The influence of native macroalgal canopies on the distribution and abundance of the non-native kelp Undaria pinnatifida in natural reef habitats

摘要

The Asian kelp Undaria pinnatifida (‘Wakame’) is one of the most widespread invasive non-native species in coastal marine habitats and is fast approaching cosmopolitan status, yet its interactions with native species are poorly understood. Within the Plymouth Sound (UK) Special Area of Conservation (SAC), Undaria has become a conspicuous and important component of assemblages in shallow rocky reef habitats, where it co-exists with native canopy-forming brown macroalgae. We examined the hypothesis that rocky reef habitats supporting dense macroalgal canopies will have more biotic resistance to the invasion of Undaria compared with reef habitats supporting disturbed or sparse native canopies. Field surveys were completed at two spatial scales and sampling resolutions, and a short-term field-based canopy removal experiment was conducted to examine the influence of native macroalgal assemblages on the abundance, cover, biomass and morphology of Undaria. Field surveys indicated that Undaria was negatively related to the cover of macroalgal ‘competitors’, particularly Laminaria spp. However, multiple, large Undaria sporophytes were observed within dense native canopies, suggesting that disturbance to, or the absence of, canopies is not a prerequisite for Undaria colonisation. The short-term canopy removal experiment indicated that Undaria functions primarily as a pioneer species in this system. Where native canopies were left intact, Undaria sporophytes were far less abundant and were generally smaller with lower biomass compared with those in disturbed patches. The spread of Undaria into natural habitats is inhibited by the presence of native competitors, particularly large perennial species such as Laminaria spp., although the persistence of intact dense canopies does not completely prevent assimilation of Undaria into native assemblages.