Co‐invasive fellows are not always friendly: synergistic effects of invasive predators and competitors on prior invader

Biological invasions lead to substantial changes in global biodiversity. Non‐native species invade local ecosystems, interacting also with already established invaders. The presence of invasive species in ecosystem may facilitate secondary invasions by positive interactions. Alternatively, the success of invasion may be diminished by negative relationships among newly and previously introduced/established non‐native species. However, few studies have determined joint effect of co‐invasive competitors and predators on previous invaders. Our study aimed to estimate potential synergistic effects of the fish predator, Babka gymnotrachelus , and stronger interspecific competitor, Dikerogammarus villosus , on population dynamics of the prior invader, D. haemobaphes , all of Ponto‐Caspian origin and invading many major European river basins. We hypothesized that abundance and distribution of prior invader would be negatively affected by presence of D. villosus and fish, either alone or simultaneously. Using mesocosm and laboratory experiments, we showed that the secondary co‐invaders affected distribution and abundance of D. haemobaphes , depending on ontogenetic stage of the latter. Abundance of juvenile D. haemobaphes was reduced in presence of D. villosus , while presence of predator had no effect. Reversely, adult D. haemobaphes avoided the predators, but not competitors. In the simultaneous presence of both co‐invaders, reactions of juvenile D. haemobaphes have reinforced: they were no longer able to hide against D. villosus and suffered greater abundance decline, pointing to synergistic negative effect of the competitor and predator. We provide experimental evidence demonstrating how negative interactions among invasive species may shape community structure. Moreover, as nowadays dispersal of species is driven by climate change, our results may be extrapolated to native communities, undergoing composition changes due to phenomena other than biological invasions.