Assessing the role of competitive processes in mediating the range expansions of tropical vagrant fish

Abstract Climate change is promoting fast poleward expansion in many tropical marine fish species. With projected increases in sea surface temperatures, there is significant scope for increased range expansions in the future. However, the strength and consequences of density‐driven competitive interactions between vagrant and resident fishes are largely unknown. Here, we investigate how the density effects resulting from the addition of tropical fishes to temperate assemblages may alter patterns of habitat and behaviour usage by both tropical vagrant and temperate resident species. We explore these impacts for two ecologically similar (diet, habitat and behaviour) pairs of vagrant and resident species (the territorial species Stegastes gascoynei and Parma microlepis, and the shoaling species Abudefduf vaigiensis and Atypichthys strigatus, respectively ) . We compare situations where the other member of each of these ecological pairs was present or absent. There was a shift in the type and structural complexity of habitat occupied, with all species displaying some characteristics of occupying seemingly less‐preferred habitats in the presence of their pair species than without them. Behavioural effects were only apparent for the territorial species, which had altered feeding and vigilance patterns in the presence of their pair. These data indicate that the increased density of ecologically similar fish which results from the annual arrival of vagrant fishes is likely driving shifts away from optimal habitat usage and alterations in activity of both residents and vagrants, suggesting resource competition. The degree to which this competition was evident depended on the life history of the species, being stronger for the more territorial species. While similar research on additional species would be required to determine to what degree these results could be generalised to other territorial or shoaling species, this work suggests to highlight the important role that competition will play in determining both the ability of the vagrant species to become established within temperate communities, as well as the impact they may have once they have done so.