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

1. Animals extending their biogeographic ranges poleward under global warming often interact with local species for limited resources such as food and shelter. Whether such novel species interactions facilitate or inhibit range extensions remains largely unknown.

2. We evaluated how range-extending tropical and co-shoaling temperate fishes modify their behaviours (aggression, foraging, and anti-predator) along a 2,000-km latitudinal gradient encapsulating tropical, subtropical and temperate reefs in a global ocean warming hotspot.

3. All five tropical fish species showed increased anti-predator behaviours and decreased bite rate at their novel temperate range compared to their native tropical and subtropical ranges. However, when shoaling with temperate fishes, three out of five tropical fish species had higher bite rates and all five tropical fish species spent less time sheltering compared to tropical-only shoals, respectively, irrespective of range zone.

4. In their subtropical ranges, tropical fish were more aggressive towards co-shoaling temperate fish compared to their poleward novel warm-ranges. This increased tropical fish aggression resulted in increased fleeing responses by the two temperate fishes at their subtropical warm trailing edges compared to their warm- and cold-temperate core ranges.

5. Our findings suggest that tropical fish species trade-off foraging efficiency for anti-predator behaviour in their novel warm- and cold-temperate ranges, independent of shoaling interactions.

6. However, shoaling with temperate species can increase the foraging efficiency of tropical fishes, which may be a mechanism (phenotypic plasticity) that enhances their performance at their leading temperate range edges.

7. Since novel species interactions enhanced the behavioural performance of some tropical fishes, we conclude that behavioural interactions between range-extending and local species can facilitate successful range-extensions of some species into novel environments.