Positive species interactions strengthen in a high-CO2 ocean

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.