Abstract
The Asian kelp, Undaria pinnatifida (Harvey) Suringar 1873, is considered to be one of the world’s most invasive species and is well established around the New Zealand coastline. However, ecological implications of U. pinnatifida for marine ecosystems remain largely unknown. This thesis aimed to provide a better understanding of the overall ecological role that U. pinnatifida plays in rocky subtidal and sandy beach ecosystems of Otago, southern New Zealand, and provide insights of the effects that it might have in community structure and benthic food webs. Information on the abundance of living U. pinnatifida on subtidal rocky reefs (2-3m depth), and of dislodged U. pinnatifida in both the surf zone (drift) and on sandy beaches (wrack) revealed that the invasive species represents an abundant although inconsistent resource for the native fauna, comprising up to 75 % of any macroalgal assemblages (living, drift, wrack) during warm months and < 10% during colder months. Chemical (C:N, organic content, protein content, calorific content) and biomechanical (toughness and elasticity) traits showed U. pinnatifida to be a potentially good food source with no apparent biomechanical properties that may prevent high consumption. Feeding assays showed the beach consumer Bellorchestia quoyana (amphipod) consumed lower amounts of U. pinnatifida than of native macroalgae when given a choice but comparable amounts when given no choice, and it strongly preferred U. pinnatifida when macrolgae were reconstituted into agar foods. Feeding choice assays performed on four common subtidal grazers showed that Haliotis iris (gastropod) and Aora typica (amphipod) consumed a variety of macroalgae including U. pinnatifida; Cookia sulcata (gastropod) also consumed U. pinnatifida but strongly preferred Ulva spp.; and Batedotea elongata (isopod) barely consumed U. pinnatifida. Finally, field studies revealed that U. pinnatifida provides a comparable habitat to the also morphologically simple macroalgae Xiphophora gladiata and Marginariella spp., but hosts only 1/4 of the density of invertebrates compared to morphologically complex species such as Carpophyllum spp., Cystophora spp. and Sargassum sinclarii. The findings reveal that the role of U. pinnatifida as a habitat and food for the native fauna varies across ecosystems and among invertebrate species but is was generally similar to some of the native macroalgae it was compared to. This work has also demonstrated that knowledge of traits specific to faunal species (i.e. trophic position, habitat use, etc.) and macroalgae (i.e. chemistry, biomechanics, morphology, etc.) need to be considered when predicting the effects of invasive species.