Establishment of the introduced kelp Undaria pinnatifida following dieback of the native macroalga Phyllospora comosa in Tasmania, Australia

The Asian kelp Undaria pinnatifida has recently invaded the coastlines of several countries across both hemispheres. Although the occurrence and subsequent spread of the alga has been well documented, the processes underpinning establishment and spread remain poorly understood. Recent work involving canopy manipulations has demonstrated that disturbance to the native algal canopy facilitates establishment of U. pinnatifida sporophytes at high densities, however, the kelp’s response to a natural disruption of the native algal canopy has not been assessed. In summer/autumn 2001, we examined the response of U. pinnatifida to the significant dieback of a common native canopy forming macroalga (Phyllospora comosa) on the east coast of Tasmania. The response of U. pinnatifida and native algae to the dieback was observed during the season for growth of U. pinnatifida sporophytes (spring 2001) and compared with adjacent areas where dieback did not occur. Undaria pinnatifida sporophytes established at high densities (6.75 ± 1.99 stipes m–2) in dieback areas, but remained rare or entirely absent in control areas where the native canopy was intact. The dieback also resulted in bleaching of encrusting algae and increased cover of understorey algae and sediment. The results support the findings of our recent artificial disturbance experiments, confirming the importance of disturbance events for the successful establishment of U. pinnatifida at high densities.