The role of temperate herbivores in facilitating expansion of a novel subtropical habitat-former on warming temperate reefsResistance, Resilience and Phase Shifts

Erin McCosker¹, Adriana Verges^1,2, Peter Steinberg¹, Mark Hay³

¹University of New South Wales, Australia - ²Sydney Institute of Marine Science, Australia - ³Georgia Institute of Technology, USA

Climate-driven phase shifts in dominant foundation species are a widespread phenomenon affecting temperate latitude reefs globally. Typically these phase shifts are visible as temperate habitat-forming species such as kelps are replaced by novel subtropical-tropical habitat-forming species that includes corals, seaweeds or turfing algae. Herbivory has emerged as one of the key processes associated with phase shifts away from kelps and canopy-forming seaweeds to novel habitat-formers on temperate reefs, where warming ocean temperatures facilitate overgrazing of kelps and seaweeds by increasingly abundant range-extending tropical herbivorous fishes. We used filmed feeding assays to examine the role of herbivores in facilitating expansion of a subtropical range-extending coral, Pocillopora aliciae, at a temperate rocky reef in southeastern Australia. Kelp assays were deployed in coral-dominated and adjacent kelp-dominated habitat and consumption of kelp and the surrounding turf algae quantified. Both the proportion and rate of kelp assays consumed was higher for assays deployed in coral than kelp-dominated habitat. Turf algae grazing rates of herbivorous fishes were also higher in coral than kelp-dominated habitat. The main actors identified as removing kelp and turf algae were commonly-occurring temperate herbivorous fishes and the temperate long-spined sea urchin. Our findings suggest an overlooked role of temperate herbivores in facilitating kelp to coral phase shifts via competitive release. These novel coral-algae-herbivore interactions present another pathway to accelerating the tropicalisation of temperate reefs.