The establishment of fishes at their expanding cold edge:
Predictive traits and niche shifts

The establishment of species in previously uncolonized regions may cause variable impacts on recipient ecosystems. Identifying which species are of high invasiveness potential and understanding which habitats they will colonize at the expanding edge are ecological challenges. Species traits, and especially trait breadth, may help distinguish between species of low and high invasiveness. Nevertheless, trait data are usually only present at coarse species-level resolution which precludes fine-scale assessments of invasiveness and niche occupancy patterns. Here, we use the Lessepsian migration case study in which Red Sea species enter the Mediterranean Sea via the Suez Canal.
We collected individual-level trait and habitat use information for marine fish species at their native range and their recently expanding range using 247 samples of stereo-baited remote underwater video surveys. Then, we compare four different assemblages: Red Sea natives that are not Lessepsians, Red Sea Lessepsians, Lessepsian species within the Mediterranean Sea, and Mediterranean Sea natives.
Our comparisons revealed that Lessepsian species at the Red Sea have a larger depth range, greater occupancy, wider habitat breadth, and higher affinity to unconsolidated substrates compared with Red Sea species that are not Lessepsians. We further found that Lessepsian species following the expansion to the Mediterranean Sea, have maintained their depth range, yet shifted towards shallower depths, and expanded their thermal range. Lessepsian species that expanded to the Mediterranean Sea had greater affinity with consolidated seabeds compared with their native range.
Taken together, our results suggest that larger depth range and higher occupancy at the native range may indicate higher invasiveness. We also reinforce previous findings suggesting that Lessepsian species are preadapted to entering the Mediterranean Sea by having higher affinities with sandy habitats. Yet, for some species, this preference may change after the settlement in the Mediterranean Sea. Thus, while we find evidence that invasiveness is predictable, niche shifts may be common hence increasing our uncertainty in predicting how species may colonize habitats at their expanding edge.