MEPS 587:93-104 (2018)  -  DOI: https://doi.org/10.3354/meps12429

ABSTRACT: Invasive species are affecting coastal ecosystems worldwide and there are many potential mechanisms that allow their spread into native communities. To investigate this phenomenon, we used an experiment in which the canopy of the southern bull kelp Durvillaea antarctica was removed in 2 seasons and community development was followed over 3 yr. The invasive kelp Undaria pinnatifida recruited almost exclusively into plots from which the natural canopy had been removed. We examined, through a series of regression tree models and change point analyses, the gradients of community responses that allowed the successful recruitment of this invasive species. Analyses revealed a range of coinciding conditions, especially a decline in fucoid cover below 20% and an increase in turf cover above 80%, that facilitated Undaria recruitment. The abundance of molluscan grazers and cover of subcanopy algae had lesser effects on Undaria recruitment. In an ecological sense, there was a clear tipping point in the interaction between canopy loss and the subsequent expansion of coralline turf that allowed communities to switch from dominance by native species to seasonal dominance of Undaria. This study illustrates the complex nature of disturbance thresholds and interactions within the native communities that can facilitate the spread and recruitment success of Undaria.

KEY WORDS: Tipping point · Threshold · Rocky reef · Native algae · Invasive algae · Regression tree