Discussion
One of our main findings is that locally diverse ant assemblages resisted W. auropunctata in both Mexico and Puerto Rico. We identified several dominant species from the local ant assemblage in the native and introduced habitats and compared their competitive interactions with W. auropunctata. In particular, we examined resource and interference competition across short and long-term time scales. We found that in Mexico the only the ground-foraging ant S. geminata displayed superiority against W. auropunctata, while S. picea, P. synantropica, and P. protensa also lost against W. auropunctata at baits. Furthermore, the long-term competitive interactions in the laboratory showed that W. auropunctata was superior against S. picea. In Puerto Rico, another exotic ant S. invicta outcompeted W. auropunctata in areas where they overlapped. Laboratory experiments with the native ant L. iniquum showed that W. auropunctata was superior at baits across the short and long-term competition experiments.
Exotic ants can displace native ant species through interference and resource competition(Sakai 2001). While these traits have been implicated in the success of exotic ants, they don’t always explain why exotic ants excel at displacing native ant species (Holway 2002a). Within native communities, competitive trade-offs between behavioral dominance and resource discovery are thought to promote species coexistence (Fellers 1987, can violate the discovery-dominance trade-off in the introduced range, in the case of Argentine ants, resulting in a competitive advantage over native species (Holway 1998). While the breaking of the discovery-dominance tradeoff has been linked with the invasive success of Argentine ants, we lack such information on other major invasive ant species. Laboratory experiments involving W. auropunctata and native ant species, showed that W. auropunctata had the poorest foraging abilities in terms of resources discovery compared competitive trade-offs and found that W. auropunctata was less efficient in discovering and recruiting worker to baits in their introduced habitat of Puerto Rico relative to their native habitat in Mexico, suggesting that competitive trade-offs are not indicative of invasive success (Yitbarek et al. in press).
The organization of exotic species within local species assemblages in their native ranges allows us to understand why some exotic species thrive in their introduced ranges (Calcaterra 2016). Here we sought to understand how W. auropunctata ranks relatively to locally dominant native ant species. In Mexico, several native ant species from the local species assemblage were able to withstand W. auropunctata. The tropical ant S. geminata showed high levels of aggressiveness  towards W. auropunctata at baits. Although W. auropunctata  was initially able to dominate resources, S. geminata’s  high recruitment and interference ability led to the displacement of W. auropunctata. Although S. geminata is frequent victim to attacks by phorid flies which can interfere with its ability to rapidly discover resources(Brown 1991, phorid flies at our site which greatly enhanced S. geminata’s dominance at baits. In comparison, P. protensa is a slow moving ground forager ant with many small nests that are widely distributed. Although W. auropunctata was initially dominant at baits, P. protensa was able to overtake the baits for some time after which W. auropunctata regained its dominance again. These cyclical patterns are illustrative of the transient nature of competitive interactions over time.
The arboreal ant S. picea is a minute ant that primarily forages on coffee where its nests can be found under the bark on the trunk of the coffee tree. S. picea was initially more dominant but as W. auropunctata recruited its workers from the leaf litter and arboreal nests to the baits it started to regain its dominance. The ability to forage and nest on the ground and the trees enables W. auropunctata to hold a competitive advantage over native species. We confirmed this pattern in the long-term experiment were W. auropunctata interference ability enabled it to invade S. picea nests. The ground-foraging ant P. synanthropica nests in the ground but forages both in the leaf litter and in the coffee trees. P. synanthropica was competitively dominant over W. auropunctata as it quickly discovered baits and vociferously defended against W. auropunctata intruders. P. synanthropica’s dominance has also been observed in competition experiments against S. geminata and P. protensa (Perfecto 2013).
In Puerto Rico, the exotic ant S. invicta was superior to W. auropunctata at ground baits. S. invicta reached high population densities at baits which forced W. auropuncatata to being pushed out at baits. While S. invicta attained a numerical advantage over W. auropunctata, the extent of their colonies was limited to areas of the farm that received more sun, which allowed W. auropunctata to retain its dominance in other areas where S. invicta did not occur. Laboratory experiments showed an oscillatory trend between W. auropunctata and L. iniquum competition. We found that L. iniquum workers were faster at discovering resources but were less efficient at guarding baits against W. auropunctata workers. As W. auropunctata recruited more workers it slowly attained dominance. These patterns are reflected across the long-term experiment were we found higher numbers of live W. auropunctata workers. W. auropunctata displaced L. iniquum by raiding their nest and killing its workers and brood, in some cases moving its entire nest into the invaded species. Our long-term experiments show that nest invasions are a mechanism by which W. auropunctata displaces native ant species. Previous laboratory experiments showed that W. auropunctata displaced the native ant species Monomorium subupacum by invading its nest and consuming workers and brood (Vonshak 2011). Similar evidence for nest raiding of ant colonies has been found for the invasive ants L. humile and P. megacephela in their introduced ranges(Holway 2002,
Recent evolutionary studies have shown that W. auropunctata invasion displays a polymorphism in its reproductive system reproductive structure first occurred within the native range of W. auropunctata from low-density populations in natural habitats (i.e. sexual populations) to high-density workers (i.e. clonal populations) in anthropogenic habitats(Fournier 2005a, human modified habitats suggests that W. auropunctata’s invasive success is associated to its clonal reproductive structure(Hufbauer 2012). It remains unclear, however, to what extent clonality in the introduced range contributes to ecological dominance in W. auropunctata populations. Our study compared the invasion dynamics of W. auropunctata populations in agricultural ecosystems. W. auropunctata populations occurring in these anthropogenic sites are mostly likely clonal (Orivel 2009).  Within our most densely populated site in Mexico we found that W. auropunctata was competitively superior against several dominant ant species, contrary to Puerto Rico where the most dominant ant L. iniquum was able to withstand competitive pressure by W. auropunctata .
Although direct competitive interactions can structure local assemblages, indirect interactions can alter competitive outcomes(Hsieh 2012). Trait-mediated-indirect effects (TMII) have been found to play an important role in structuring ecological communities can be highly host specific, by using ant pheromones, which as a consequence reduce the number of ant foragers at resources (Feener Jr 2000). This can have important ramifications in ecosystems when phorids reduce competitive hierarchies (Hsieh 2012).  Several studies have reported that Pseudacteon phorids affect resource retrieval by S. geminata species didn’t observe phorids during direct encounters involving S. geminata and W. auropunctata but that may be heightened by biogeographical disparities reductions in foraging activities of Linipethema species have been recorded in the presence of phorid species(Orr 2003). Future studies should evaluate whether L. iniquum species get attacked by phorid flies in the field and how this indirect effect impacts competitive interactions with W. auropunctata in Puerto Rico.
A central question in invasion biology is to what extent exotic species are limited by their competitors in their native ranges and how release from interspecific competition can explain their invasive success in the introduced range. We found that W. auropunctata was able to outcompete dominant ants in Mexico, but that it was restricted by native competitors in its introduced range of Puerto Rico. These findings suggest that release from interspecific competition does not explain the invasive success of W. auropunctata in novel environments. An alternative explanation for W. auropunctata’s invasive success is the escape from natural enemies in the introduced range (Torchin 2003). Recent work suggests that a purge from endosymbiotic bacteria might have contributed to W. auropunctata’s invasion success because of lower parasite pressure in the introduced range (Rey 2013). The loss of parasites during invasion has been reported for several exotic ant species (Reuter 2004, endosymbiotic bacteria within W. auropunctata populations between Puerto and Mexico. In particular, experiments that seek to determine how changes in W. auropunctata’s microbiota influence interspecific interactions with native species.

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References

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