The alien flora of Brazilian Caatinga: deliberate introductions expand the contingent of potential invaders.
Alien floras have been examined at regional and continental scales, but the connections between the cause of introduction and the nature and invasiveness of alien floras remain poorly explored. This is despite the fact that initial introduction determines the alien species pool from which the invasion proceeds. Here we examine the profile of the alien flora of the Brazilian Caatinga, a semi-arid tropical vegetation, in order to define the major connections between economic activities, introduction pressure and nature of the alien flora in terms of taxonomy, native ranges and economic use. Alien plant species introduced in the Caatinga ecosystem were compiled from the literature as well as from herbaria surveys. A total of 205 alien plants species were recorded, distributed across 135 genera and 48 families. The alien flora of the Caatinga ecosystem is clearly explained in terms of taxonomy, native ranges and use by human populations. The highest numbers of alien species belong to the families Poaceae (61 spp.) and Fabaceae (33). Nearly one-third of the alien flora is represented by tropical forage plants, particularly grasses from Africa and America, which were introduced deliberately, particularly as forage for grazing livestock. Finally, 20 alien species were considered invasive, nine of them deliberately introduced, such as those species serving as forage for livestock. The Caatinga "case" calls attention to the socio-ecological drivers of alien floras and to which extent particular regions or biotas are susceptible to experiencing further biological invasion due to deliberate introductions.