Shifting mechanisms of biotic resistance across multiple life-history stages reduce the abundance of a non-indigenous marine invertebrate

Background/Question/Methods

Ecological succession and invasion biology are inextricably linked processes, and are used to study community change. Research on ecological succession has traditionally focused on understanding species’ interactions during one or few life-history stages, primarily among adults or effects of adults on new recruits. This is surprising considering that many organisms have complex life cycles with multiple life-history stages (and associated critical transitions) before reaching reproductive maturity. Here, we investigate how biotic interactions with resident competitors and predators at various life-history stages affect the success of a marine non-indigenous species. Specifically, we conducted additive and replacement design experiments in the laboratory covering interactions from gamete release to post-metamorphic stages between the introduced solitary ascidian Ciona intestinalis and the native solitary ascidian Ascidia ceratodes. We also placed new metamorphs of both species in the field to examine direct and indirect effects of third-party neighbors and predators on longer-term survival and growth. Ciona is a major pest species that forms dense monocultures in much of its introduced range, but not in northern California. Our study examines potential mechanisms of biotic resistance among the resident fauna.

Results/Conclusions

Small predators such as amphipods were highly effective at removing larvae and recent settlers less than few days old, but had little effect on 2-week old individuals. Larger juveniles of Ciona suffered complete mortality in the field due to larger predators such as fish and crabs, but significant numbers of Ascidia juveniles survived. Thus, predation on earlier life-history stages more strongly affected survival of Ciona than Ascidia. Where predators were excluded, field experiments found no effects of competition from colonial ascidians on either Ciona or Ascidia. Ciona grew significantly faster than Ascidia in these experiments, suggesting it might have a competitive advantage at early stages. However, after 14 weeks, Ascidia became the clear competitive dominant in assemblages containing both species, allowing persistence of Ciona and other colonial ascidians only in patches where Ascidia had become dislodged from the substrate. Overall, predation restricts Ciona establishment at early life-history stages, but even when and where predation is reduced, competition from a dominant native greatly reduces invasion success. Thus, the failure of Ciona to establish monocultures as seen elsewhere appears due to biotic resistance of the resident community at multiple life-history stages.