Abiotic and biotic drivers of exotic species success in marine fouling communities of Washington, U.S.A

Understanding how and why exotic species succeed in new biological communities is vital for both conservation and basic ecology. Using a combination of surveys, experiments and demographic modeling, I teased apart the abiotic and biotic drivers of exotic species success in marine fouling communities of Washington, U.S.A. I first documented native and exotic species diversity of fouling communities throughout the region by conducting extensive surveys of floating dock habitats. With this data I also tested two classical ecological theories, diversity-stability theory and diversity-resistance theory. I found no support for diversity-stability theory, equivocal support for diversity-resistance theory, and evidence that native and exotic species prefer different abiotic environments. I then conducted manipulative experiments to test the effects of abiotic environment and direct-contact interactions to the vital rates and population growth rates of the most common exotic species, the colonial ascidian Botrylloides violaceus. Results showed that both abiotic environment and direct-contact interaction influenced the survival and growth of B. violaceus. However, abiotic effects on population growth rates were much larger than direct-contact effects. I subsequently explored the effects of pre-emptive competition for space by testing the effect of natural communities on B. violaceus larval recruitment rates and incorporated this effect into a space-dependent model. I showed that space-dependent B. violaceus populations are likely to exhibit phase-locked cycles and that dynamics are sensitive to inter-specific space-occupation and the growth rates of medium-sized colonies in the summer. Finally, I used cage-exclosure experiments to test the enemy-release hypothesis, which posits that native predators limit native species to a greater extent than they do exotic species. I found that large molluskan predators do not limit B. violaceus at any site and that their effect on native species depends on the biomass of large molluskan predators at each site. Thus the enemy-release hypothesis was only supported only at sites with high molluskan biomass. Overall, I conclude that pre-emptive competition for space is the most important biotic interaction to the exotic B. violaceus , but that the abiotic environment is the main driver of the success of this exotic species in Washington fouling communities.