Importance of the invasive macroalga Undaria pinnatifida as trophic subsidy for a beach consumer

Food webs on sandy beaches rely heavily on spatial subsidies of macroalgae and other detritus. Invasive macroalgal species are modifying many ecosystems worldwide, so the potential exists for them to alter the quantity and quality of food available to sandy beach consumers. We evaluated the suitability of the invasive kelp Undaria pinnatifida as a food for the talitrid amphipod Bellorchestia quoyana, an abundant consumer on sandy beaches in southern New Zealand. U. pinnatifida had similar gross nutritional and biomechanical properties to the three native macroalgal species (the kelps Macrocystis pyrifera and Durvillaea antarctica, and the green Ulva spp.). B. quoyana consumed U. pinnatifida at similar or higher rates than the native kelps in laboratory feeding assays (choice and no-choice assays with fresh tissues, and choice assays with tissue reconstituted into agar foods to remove structural properties). Our results indicate that U. pinnatifida is a usable alternative food source for B. quoyana and could, therefore, contribute to the local sandy beach food web. Understanding the role that invasive macroalgae play as a food source for beach consumers is essential to better comprehend how they may affect these subsidy-dependent ecosystems.