Parasites play an important role in invasion success with important consequences for biodiversity and community structure. While much research has focused on direct effects of parasites on biological invasions, parasites can also indirectly influence interactions within the invaded community across trophic levels. For instance, parasites can mediate competitive interactions between native and exotic species through trait-mediated indirect effects. We consider the interactions between the parasitoid fly Pseudacteon sp. (Diptera: Phoridae), and its native host ant Linipethema iniquum, and the exotic ant Wasmannia auropunctata in the introduced range of Puerto Rico. We examined the effects of phorid flies on the competitive outcome between the arboreal ants W. auropunctata and native ant L. iniquum. Furthermore, we investigate the searching efficiency of phorid flies in detecting L. iniquum nests. To study the indirect effects on ant competition, we monitored ant recruitment to baits over a 60-min time interval in the presence and absence of phorid fly parasitoids. We then performed field experiments and measured phorid arrival time to arboreal nests of L. iniquum located in both a) W. auropunctata patches and in b) isolated patches dominated by L. iniquum nests. We found that the presence of phorid fly significantly reduced recruitment of L. iniquum workers to baits through induced behavioral changes thereby increasing the ability of W. auropunctata to acquire resources. In addition, we found that phorid arrival time in isolated patches of L. iniquum patches was faster as compared to L. iniquum nests located within W. auropunctata patches. Our results show that phorid fly parasitoids indirectly may influence competitive interactions by attacking the host-ant L. iniquum and consequently providing an advantage to local spread of W. auropuntata populations in Puerto Rico. However, the spatial dynamics of arboreal ants shows that L. iniquum seeks protection from phorid fly parasotoids by moving their nests to W. auropunctata dominated patches..
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