The cultivation and dissemination of alien ornamental plants increases their potential
to invade. More specifically, species with bird-dispersed seeds can potentially infiltrate
natural nucleation processes in savannas.
To test (i) whether invasion depends on facilitation by host trees, (ii) whether
propagule pressure determines invasion probability, and (iii) whether alien host plants
are better facilitators of alien fleshy-fruited species than indigenous species, we mapped
the distribution of alien fleshy-fruited species planted inside a military base, and
compared this with the distribution of alien and native fleshy-fruited species established
in the surrounding natural vegetation.
Abundance and diversity of fleshy-fruited plant species was much greater beneath
tree canopies than in open grassland and, although some native fleshy-fruited plants
were found both beneath host trees and in the open, alien fleshy-fruited plants were
found only beneath trees.
Abundance of fleshy-fruited alien species in the natural savanna was positively
correlated with the number of individuals of those species planted in the grounds of the
military base, while the species richness of alien fleshy-fruited taxa decreased with
distance from the military base, supporting the notion that propagule pressure is a
fundamental driver of invasions.
There were more fleshy-fruited species beneath native
sp. trees of the equivalent size. Although there were significant differences
in native plant assemblages beneath these hosts, the proportion of alien to native fleshyfruited
species did not differ with host.
Synthesis. Birds facilitate invasion of a semi-arid African savanna by alien fleshyfruited
plants, and this process does not require disturbance. Instead, propagule
pressure and a few simple biological observations define the probability that a plant will
invade, with alien species planted in gardens being a major source of propagules. Some
invading species have the potential to transform this savanna by overtopping native
trees, leading to ecosystem-level impacts. Likewise, the invasion of the open savanna by alien
host trees (such as
sp.) may change the diversity, abundance and species composition
of the fleshy-fruited understorey. These results illustrate the complex interplay between
propagule pressure, facilitation, and a range of other factors in biological invasions.||en