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Dryad

Positive species interactions strengthen in a high-CO2 ocean

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Jun 20, 2021 version files 55.81 KB

Abstract

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 CO2 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 CO2. The accelerated primary productivity translated into 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 co-existing herbivore, resulting in altered food web interactions and predator populations under future ocean acidification.