Body size mediates trophic interaction strength of novel fish assemblages under climate change

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.