Aim The aim of this study was to determine the number of successful
establishments of the invasive Argentine ant outside native range and to see
whether introduced supercolonies have resulted from single or multiple
introductions. We also compared the genetic diversity of native versus
introduced supercolonies to assess the size of the propagules (i.e. the number
of founding individuals) at the origin of the introduced supercolonies.
Location Global.
Methods We used mitochondrial DNA (mtDNA) markers and microsatellite loci
to study 39 supercolonies of the Argentine ant Linepithema humile covering both
the native (n = 25) and introduced range (n = 14).
Results Data from three mitochondrial genes and 13 nuclear microsatellites
suggest that the introduced supercolonies studied originated from at least seven
founding events out of the native area in Argentina (primary introductions). The
distribution of mtDNA haplotypes also suggests that supercolonies in the
introduced range each derive from a single source supercolony and that one of
these source supercolonies has been particularly successful, being the basis of
many introduced populations spread across the world. Comparison of the genetic
diversity of supercolonies based on the five most diverse loci also revealed that
native and introduced supercolonies have greatly overlapping ranges of diversity,
although the genetic diversity is on average less in introduced than in native
supercolonies.
Main conclusions Both primary introductions (from the native range) and
secondary introductions (from sites with established invasive supercolonies) were
important in the global expansion of the Argentine ant. In combination with the
similar social organization of colonies in the native and introduced range, this
indicates that invasiveness did not evolve recently as a unique and historically
contingent event (e.g. reduction of genetic diversity) in this species. Rather, native
L. humile supercolonies have characteristics that make them pre-adapted to
invade new – and in particular disturbed – habitats when given the opportunity.
These results have important implications with regard to possible strategies to be
used to control invasive ants.