Alien vs. predator: influence of environmental variability and predation on the survival of ascidian recruits of a native and alien species

Abstract

Drivers of successful introduction of exotic species remain a major headline in marine invasion biology. We ran two experiments aiming to disentangle the effects of abiotic factors like contaminants and the effect of predation on recruits’ survival of one native and one alien ascidian species. A feeding experiment allowed us to monitor microscale variation of generalist fish predation, which varied significantly within a marina. We also monitored the in situ survival of lab-grown ascidians at three locations within the marina, half with predator cage exclusion. The survival of the native Ciona intestinalis was conjointly highly influenced by location and caging. We were able to identify a link between predation intensity exerted by mobile generalist macropredators and C. intestinalis survival, whereas none of the measured contaminants accounted for site variability of survival. The non-indigenous Styela clava had significant higher survival and biomass when uncaged, suggesting a positive effect of predation for this species. The natural in situ recruits of C. intestinalis showed higher biomass when caged and may have competed with lab-grown S. clava. Our results suggest that generalist fish predation may play a crucial role in the success of non-indigenous species due to facilitation through competitive release.