Implications of Climate-Induced Localised Extinction of Bull Kelp (Durvillaea amatheiae) for Holdfast Macrofauna

Globally, work is being undertaken to preserve biodiversity and prevent further species extinctions. Yet, without adequate data about the presence of species within ecosystems, hidden extinctions can occur without our knowledge. Given that poleward range retractions are projected for bull kelp (Durvillaea amatheiae) in southeastern Australia, there is the potential for hidden extinctions to occur in associated macrofaunal species. However, this risk cannot be assessed due to the limited data on the invertebrate macrofauna associated with D. amatheiae. We documented macrofauna within D. amatheiae holdfasts, with the objective of establishing a first analysis of the holdfast fauna for this kelp and identifying, where possible, any host-specific taxa. Holdfasts were collected at six locations, spanning most of the range of D. amatheiae on the southeastern Australian mainland. Macrofauna were removed from holdfasts and identified to the lowest possible taxonomic level. Modelling was applied to determine factors correlated with patterns of assemblages among locations. Holdfast assemblages were found to vary significantly among locations and exhibited a latitudinal gradient in assemblage structure. The northernmost location (Aragunnu), which has the highest risk of D. amatheiae range retractions, was found to have distinct species assemblages and generally higher diversity and abundance than locations further south. However, no host-specific taxa were identified in samples, indicating that there is a low risk of multiple hidden species extinctions occurring in holdfast macrofauna, due to future range retractions of D. amatheiae. Rather, most of the macrofauna taxa in D. amatheiae holdfasts were found to also occur in association with the sympatric golden kelp Ecklonia radiata, which is not expected to undergo range retractions at the latitudes where losses of D. amatheiae are projected. Overall, there is an urgent need to develop strategies to combat projected future range retractions of D. amatheiae to reduce impacts on the species that rely on D. amatheiae for habitat.