Stage-based biotic resistance to a non-native species

Biotic resistance theory posits a negative relationship between diversity and invasibility. The mechanisms behind this can be variable, but studies are often focused on competition between many resident and introduced species. However, individual species may interact more strongly in communities, some of which may be effective predators and, thus, could offer stronger biotic resistance, broadening biotic resistance theory beyond diversity. In species with complex life histories, biotic resistance may be stage-specific, with resisting species affecting select life stages. The relationship between native eastern mosquitofish ( Gambusia holbrooki ) and the non-native tropical clawed frog ( Xenopus tropicalis ; TCF), provides a model system to investigate stage-specific biotic resistance. Preliminary observations suggest TCFs in their introduced range in Florida, USA are more likely to occupy ponds without fish and we hypothesise that mosquitofish provide predatory biotic resistance to early TCF life stages. We tested the hypothesis that the strength of predatory biotic resistance to species with complex life histories would be based on life stage. We also examined how the strength of predatory biotic resistance can be mitigated by predator density, cannibalism by adult life stages and habitat complexity. Hypotheses were tested using wild-caught and lab-reared individuals in controlled laboratory experiments. Early TCF life stages, lacking defensive traits, suffer near-total mortality due to predation and harassment, with vulnerability decreasing during and after metamorphosis. Mosquitofish density and habitat complexity had minimal effects, while the combination of adult frogs and mosquitofish resulted in facilitation between the two predators, increasing tadpole predation. This study underscores the need for thorough testing of the biotic resistance hypothesis across life stages and shows promise for the control of TCFs in Florida. Mosquitofish could be used as a biological control to reduce the recruitment of TCFs by eliminating the larval stage.