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Invasive species pose a significant threat to marine environments around the world. Monitoring and research of invasive species is needed to provide direction for management programmes. This thesis is a continuation of research conducted on the invasive alga Undaria pinnatifida following its discovery on Wellington’s south coast in 1997. By compiling the results from previous monitoring surveys (1997-2000 and 2008) and carrying out additional seasonal surveys in 2018, I investigate the distribution and spread of U. pinnatifida on Wellington’s south coast, how this may have changed over time and what impacts it may have had on native macroalgal and invertebrate grazer communities. Intertidal macroalgal composition and U. pinnatifida abundance was recorded on fifteen occasions between 1997 and 2018 at two sites at Island Bay and two sites at Owhiro Bay. In addition, the subtidal abundance of six invertebrate grazers was recorded eight times within the same sampling period. Microtopography was also measured at each site to determine if topography had an influence on macroalgal composition. From 1997 to 2000 U. pinnatifida abundance gradually increased per year, but its spread remained localised to Island Bay. In 2008 U. pinnatifida had spread westward to Owhiro Bay where it was highly abundant. However, in 2018 no U. pinnatifida was recorded at any of the sites indicating a collapse of the invasion front. Further investigation revealed that U. pinnatifida was still present along the south coast with the nearest population only 500 m away from the nearest study site. The cause of the U. pinnatifida collapse is not known for certain, but it is unlikely that biotic resistance in the form of competitive exclusion or grazing or a change in environmental parameters such as temperature or nutrient concentration were contributing factors. It is speculated that the collapse arose from a multitude of confounding effects of which further research is needed to identify the exact cause. U. pinnatifida had no impact on macroalgal or grazer community composition. Additionally, microtopography also had no significant impact on macroalgal composition.  This study reports the first ever invasion front collapse of U. pinnatifida in the world, and as a result, provides a new insight on U. pinnatifida distribution and invasion ecology. These findings can assist in predicting the future spread of U. pinnatifida populations as well as aid in formulation of new management strategies.

Localised population collapse of the invasive brown alga, Undaria pinnatifida: Twenty years of monitoring on Wellington's south coast