Both abiotic and biotic factors can influence the success of an exotic species in its new range. Understanding the relative contribution of each factor is paramount for both predicting species distributions under different climate-change scenarios and for recognizing the selective pressures shaping exotic populations. Here I utilize comparative demographic techniques to tease apart the effects of the temperature and competition on the population dynamics of an exotic colonial ascidian, /Botrylloides violaceus/, in marine fouling communities of the northeast Pacific/./ Vital rates were estimated from individual colonies grown in either the absence or presence of competitors at four different sites that vary in temperature and competition intensity. Size-structured matrix models were constructed with each set of vital rates to compare the effect of competitor presence on expected population growth rate (λ), size distributions and vital rate elasticities for populations at each site. Projections of these size-structured matrices with and without space-limitation and temperature-dependent vital rates were also fit to population, community diversity and temperature data collected at regular intervals at each site over three years.

Results/Conclusions

Results clearly show that temperature drove population growth rates across sites and within years. Competition intensity alone altered population size-distributions but only slightly decreased growth rates within sites. The implications of these results will be discussed in a life-history evolution context. Another interesting result of this study is that competition intensity was not correlated with species diversity, which conflicts with Elton’s popular and well-supported hypothesis that more diverse communities more thoroughly utilize resources and thereby limit exotic species success. This highlights the need to explicitly incorporate abiotic factors, such as temperature, when predicting future population dynamics and spread of exotic species.