Condition‐specific competition between invasive Argentine ants and Australian Iridomyrmex

Abstract

Summary Biotic interactions and abiotic suitability are often emphasized as factors influencing invasion success. Here we examine how these factors might interact to affect the competitive performance of invasive Argentine ants (Linepithema humile) in Western Australia. In standardized bait surveys Argentine ants controlled twice as many baits in the morning (ground temperatures: 18–25 °C) compared to the number they controlled in the afternoon (ground temperatures: 30–40 °C). Moreover, Argentine ants displaced native ants from baits only in the morning and were, in turn, displaced by native ants only in the afternoon. We used a factorial field experiment to test the relative importance of interspecific competition (±) and ground temperature variation [warm (25 °C)/hot (33 °C)] as determinants of competitive performance among established colonies of Argentine ants and two species of native Iridomyrmex (I. bicknelli and I. rufoniger suchieri). We observed a condition‐specific competitive asymmetry between I. bicknelli and L. humile. In the absence of each other as competitors, these two species responded to ground temperature variation in an opposite manner. I. bicknelli foraged more actively when ground temperatures were hot, whereas L. humile foraged more actively when ground temperatures were warm. In competition with I. bicknelli, Argentine ants monopolized two‐thirds of all baits but controlled more than twice the number of baits under warm conditions than under hot conditions. Iridomyrmex bicknelli dominated 22% of all baits but did so primarily when conditions were hot. Linepithema humile and I. r. suchieri responded to abiotic variation in a qualitatively similar manner. Both species foraged less actively when ground temperatures were hot than when ground temperatures were warm, but L. humile showed stronger declines when ground temperatures were hot than did I. r. suchieri. In competition with I. r. suchieri, Argentine ants again monopolized two‐thirds of all baits. Our findings suggest that physical conditions unsuitable for L. humile may combine with interspecific competition from native ants that are more tolerant to hot, dry conditions to reduce the extent to which natural areas in Australia become invaded.