Abstract
Undaria pinnatifida is a Laminarian kelp originally from the Sea of Japan that has spread throughout temperate harbours and coastlines around the world. Due to its plasticity, fast growth rate, and high reproductive output, U. pinnatifida is widely considered one of the worst invasive species in the world. After an introduction of U. pinnatifida occurred at one of New Zealand’s most remote islands, marine biosecurity measures to manage populations were set into legislation by the central government of New Zealand over twenty years ago when U. pinnatifida was officially designated an “unwanted organism”. Yet, the invasive kelp is now present in many rocky reefs throughout New Zealand and its persistent presence has led some to believe that management is not feasible. Additionally, because the ecological impact of U. pinnatifida is unclear, there is a lack of evidence to support management action being necessary. Studies on U. pinnatifida are also limited to mostly intertidal ecosystems for New Zealand even though populations appear to reach subtidal depths in other invaded regions. Therefore, to determine whether an U. pinnatifida population can be controlled once it has established on natural substratum, three removal experiments were completed across three regions: one newly introduced population in a remote location on a pristine offshore island, one well-established and widely spread population with high biomass in a marine protected area, and one recently established but less widely distributed population in a fjord. The spatial distribution of U. pinnatifida across the rocky reef of the new incursion site was tracked for two years to confirm the vector of the new incursion and map its spread into a pristine kelp forest on the rocky reef over time. Undaria pinnatifida was also removed from this site to assess the effectiveness of early detection and rapid response. For the other two experiments, the impacts of large-scale removal were assessed by surveying macroalgal species density at multiple sites in each area using a before-after control-impact experimental design over three years across multiple 1000 m2 removal plots. Approximately 20 tonnes of U. pinnatifida was removed across all sites during these studies.
Initial biomass of U. pinnatifida was lowest at the newly introduced site. Then there were no additional U. pinnatifida sporophytes observed after the manual removal of sporophytes. Removed biomass was highest at the well-established area. After three years of removal, the biomass of the well-established sites decreased by 80%. Similarly, the total biomass of the recently established sites decreased by 70% after two years of removal. The recently established sites also shifted from an adult to a juvenile-dominated population. During all of the removal experiments, there were few observable impacts on native macroalgal species. These experimental manipulations across multiple sites and years provided robust causal evidence of the spatial and temporal extent of large-scale U. pinnatifida biomass removal. However, while the experiments showed that manual removal was able to reduce U. pinnatifida biomass and maintain U. pinnatifida density, any relationship between U. pinnatifida and native macroalgal species was still unclear. Therefore, to assess the ecological role of U. pinnatifida in Te Waipounamu (the South Island, New Zealand) and to determine if there was a relationship between the invasive macroalgal species and native macroalgal species, in situ surveys were conducted across six invaded regions by directly surveying and sampling macroalgal biomass. These surveys indicated that U. pinnatifida was the most dominant macroalgal species across the regions and suggested direct competition with other large brown macroalgal species. Overall, this thesis presented novel ecological research on the removal and monitoring of an invasive marine species. The work presented here may help to inform environmental managers about the impacts of sustained control efforts, with particular attention to Te Waipounamu as well as implications for regions where U. pinnatifida populations are also spreading. The results discussed throughout this thesis can also be used to support management actions for invaded marine ecosystems in general, which is likely to benefit multiple species across productive and diverse marine areas that are increasingly influenced by other cumulative environmental stressors.