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 amongst species, in particular for species with different thermal niches. Temperate rocky reefs along the southeast coast of Australia are climate hotspots 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 isotopes 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. The strengthening of these novel competitive interactions are likely to drive changes to temperate food web structures and reshuffle existing species community structures.

We used a small hand net and a mixture of ethanol and clove oil to capture juveniles of six native temperate species (Ambassis jacksoniensis, Atypichthys strigatus, Microcanthus strigatus, Parma microlepis, Pempheris affinis, Trachinops taeniatus) and of three tropical vagrant species (Abudefduf sexfasciatus, A. vaigiensis, Pomacentrus wardi) along the southeast coast of Australia between latitudes 30.8º and 36.8ºS.

For stomach content analysis, the entire gut was removed from each frozen fish, and the abundance of each prey item (excluding vegetation) inside the stomach was visually counted, their maximum lengths measured, and identified at family level using a stereo microscope. Autotroph prey items including algae and seagrass were grouped together, as the partly digested vegetation could not be identified, and their contribution to the diet was estimated as a count. Then, we calculated Bray-Curtis similarity index using on stomach contents.

For stable isotope analysis, white muscle tissue free from scales, skin and bone was extracted from each fish, and freeze-dried for at least 36 hrs in individual Eppendorf tubes. Dried tissue was ground using a ball mill. Powdered muscle tissues of each fish were weighed in tin capsules individually and analysed for stable δ ¹⁵N and δ¹³C isotope, using a Nu Instruments NuHorizon Continuous Flow IRMS (CF-IRMS) at the University of Adelaide. We then calculated Euclidean similarity index based using stable isotope signatures.

For prey size similarity, we calculated proportional similarity in median prey size per fish stomach.