Does seawall greening facilitate non-indigenous species?

Artificial structures such as seawalls, typically support less biodiversity than the natural habitats they replace and can harbour non-indigenous species (NIS). Greening projects enhance biodiversity by adding complexity and surface area. It is, however, unclear to what extent they facilitate NIS. In one of the largest attempts to green seawalls through retrofitting, two 12m stretches were fitted with habitat enhancing concrete panels of six designs - five complex and one flat. Sampling of whole panels and the microhabitats within these were assessed as to - (1) whether colonisation of NIS was enhanced on the complex panels when compared to the flat panel, (2) whether there were particular designs and microhabitats that promoted NIS, and (3) whether colonisation patterns differed among tidal elevations. At high and mid intertidal elevations, the contribution of NIS to total abundance and richness was generally very small on both complex and flat panels. At the low intertidal elevation by contrast, NIS contributed approximately 75% sessile cover, 50% richness and were in some instances 50% more abundant and diverse in growth on the complex rather than flat panels. Within the panels, NIS were particularly abundant in moist, shaded microhabitats. Knowledge of these factors that promoted NIS colonisation will assist in designing future greening interventions that do not facilitate NIS.