Invasive species represent an increasing fraction of aquatic biota. However, studies on the role and consequences of facilitative interactions among aliens remain scarce. Here, we investigated whether the spread of the alien water boatman Trichocorixa verticalis verticalis in the Iberian Peninsula is related to reduced mortality from predation compared with native Corixidae, especially since Trichocorixa co-occurs with the invasive fishes Gambusia holbrooki and Fundulus heteroclitus. All three invaders have a common native range in North America and are widespread in and around Doñana in SW Spain. Using laboratory experiments, we compared the predation rates by the two exotic fish and native Odonata larvae on Trichocorixa and the native Sigara lateralis. We found no evidence to suggest that Trichocorixa suffers lower predation rates. However, when both corixids were mixed together, predation of Trichocorixa by Odonata larvae was higher. Odonata larvae were size-limited predators and the proportion of corixids ingested was positively correlated with mask length. Since Trichocorixa is smaller than its native competitors, this may explain their higher susceptibility to predation by Odonata. This may be one of various factors explaining why Trichocorixa is particularly dominant in saline habitats where Odonata are rare, while it is still scarce in fresh waters. © 2014 Springer International Publishing Switzerland.

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