Persistence of the exotic kelp Undaria pinnatifida does not depend on sea urchin grazing

Abstract

We investigated mechanisms enabling persistence of the introduced Asian kelp Undaria pinnatifida on the sea urchin (Heliocidaris erythrogramma) 'barrens' on the east coast of Tasmania. Development of dense stands of U. pinnatifida requires disturbance to reduce the cover of native algae. Observations of U. pinnatifida occurring abundantly on sea urchin barrens suggests that disturbance in the form of grazing by sea urchins prevents recovery of native canopy-forming species, allowing dense stands of U. pinnatifida to persist. We examined this hypothesis over a 30 mo period in a manipulative experiment in which the response of native algae and U. pinnatifida was examined in treatments comprising all possible combinations of +/- urchins, +/- U. pinnatifida and +/- enhanced native algal spore inoculum. The results demonstrated that the sea urchin H. erythrogramma can have a significant impact on U. pinnatifida abundance. The response was most dramatic in the 2001 sporophyte growth season, when sea urchins destructively grazed U. pinnatifida sporophytes in experimental plots on the urchin barren. In other years, when there was higher recruitment of U. pinnatifida sporophytes, urchins reduced sporophyte abundance but did not prevent development of a U. pinnatifida canopy. Removal of sea urchins resulted in a slow increase in cover of understorey red algae, but only limited recovery of native canopy-forming species. In treatments where both sea urchins and U. pinnatifida were removed, cover of canopy-forming species did not exceed 6% over the duration of the study. Thus, in the absence of sea urchin grazing, there was no evidence of inhibition of U. pinnatifida by native algae. While the intensity of sea urchin grazing may directly influence the extent of the U. pinnatifida canopy, recovery of native canopyforming species was apparently influenced by a combination of factors including seaurchin grazing, depth and, most importantly, the degree of sediment accumulation on the rocky substratum. The manipulations showed that removal of the factor (i.e. sea urchin grazing) that ostensibly facilitated replacement of native canopy-forming algae by U. pinnatifida did not realise recovery of native canopy-forming species.