LAUSR

ABSTRACT Aquatic ecosystems are often negatively affected by invasive species. However, biotic resistance by native species, either by competition or predation, can reduce the impacts of invasions by non‐native species. The Western Mosquitofish (Gambusia affinis) is one of the most impactful invasive species of freshwater fish and cause declines in native fish populations. Using two mesocosm experiments conducted in different years, we examined the ecological interactions between juveniles of the native fish, Bluegill Sunfish (Lepomis macrochirus), and adults of the invasive fish, G. affinis. We found evidence for interactions between L. macrochirus and G. affinis. However, interactions did not appear symmetric, with L. macrochirus generally more affected by intraspecific interactions than interspecific interactions whereas G. affinis was more affected by interspecific interactions than intraspecific interactions. The presence of either species of fish led to a decrease in the number of large zooplankton and a tendency for a decrease in the total number of zooplankton. Based on these results, native L. macrochirus appear to be able to reduce the ability of non‐native G. affinis to establish or maintain populations through both competition and predation (i.e., acting as an intraguild predator). The consistency of our results across both experiments, with their different designs and their occurring in different years, gives weight to these conclusions. The reduction of or prevention of establishment of populations of invasive G. affinis would likely benefit the aquatic communities of ponds with fish, especially small‐bodied native fish.