Can separation along the temperature niche axis promote coexistence between native and invasive species?

Aim Western mosquitofish (Gambusia affinis) have been linked with the decline of native fish and amphibians throughout the world. Separation along the temperature niche axis may promote the long‐term coexistence of introduced western mosquitofish, with native species in temperate regions. Recent research has shown that western mosquitofish can reduce the recruitment of native least chub (Iothichthys phlegethontis) endemic to the Bonneville Basin. We tested the hypotheses that cold temperatures (≤ 15 °C in the summer, freezing winters) would: (1) reduce the aggressive and predatory effects of western mosquitofish on least chub, and (2) eliminate the overwinter survival and recruitment of western mosquitofish while having little effect on least chub recruitment. Location Bonneville Basin of Utah, USA. Methods We used short‐term tests in the laboratory at the level of individuals and manipulated temperature (warm, cold and seasonal treatments) in long‐term experiments using mesocosms at the population level. Results Cold temperatures (≤ 15 °C) reduced the aggression and predation of western mosquitofish on least chub at the level of individuals. At the population level, however, cool summers (≤ 15 °C) eliminated recruitment in both species because they required warm summers (c. 20–30 °C) to survive freezing winters. Although least chub had an overwinter advantage in survival (75% least chub, 45% western mosquitofish), it was overwhelmed by the rapid reproduction of western mosquitofish as temperatures increased in the summer. Main conclusions Studies at the level of populations are necessary to understand the ultimate effects of introduced species on native taxa. Separation along the temperature niche axis was not sufficient to promote coexistence between these species in habitats with warm summers (c. 30 °C). Although coexistence may be possible in habitats with cool summers (≤ 20 °C) and freezing winters, the ability of niche separation to promote long‐term coexistence between native and introduced species may ultimately depend on their respective rates of evolution. Long‐term coexistence may not be possible if introduced species can adapt to new environmental conditions faster than native species can evolve mechanisms to reduce their harmful effects.