The effect of density on aggression between a highly invasive and native fish

Population densities of invasive species fluctuate spatially and temporally, suggesting that the intensity of their aggressive interactions with native species is similarly variable. Although inter‐specific aggression is often thought to increase with population density, it is often theorized that it should be exceeded by intra‐specific aggression since conspecifics share a greater degree of resource overlap. Yet, the magnitude of intra‐specific aggression is seldom considered when examining aggressive interactions, particularly those between invasive and native species. Here, we manipulated the density of the invasive eastern mosquitofish, Gambusia holbrooki , and observed its aggressive interactions with juveniles of the native Australian bass, Macquaria novemaculeata in a laboratory setting. For both species, the magnitudes of intra‐ and inter‐specific aggression were recorded. Regardless of density, the native M. novemaculeata was more aggressive towards heterospecifics than G. holbrooki was. In addition to this, M. novemaculeata was more aggressive to G. holbrooki than towards conspecifics, at both low‐ and high‐density conditions. In contrast, G. holbrooki was similarly aggressive towards M. novemaculeata and G. holbrooki at a high density, yet at low density, displayed significantly more aggression towards conspecifics than M. novemaculeata . These findings demonstrate the importance of considering intra‐specific aggression when exploring behavioural interactions between native and invasive species.