The successful establishment of species introduced to new regions is influenced by their traits as well as those of species in the recipient community through biotic interactions that create resistance to invasion or may facilitate it. Predation, for example, has been shown to create resistance to colonizing non-native ascidians in benthic habitats although they are dominant on docks. Following a survey of Charleston Marina, Oregon, USA, that revealed predators on floating docks were found consuming native but not non-native ascidians, we tested the hypothesis that benthic predators create biotic resistance to non-native ascidians, whereas suspended predators indirectly facilitate them by mainly depredating native ascidians. We conducted surveys, feeding assays, and caloric analysis to explore if the observed predation pattern on artificial substrata is attributable to differences in palatability characteristics of native versus non-native ascidians and whether predation pattern changes in the benthic environment with its more diverse predator community.
Results/Conclusions
Feeding assay data suggest that tunic structure may be a key factor influencing predation rates of one of the predators found on docks, a nudibranch, which preyed upon the native ascidian Distaplia occidentalis more than the non-native Botrylloides violaceous. A second suspended predator, a flatworm, specialized on whole colonies D. occidentalis. Benthic crabs, however, ate the non-native ascidians. Hence, non-native ascidians seem to be facing reduced predation in suspended but not benthic environments due to predator specialization and differences in palatability characteristics. The suspended predators may have indirectly facilitated the invasion of docks whereas the benthic ones provided resistance to the invasion of benthic areas. Differing levels of predator diversity and specialization between anthropogenic introduction areas to the surrounding community are likely shaping community susceptibility to invasion and affecting lag times to escape from initially colonized habitats.