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Understanding the mechanisms that facilitate or inhibit invasion of exotic seaweeds is crucial in assessing the threat posed by their incursion and to define control options. In this paper, we consider how life history characteristics of the invading species and properties of the recipient environment influence the likelihood of invasion, giving particular emphasis to how disturbance influences the establishment, spread and persistence of introduced seaweed populations. Very few commonalities in key life history traits emerge since each species possesses a unique set of traits that confers a high capacity for invasiveness. Consequently, for seaweeds at least, predictions of invasibility based on life history characters alone are unlikely to be useful. In contrast, it is clear that disturbance is an important process in the establishment of these invasive species. With the possible exception of Caulerpa taxifolia, disturbance appears to be a critical factor that is either a key requirement (e.g., Codium fragile ssp. tomentosoides, Sargassum muticum and Undaria pinnatifida), or which accelerates (e.g., Fucus serratus) establishment and spread. The role of disturbance in the persistence of the invaders is more complex and depends on the species concerned. In several cases there is substantial evidence for positive feedback mechanisms that enable introduced species to persist in the absence of the disturbance factor that facilitated establishment in the first place. These circumstances define examples of ecological hystereses that pose particular challenges for management and control. The evidence suggests that, in several cases, preventing anthropogenically mediated disturbance to canopies of native seaweeds should be considered as a potential control option to minimise the risk of establishment of exotic species at high densities. However, for these kinds of introduced species, once they are established, control options that primarily target the disturbance are unlikely to represent viable management options.

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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.

Cover sheet Cover sheet Title Persistence of sea urchin (Heliocidaris erythrogramma) barrens on the east coast of Tasmania: inhibition of macroalgal recovery in the absence of high densities of sea urchins Author Valentine, JP, Craig Johnson Bibliographic citation Bibliographic citation Valentine, JP; Johnson, Craig (2005). Persistence of sea urchin (Heliocidaris erythrogramma) barrens on the east coast of Tasmania: inhibition of macroalgal recovery in the absence of high densities of sea urchins.