Positive species interactions strengthen in a high-CO 2 ocean

• Published in: Proceedings of the Royal Society B: Biological Sciences
• Main Authors: Ferreira, Camilo M., Connell, Sean D., Goldenberg, Silvan U., Nagelkerken, Ivan
• Other Authors: Australian Research Council
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 2021
• Subjects: Parma; Ocean acidification
• Online Access: http://dx.doi.org/10.1098/rspb.2021.0475; https://royalsocietypublishing.org/doi/pdf/10.1098/rspb.2021.0475; https://royalsocietypublishing.org/doi/full-xml/10.1098/rspb.2021.0475

Negative interactions among species are a major force shaping natural communities and are predicted to strengthen as climate change intensifies. Similarly, positive interactions are anticipated to intensify and could buffer the consequences of climate-driven disturbances. We used in situ experiments at volcanic CO 2 vents within a temperate rocky reef to show that ocean acidification can drive community reorganization through indirect and direct positive pathways. A keystone species, the algal-farming damselfish Parma alboscapularis, enhanced primary productivity through its weeding of algae whose productivity was also boosted by elevated CO 2 . The accelerated primary productivity was associated with increased densities of primary consumers (herbivorous invertebrates), which indirectly supported increased secondary consumers densities (predatory fish) (i.e. strengthening of bottom-up fuelling). However, this keystone species also reduced predatory fish densities through behavioural interference, releasing invertebrate prey from predation pressure and enabling a further boost in prey densities (i.e. weakening of top-down control). We uncover a novel mechanism where a keystone herbivore mediates bottom-up and top-down processes simultaneously to boost populations of a coexisting herbivore, resulting in altered food web interactions and predator populations under future ocean acidification.