Establishment and persistence of dense stands of the introduced kelp Undaria pinnatifida
Creators: name: Valentine, JP
Publisher: name: University Of Tasmania
Year: 2023

description: Despite high rates of occurrence of non-indigenous organisms in the marine environment, few studies have critically examined mechanisms underpinning the invasion process. In this study manipulative experiments and observations of a natural disturbance to native marine algae were used to examine the invasion dynamics of the Asian kelp Undaria pinnatifida on the east coast of Tasmania. Disturbance to reduce cover of the native algal canopy was found to be a critical stage in the establishment of U. pinnatifida, while the presence of a stable canopy of native algae inhibited sporophyte development. In the first season of sporophyte growth following artificial canopy removal, U. pinnatifida recruited at high densities (up to 19 plants m-2) while remaining rare or absent in unmanipulated plots. A similar response was recorded in areas where native macroalgae declined through natural processes. These results suggest that microscopic U. pinnatifida gametophytes or sporophytes presently occur throughout these native algal beds, but do not develop into visible sporophytes while the canopy is intact. The timing of disturbance was also an important factor. U. pinnatifida recruited in higher densities in plots where the native canopy was removed just prior to the sporophyte growth season (winter), compared to plots where the canopy was removed six months earlier during the period of spore release (spring). In the second year following canopy removal, U. pinnatifida abundance declined significantly, associated with a substantial recovery of native canopy-forming species. This supports the hypothesis that continued disturbance or stress to reduce cover of native algae is required for persistence of dense stands of U. pinnatifida. Recovery of native algae after infestation by U. pinnatifida was investigated in greater detail in a large manipulative experiment conducted on a sea urchin 'barren' (Heliocidaris erythrogramma) seasonally dominated by dense cover of U. pinnatifida. This habitat was chosen as a model system to investigate persistence of U. pinnatifida for two reasons. Firstly, U. pinnatifida occurs most abundantly in Tasmanian waters on these urchin barrens and secondly, the level of disturbance could be easily manipulated in this system by controlling sea urchin density. The experiment examined the response of U. pinnatifida and native macroalgae to treatments comprising all combinations of presence and absence of sea urchins, presence and absence of U. pinnatifida sporophytes, and presence and absence of fertile native macroalgae. U. pinnatifida not only persisted in the absence of sea urchin grazing, but was significantly more abundant compared to areas where urchin densities remained un-manipulated. Recovery of native canopy-forming species was minimal, even in treatments from which sea urchins and U. pinnatifida were removed, and an enhanced supply of native algal spores provided. Thus, factors other than urchin grazing were limiting development of native algae, consequently there was no evidence of inhibition of U. pinnatifida by native canopy species. Recovery of native canopy-forming species was also examined in a transplant experiment. Settlement pavers colonised by high densities of native canopy-forming species were transplanted from an algal bed to sea urchin removal areas on an adjacent urchin barren. Following transplantation a marked reduction in cover of canopy-forming algal recruits occurred in the absence of high densities of sea urchins, while cover was not affected on handling controls (pavers treated similarly but re-deployed into the algal bed). These results suggest that U. pinnatifida may persist in the absence of disturbance in some circumstances, because other factors are limiting the recovery of native algae on the urchin barren. Canopy removal led to accumulation of a sediment matrix on the substratum, which is likely to influence settlement and development of early developmental stages of native algae. A consistent cover of sediment was observed on a large scale on the urchin barren, and rapidly developed on pavers transplanted to the urchin barren. Accumulation of sediment on the natural substratum, beneath dense algal cover was minimal by comparison. It appears that sediment accumulation plays a major role in inhibiting recruitment of native canopy-forming species. The results of this work provide management options for control of U. pinnatifida. Where disturbance is linked to anthropogenic activity, managing the disturbance is likely to prove a more practical and cost-effective method of controlling invasion of U. pinnatifida at high densities than targeting the plant directly. On the east coast of Tasmania, preventing formation of H. erythrogramma barrens is of fundamental importance in this context.