Tropical fishes benefit more from novel than familiar species interactions at their cold-range edges

Animals that extend their biogeographic ranges in response to global warming are often forced to interact with local species for limited resources. Whether these novel species interactions facilitate or inhibit range extension success in novel environments remains largely unknown. Here, we evaluate in the wild how range-extending coral reef fishes modify their behaviours (shoaling dynamics, foraging, and anti-predator) at their cold-water ranges to overcome biotic resistance by temperate species and adjust to novel species interactions resulting from climate change. We study range-extending tropical as well as their co-shoaling temperate fishes across a 2,000-km latitudinal gradient from tropical to temperate rocky reefs in a global warming hotspot. In their novel temperate ranges, tropical fishes showed increased anti-predator behaviours (increased shoal cohesion and shoal alignment, increased flight initiation distance, and increased lateralisation) and decreased bite rates compared to their native tropical and subtropical ranges. However, tropical fish in novel mixed-species shoals had higher bite rates compared to tropical-only shoals. In contrast, temperate fish behaviours and shoaling dynamics were unaffected by novel shoaling interactions across their subtropical and temperate ranges studied. Our findings reveal that tropical fish trade-off foraging efficiency for predator vigilance in their new and unfamiliar warm- and cold-temperate ranges, independent of novel shoaling interactions. However, novel mixed-species shoaling can increase the foraging efficiency of tropical fish, which may be a mechanism (phenotypic plasticity) that allows them to perform better at their novel cold-temperate range edges. Since these novel species interactions can enhance species fitness of tropical fishes, we conclude that behavioural interactions between invading and local species can accelerate successful range-extensions of some species  into novel climatic environments.