Joseph P. Valentine

Establishment of the introduced kelp Undaria pinnatifida in Tasmania depends on disturbance to native algal assemblages.

Abstract Despite recent rapid increases in the occurrence of nonindigenous marine organisms in the marine environment, few studies have critically examined the invasion process for a marine species. Here we use manipulative experiments to examine processes of invasion for the Asian kelp Undaria pinnatifida (Harvey) Suringar at two sites on the east coast of Tasmania. Disturbance to reduce cover of the native algal canopy was found to be critical in the establishment of U. pinnatifida , while the presence of a stable native algal canopy inhibited invasion. In the first sporophyte growth season following disturbance of the canopy, U. pinnatifida recruited in high densities (up to 19 plants m −2 ) while remaining rare or absent in un-manipulated plots. The timing of disturbance was also important. U. pinnatifida recruited in higher densities in plots where the native canopy was removed immediately prior to the sporophyte growth season (winter 2000), compared with plots where the canopy was removed 6 months earlier during the period of spore release (spring 1999). Removal of the native canopy also resulted in a significant increase in cover of sediment on the substratum. In the second year following canopy removal, U. pinnatifida abundance declined significantly, associated with a substantial recovery of native canopy-forming species. A feature of the recovery of the native algal canopy was a significant shift in species composition. Species dominant prior to canopy removal showed little if any signs of recovery. The recovery was instead dominated by canopy-forming species that were either rare or absent in the study areas prior to manipulation of the canopy.

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.

Mechanisms of invasion: establishment, spread and persistence of introduced seaweed populations

Abstract 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.

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

Abstract Sea urchin barrens occur commonly in temperate regions throughout the world and have significant implications for ecological processes on subtidal rocky reefs because they constitute areas of low productivity and diversity compared with habitats dominated by macroalgae. On the east coast of Tasmania, the occurrence of sea urchin (Heliocidaris erythrogramma) barrens in sheltered bays has additional implications in that they represent an important habitat of the introduced kelp Undaria pinnatifida. Identifying the factors responsible for ongoing maintenance of the barren habitat is essential in defining management options to promote recovery of native canopy-forming species. We used transplant experiments to investigate whether inhibition of recovery of native canopy-forming algae can occur in the absence of intense sea urchin grazing. High densities of native canopy-forming species successfully colonised paving blocks deployed in a dense algal bed adjacent to a sea urchin barren. Transplanting these paving blocks to plots on the barren from which sea urchins were removed resulted in >80% mortality of recruits after three months, and 100% mortality after seven months. The decline in macroalgal recruits on paving blocks transplanted to the urchin barren was associated with an increase in the cover and depth of sediment. A persistent cover of sediment also developed on paving blocks deployed on the urchin barren, where no native canopy-forming algal recruits were observed. While sea urchins are undoubtedly important in creating urchin barrens, our results suggest that other mechanisms can influence recovery of native canopy species. In sheltered and semi-exposed bays on the east coast of Tasmania, sedimentation appears to play a critical role in inhibiting early developmental stages of native macroalgae, thereby contributing to a positive feedback that acts to maintain the barren habitat.