Tropical range extending herbivorous fishes gain foraging benefits by shoaling with native temperate species

Tropicalization creates novel species assemblages in temperate ecosystems as range-extending species move. The sociality of range-extending species could facilitate their establishment, particularly if they associate with natives. The eastern Mediterranean Sea has witnessed widespread collapses of vegetated ecosystems since the arrival of the tropical rabbitfishes Siganus rivulatus and Siganus luridus. We explored whether mixed-species shoaling influenced the fish foraging activity of the novel herbivore assemblage. We recorded 250 shoals, 30% composed exclusively of native species (i.e., Sarpa salpa and Sparisoma cretense), 43% of rabbitfish, and 27% of species from both origins. S. rivulatus was the most gregarious species, establishing positive shoaling associations with S. luridus and S. salpa, whereas S. cretense was the least sociable. Foraging differed between native and range-extending species. Compared to native species in mixed-species shoals, range-extending species increased their bite rates with shoal size, both in mono- and multi-specific groups, indicating that they boosted their foraging efficiency (i.e., increased bites per minute) when foraging in large groups, regardless of which species they shoaled with. Native species, in contrast, only increased their bite rates with shoal size while foraging in mono-specific groups. Thus, our study shows that, unlike natives, range-extending species may gain foraging benefits from facilitative associations in mixed-species shoals. This gregarious behaviour could help explain the disproportionate herbivory pressures range-extending species impose on tropicalized ecosystems.