Characterizing the impacts of exotic species on the morphology of solitary threespine stickleback ( Gasterosteus aculeatus ) populations in southwestern British Columbia

Exotic species are one of the greatest threats to native species, communities, and ecosystems. Introductions of multiple exotic species into an environment may have different effects on native populations compared to when exotic species are introduced individually. Threespine stickleback species pairs ( Gasterosteus aculeatus ) and neighbouring solitary populations in southwestern British Columbia, a textbook example of an adaptive radiation, are now under threat from multiple exotic species. We assessed whether variation in morphological characters and body shape among solitary threespine stickleback populations was associated with different combinations of introduced smallmouth bass ( Micropterus dolomieu ) and signal crayfish ( Pacifastacus leniusculus ). We also examined morphological changes over time spans of 18-43 years to determine whether contemporary characteristics have responded to the presence of exotic species. We found clear differences in stickleback traits and body shape among exotic species combinations. Stickleback coexisting with bass and crayfish were highly armoured, whereas bass-only lakes contained stickleback with reduced armour. Two stickleback populations that coexist with signal crayfish showed significant increases in size over time. These patterns suggest that smallmouth bass and signal crayfish may have significant and different impacts on stickleback morphology.