Positive cascading effects of epiphytes enhance the persistence of a habitat-forming macroalga and the biodiversity of the associated invertebrate community under increasing stress

Enhanced persistence of stress sensitive species in harsh environments due to amelioration of physical stress by habitat-forming species has been widely documented. By contrast, less attention has been given to positive species interactions that may enhance stress tolerance in habitat-forming species, with positive cascading effects on biodiversity and ecosystem functioning. We used a combination of field surveys and experiments to assess the effects of the epiphyte Jania rubens, on its subtidal macroalgal host Halopithys incurva, and their cascading effects on the associated invertebrate community, along a depth gradient associated with increasing stress (i.e. light stress at shallower depths). Correlative studies showed that the biomass and photosynthetic efficiency of H. incurva increased with depth, along with a decrease of epiphyte biomass. The experimental manipulation of epiphytes (removal in shallow water and addition in deep water) revealed that their effects on H. incurva physiology varied from positive in shallow habitats to negative in deep habitats. These results suggest that reduction of light/UV stress by J. rubens allows H. incurva to persist in shallow-water habitats that would be otherwise unsuitable. In addition, colonization by large clumps of J. rubens increased the abundance and diversity of associated invertebrates in shallow habitat, suggesting that positive feedbacks between H. incurva, acting as the primary habitat-former, and J. rubens, acting as a secondary habitat-former, generate a facilitation cascade. Synthesis. Our study suggests that the persistence of some habitat-forming species and their ability to support biodiversity may rely upon physical stress reduction by other species (e.g. epiphytes in our case) that are generally neglected when planning conservation strategies. A deeper understanding of context dependency in positive species interactions is essential for predicting patterns of species distribution under increasingly stressful climate scenarios.