Relative effects of environment and direct species interactions on the population growth rate of an exotic ascidian

The success of exotic species can be influenced by both the abiotic environment and species interactions. Many studies have demonstrated significant effects of either type of factor on aspects of exotic success, but few have considered their relative effects on population growth rate, a more holistic measure of success. To quantify the relative effects of environment and direct competition on an exotic ascidian, Botrylloides violaceus, I manipulated direct contact interactions at four sites with different abiotic environments and tracked individual colonies over 3 years. I tested site and contact treatment effects on survival, growth and fecundity, and then conducted a life table response experiment on a periodic, size-structured population matrix model to test their effects on population growth rate. Both site and contact interaction were important to explaining variation in survival and growth. Contact interactions decreased the survival and growth of larger colonies but unexpectedly increased the survival of small colonies at some sites, which led to relatively weaker and spatially variable effects on overall population growth rates. Site effects on population growth rates were an order of magnitude larger than contact effects, and site variation in winter vital rates made the largest contributions to changes in population growth rate. The results of this study suggest that the abiotic environment plays a larger role in the success of B. violaceus. Thus, environmental variables, such as temperature and salinity, could be used to predict this exotic species’ success under different environmental scenarios, including global climate change.