Do native subtidal grazers eat the invasive kelp Undaria pinnatifida?

Key to understanding the impacts of invasive macroalgae on local food webs is determining the extent to which native herbivores consume the invasive macroalga. We used multiple-choice feeding assays to ascertain the relative feeding preferences of four subtidal grazers (the amphipod Aora typica, the isopod Batedotea elongata and the gastropods Cookia sulcata and Haliotis iris) for the invasive macroalga Undaria pinnatifida and six native macroalgae (Macrocystis pyrifera, Durvillaea antarctica, Carpophyllum flexuosum, Cystophora scalaris, Marginariella boryana and Ulva spp.) that are all abundant along the Otago coast of southern New Zealand. Multiple-choice feeding assays were run under laboratory conditions during the austral autumn (April and June) of 2013. The relative abundances of the macroalgae in the field were also determined. All of the grazers ate U. pinnatifida at rates comparable to most of the native macroalgae, except for B. elongata, which barely consumed it. This indicates that U. pinnatifida, which was shown to be more abundant than native macroalgae in subtidal habitats, has the potential to contribute organic matter to the local food web and may be an undesirable food for some group of grazers. We suggest that U. pinnatifida could potentially alter existing trophic relationships.