Compared stress tolerance to short-term exposure in native and invasive tunicates from the NE Atlantic: when the invader performs better

The combined impact of invasive species and climate change threatens natural systems worldwide, often facilitating the expansion of harmful invasive species. It is imperative to understand the mechanisms behind why species become invasive and widespread. Traditionally, it is thought that invasive species have greater tolerances to a wider array of environmental conditions than natives. We, therefore, tested the hypothesis that invasive species are more tolerant to the effects of short-term exposure to temperature and salinity stress. Using unifactorial experiments, we compared the tolerances of two common fouling NE Atlantic ascidians, the native Ciona intestinalis and the invasive Styela clava, to increased temperature and decreased salinity. We measured lethal and behavioural responses affecting 50% of populations to give an indication of the tolerance limits for temperature (LT50) and salinity (EC50), and respiration rate to give an indication of the change in metabolic response. The invasive S. clava was more tolerant to increased stress (LT50 = 29.5 A degrees C, EC50 = 19.5) compared with C. intestinalis (LT50 = 27.0 A degrees C, EC50 = 22.7), whereas both species displayed similar metabolic responses observed through increased respiration rates. This study is among the first to experimentally determine limits for temperature and hyposalinity stress for either species and supports the hypothesis that the invader performs better under extreme conditions. Future environmental changes caused by events such as heat waves and climate change could push species to the edge of their physiological limits, potentially facilitating competitive shifts between native and invasive species.