RESUMO
Biological invasions have significant ecological, evolutionary and economic consequences. Ants are exemplary invaders and their invasion success is frequently attributed to a shift in social structure between native and introduced populations. Here, we use a multidisciplinary approach to determine the social structure, origin and expansion of the invasive Argentine ant, Linepithema humile, in Australia by linking behavioural and genetic studies with indicators of dispersal pathways and propagule pressure. Behavioural assays revealed a complete absence of aggression within and between three cities - Melbourne, Adelaide and Perth - spanning 2700 km across Australia. Microsatellite analyses showed intracity genetic homogeneity and limited but significant intercity genetic differentiation. Exceptions were two Perth nests that likely represent independent translocations from Adelaide. These patterns suggest efficient local gene flow with more limited jump dispersal via transport corridors between cities. Microsatellite analyses of L. humile from potential source regions, combined with data from port interceptions, trade pathways and the timeline of spread within Australia, implicate the main European supercolony as the source of L. humile in Melbourne. Such an introduction probably then redistributed across Australia and spread to New Zealand to form an expansive Australasian supercolony.
RESUMO
The indirect effects of biological invasions on native communities are poorly understood. Disruption of native ant communities following invasion by the Argentine ant (Linepithema humile) is widely reported to lead indirectly to the near complete collapse of seed dispersal services. In coastal scrub in southeastern Australia, we examined seed dispersal and handling of two native and two invasive alien plant species at Argentine ant-invaded or -uninvaded sites. The Argentine ant virtually eliminates the native keystone disperser Rhytidoponera victoriae, but seed dispersal did not collapse following invasion. Indeed, Argentine ants directly accounted for 92% of all ant-seed interactions and sustained overall seed dispersal rates. Nevertheless, dispersal quantity and quality among seed species differed between Argentine ant-invaded and -uninvaded sites. Argentine ants removed significantly fewer native Acacia retinodes seeds, but significantly more small seeds of invasive Polygala myrtifolia than did native ants at uninvaded sites. They also handled significantly more large seeds of A. sophorae, but rarely moved them >5 cm, instead recruiting en masse, consuming elaiosomes piecemeal and burying seeds in situ. In contrast, Argentine ants transported and interred P. myrtifolia seeds in their shallow nests. Experiments with artificial diaspores that varied in diaspore and elaiosome masses, but kept seed morphology and elaiosome quality constant, showed that removal by L. humile depended on the interaction of seed size and percentage elaiosome reward. Small diaspores were frequently taken, independent of high or low elaiosome reward, but large artificial diaspores with high reward instead elicited mass recruitment by Argentine ants and were rarely moved. Thus, Argentine ants appear to favour some diaspore types and reject others based largely on diaspore size and percentage reward. Such variability in response indirectly reduces native seed dispersal and can directly facilitate the spread of an invasive alien shrub.