Mechanisms of invasion and persistence of the invasive kelp Undaria pinnatifida (Harvey) Suringar within intertidal areas of southern New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Very few studies have examined the mechanisms of local establishment and persistence of invasive marine species, especially invasive seaweeds. Undaria pinnatifida, an invasive kelp from Japan, Korea and China was first discovered in New Zealand in 1987. Although U. pinnatifida's spread has been rapid, particularly along the east coast of southern New Zealand very little is known about how this species establishes and persists within invaded areas. This study is unique in that it determines some of the characteristics and mechanisms that enable Undaria pinnatifida to invade and persist in intertidal areas of southern New Zealand. Demographic characteristics of U. pinnatifida were examined within the intertidal zone at four sites along the east coast of the South Island, New Zealand. Continual recruitment over this period resulted in the presence of mature plants and recruit plants during most of the year. However, although plants could be found year round at most sites, distinct cohorts were observed between autumn and spring at all sites. Plants were absent during most months of the summer at Rapaki Bay. Maximal growth occurred during the spring but periods of peak growth were variable, both temporally and spatially. Reproducing plants were found from mid winter to mid summer at most sites with peak reproduction occurring between the late spring and mid summer. Potential lifetime reproductive output of individual plants was approximately 10⁸-10⁹ spores and was related to sporophyll size. Plants with the largest sporophylls were found within the low zone at Moeraki Platform. The appearance of spring recruits suggests that autumn recruits may be able to reproduce by winter and provide a second generation during the same year. Substratum removal experiments examining the ability of U. pinnatifida to recruit onto bare space and coralline turfs showed that coralline turfs facilitated recruitment. Large numbers of recruits were found within the coralline turfs regardless of timing of clearance or the size of clearance. Experiments on the survival of early postsettlement stages showed that facilitation may occur by the coralline turfs protecting embryonic sporophytes from harsh physical conditions such as desiccation whereas grazers had no effect on the survival of embryonic sporophytes. Canopy removal experiments within stands of Carpophyllum maschalocarpum revealed that a combination of size and substrata were important in the recruitment of U. pinnatifida. After 12 months, small (5 x 5 cm) clearances had recovered to preinitiation canopy coverage. In contrast, the medium (25 x 25 cm) and larger (50 x 50 cm) clearances had a mixture of both C. maschalocarpum and U. pinnatifida. The differences observed between the size treatments were attributed to contrasting life histories and demographic characteristics. Experimental removal of intra-specific and inter-specific canopies showed that U. pinnatifida is capable of recruiting very quickly after a canopy disturbance. This suggests that some form of "seed bank" may be present. Further experiments showed that it was likely the embryonic sporophytes were providing the means for rapid recruitment. Newly-developed embryonic sporophytes placed out in the field were visible after 3 months. This suggests a development period of approximately four months from spore release to visible recruit. The results from this study suggest that demographic characteristics including rapid growth, high reproductive output and extended recruitment periods are important factors in the establishment and persistence of U. pinnatifida. However, life history characteristics including the ability of embryonic sporophytes to delay recruitment and the ability to produce two generations in one year are also considered important. The dominance of U. pinnatifida within the intertidal at the study sites is largely due to the ability to coexist with coralline turfs. Coralline turfs appear to facilitate recruitment of U. pinnatifida and coupled with its life history and demographic characteristics are probably the reason why U. pinnatifida can establish and persist within the intertidal along the east coast of the South Island, New Zealand.