An experimental test of intra- and inter-specific competition between invasive western mosquitofish (Gambusia affinis) and native plains topminnow (Fundulus sciadicus)

Invasive species are a major threat to freshwater conservation. The coexistence of species in invaded habitats depends on the relative strength of intra- versus inter-specific competition, where inter-specific competition from invasive to native species is often stronger than intra-specific competition, jeopardising their coexistence. In this study, we conducted a laboratory experiment to test for the relative strength of interference competition between native plains topminnow (Fundulus sciadicus) and invasive western mosquitofish (Gambusia affinis) at three experimental temperatures. Intra- and inter-specific competition was quantified using an isodar approach, which assumes that animals are ideally distributed to maximise their fitness. Thus, their distributions measure the quality and quantity of habitat patches. This was supplemented by behavioural observations of intra- and inter-specific competition. Contrary to our predictions, we did not find evidence that competition was asymmetrical from the invasive mosquitofish to the native plains topminnow. Instead, more individuals occupied their shared preferred habitat (a slow-moving pool) in sympatry compared to allopatry, and the isodar analysis demonstrated that intra-specific interference competition was significantly stronger than inter-specific competition at all temperature levels. Behavioural observations corroborated this analysis of habitat selection that aggression was most frequent among plains topminnow in sympatry. This study shows that the widely perceived aggression of adults might not be the only key mechanism of global invasion success by mosquitofish. Other ecological traits, such as rapid reproduction, environmental tolerance, and interactions with early life stages of native species, might also be responsible for their invasion success. Additional investigations are warranted to determine whether their invasions directly affect native species or they invade degraded ecosystems opportunistically.