Investigating large‐scale invasion patterns using‐small scale invasion successions—phenotypic differentiation of the invasive round goby ( Neogobius melanostomus ) at invasion fronts

Species invasions are an ever‐growing problem that increases with globalization through increased frequency of unintentional introductions. Between establishment and spread, a lag phase often occurs in which population growth is exponential and dispersal frequency low. Individual variation in behavioral traits, consistent through time and context, have been found crucial for understanding ecological processes such as density dependent dispersal during species invasions. In a previous study of the invasive round goby ( Neogobius melanostomus ), we found population differentiation between old and new populations in the Baltic Sea with individuals in new populations being more asocial, bold, and active. Here, we investigate if behavioral differentiations are created already during the initial spread from newly established populations. Hence, we monitored population growth and subsequent small‐scale spread (< 800 m) in two newly invaded areas, as well as the behavioral traits previously connected to dispersal, over two successional seasons. We found phenotypic differentiation between dispersing and resident individuals with small‐scale dispersers being smaller and more asocial. In addition, our catch‐per‐unit‐effort data suggest a lag‐phase of 3–5 yr, following initial colonization, before the round goby start spreading into the surrounding environment. This suggests that, at least in species that grow to high densities fast, sociability is more important than boldness and activity for triggering density‐dependent dispersal.