Complex positive interactions among rocky shore sessile species

Positive interactions are important drivers of community structure and ecosystem functioning and may involve benefactor and beneficiary species directly in contact with each other, or spatially separated. On a Mediterranean coast, we examined the possible set of interactions linking 3 key taxa: the barnacle Chthamalus stellatus, the annual alga Rissoella verruculosa, which forms a belt at lower shore levels, and epilithic microphytobenthos (EMPB), widely distributed on all substrata across the mid- and high intertidal zones. We predicted that barnacles could stimulate R. verruculosa growth (through nutrient-rich excretions washed from high to low shore) and EMPB biomass and photosynthetic activity (through the same mechanism or by providing favourable habitat among calcareous shells). In turn, R. verruculosa could act as a secondary facilitator for EMPB by buffering physical stress underneath its fronds. We tested this model through an experiment involving the complete removal or killing (without removal) of C. stellatus crossed with the removal of R. verruculosa. We did not find any effect of barnacles on R. verruculosa during the period of development of algal fronds (March, April, May). In summer (July and August, when algal fronds had been lost), EMPB biomass was smaller where barnacles and algae had been removed. These results negate facilitation of R. verruculosa by C. stellatus, but they indicate positive effects of both species on EMPB. Our study provides an example of adjacent facilitation on intertidal rocky shores, likely mediated by physical mechanisms, and suggests that effects on beneficiary species can persist after the loss of the macroscopic form of the benefactor.