Demography and population biology of the invasive kelp <i>Undaria pinnatifida</i> on shallow reefs in southern New Zealand

The ability of alien species to gain traction in native communities and expand along coastlines depends on their life histories interacting with local environments. This includes their demographies and formation of viable populations. Here we assess the patterns of recruitment, growth, reproduction and survival of the invasive kelp Undaria pinnatifida in the low intertidal zone of four sites in two areas of southern New Zealand. All populations produced two overlapping cohorts annually, with maximum recruitment in early autumn and the following spring. The autumn cohort largely died off by late spring and it was therefore likely that it was the spring cohort that produced the following years' recruits. There were few visible sporophytes remaining during the hot summer months and so it was the microscopic life stages that persisted over summer to produce populations in the following year. Maximum lengths of > 1.5 m occurred in late winter, and maximum erosion rates of laminae in spring and early summer. Potential reproductive output was very large, ranging from 2.6 − 7.0 × 10⁸ spores per plant among populations at four sites. The effective dispersal distance of spores settling on the substratum in quiescent conditions was less than 20 cm from source sporophylls. Undaria showed strong seasonal and annual cycles, fast growth, high reproductive output and, despite being an annual, an ability to persist among years in local populations as it slowly expands along coastlines. There was no evidence of direct competition with native species but instead it acted as a fugitive species with an ability to recruit into almost any available small patch and, unlike the fucoids in surrounding populations, particularly those inhabited by short turfing coralline algae. The expansion of Undaria along the coastline is clearly aided by its ability to take advantage of patches in the native canopies and to find refuge for its microscopic stages in habitats unsuited to most of the dominant fucoids.