Marine coastal systems are threatened by a wide range of anthropogenic pressures. Of particular concern are increased sediment and nutrient runoff. Moreover, hotspots of anthropogenic pressures are also where invasive species tend to be. The spread of invaders can directly translate to a loss of biodiversity and associated ecosystem services. There is a growing awareness that the spread of native species can have similar large impacts on biodiversity and ecosystem function to their famous non-native counterpart. Yet the how, when and why a native species transitions to a ‘native-invader’ (sensu Simberloff and Rejmánek 2010) are poorly known. This is especially true in the marine environment. In New South Wales (NSW), eastern Australia, a native green macroalga from the notorious Caulerpa genus, C. filiformis, has spread both inside and outside its native range.
This study investigated the mechanisms that may have promoted the success of C. filiformis. The main aims were to increase our understanding on why this species has become so abundant, the habitat associations of the alga across large and local spatial scales, and the potential impacts on the seaweed community. In Chapter 2, large scale surveys showed that adult population abundance was related to sedimentation at multiple spatial scales, such as Reef Beach Index (RBI), reef width and the presence of a sediment veneer on top of the rocky reef (r+s habitat). In contrast, a low association with turf habitat was found, which is opposite to the observations for several other Caulerpa invaders. A model was proposed where the adult association with r+s represents an end stage of succession after outcompeting turfing alga. Moreover, it was shown that the seaweed community showed reduced morpho-functional richness in presence of C. filiformis. To test the proposed model, habitat association of the recruitment stage was investigated in Chapter 3. Surveys at several locations throughout the alga’s distribution showed that C. filiformis’ recruit abundance was found to be high on turfing algae habitat and not on r+s habitat. This positive association of the recruitment stage was observed for coralline forms of turfing alga, but not for filamentous forms. Those results support the model hypothesized. Additionally, the mechanisms behind this positive association were further explored in this chapter. A laboratory experiment revealed that the high association of C. filiformis’ recruits with coralline alga was explained by the increased attachment performance of C. filiformis fragments on this substrate. Namely, the increased structural complexity of coralline alga aided in rapid and strong attachment of fragments.
Finally, mechanisms that may benefit post-recruitment processes of C. filiformis, and in particular the role of turf habitat, were investigated. With the use of long term field observations in Chapter 5, it was shown that established Caulerpa appears a poor competitor, and possibly requires further disturbance to spread. For example, a laboratory experiment showed that C. filiformis is highly tolerant to large sedimentation rates, aided by rapid morphological plasticity (Chapter 4). This may benefit the alga indirectly if sediment more negatively affects competitors. Moreover, increased sediment-nutrient availability was shown to promote growth rates, benefitting the alga’s competitive strength (Chapter 5).
Overall, my thesis indicates that disturbance to native macrophytes indirectly promotes recruitment success by promoting turf habitat and that its continued spread is reliant on further disturbance to native communities – a model that has been demonstrated to invasive marine algae but not previously for a rapidly spreading native alga.