Resistance and facilitation by native algal communities in the invasion success of Undaria pinnatifida

The mechanisms by which non-indigenous species overcome resistance in native communities are not well understood but are fundamental to the ability of invaders to persist and spread once they reach alien shores. One of the most widespread invaders is the kelp Undaria pinnatifida. Here we test disturbance, competition and facilitation in its recruitment into intertidal algal communities in southern New Zealand. In field-based experiments, we manipulated native fucoid and U. pinnatifida canopies, as well as the abundance and type of understory turfing species, and provided various gap sizes in native communities. Undaria pinnatifida recruitment occurred almost exclusively on coralline turf and in the absence of canopies, both native and conspecific. Where the native fucoid canopy was removed, U. pinnatifida recruited into gaps of all sizes (5 x 5 cm, 25 x 25 cm, 50 x 50 cm), but the smallest gaps recovered their native canopy within several months and precluded further recruitment. Experimental manipulations of coralline turf showed that recruitment of U. pinnatifida was orders of magnitude greater where turf was present, regardless of the timing or sizes of clearances. Facilitation did not occur with other turfing species. Overall, native fucoids showed high resistance to invasion, but there was great facilitation by native coralline turf. Furthermore, as annual U. pinnatifida canopies disappear over summer, it is clear that coralline turf harbors the microscopic stages of U. pinnatifida that will form the following year's populations. As turfing species increase and persist following disturbances of native canopies worldwide, it can be expected that the recruitment dynamics of non-indigenous species will also change.