species as ecological players and as purveyors of early information on future biotic change. © 2008 Springer Science+Business Media B.V.