Ocean Acidification Enhances Fish Herbivory on Seagrasss

Herbivory largely determines the abundance and composition of plants and can have dramatic cascading effects when it involves foundation species such as corals, kelps, or seagrasses. Plants have evolved different strategies (e.g. low nutritional quality, chemical defences) to decrease such herbivory. These strategies may vary across tissues (e.g. reproductive vs. vegetative) and ages (e.g. old vs. young), as different parts of the plant may contribute asymmetrically to plant fitness. In aquatic systems, the presence of epibionts may further alter within-plant susceptibility to herbivory, as their presence can both increase or decrease herbivory pressure on the plant. We quantified abundance and within-plant feeding choices of an herbivorous fish across a natural gradient of ocean acidification (OA). Susceptibility of seagrass to herbivory was higher under acidified conditions, where fish were more abundant and actively feeding. Increased herbivory was due to the disappearance of within-plant differences in susceptibility to herbivory, making all leaves equally and highly susceptible. Such feeding behaviour appears to be driven by changes in epibiosis associated with acidification (i.e. decrease in epiphyte load) rather than seagrass defense traits. Thus, OA may critically modify herbivory pressure on seagrass by removing the “associational resistance” provided by epibionts.