Ivan Nagelkerken

Ecological similarity plays an important role in biotic interactions. Increased body size similarity of competing species, for example, increases the strength of their biotic interactions. Body sizes of many exothermic species are forecast to be altered under global warming, mediating shifts in existing trophic interactions among species, in particular for species with different thermal niches. Temperate rocky reefs along the southeast coast of Australia are located in a climate warming hotspot and now house a mixture of temperate native fish species and poleward range‐extending tropical fishes (vagrants), creating novel species assemblages. Here, we studied the relationship between body size similarity and trophic overlap between individual temperate native and tropical vagrant fishes. Dietary niche overlap between vagrant and native fish species increased as their body sizes converged, based on both stomach content composition (short‐term diet), stable isotope analyses (integrated long‐term diet) and similarity in consumed prey sizes. We conclude that the warming‐induced faster growth rates of tropical range‐extending fish species at their cool water ranges will continue to converge their body size towards and strengthen their degree of trophic interactions and dietary overlap with co‐occurring native temperate species under increasing ocean warming. The strengthening of these novel competitive interactions is likely to drive changes to temperate food web structures and reshuffle existing species community structures.