How climate-driven changes in disturbance frequency affect the recovery of intertidal mussel beds

Abstract

Community and population recovery from disturbance depends on the intensity, spatial scales, and frequency of disturbance events. Moreover, foundation species, like bed-forming mussels in rocky intertidal shores, are especially prone to disturbance associated with extreme climatic events. The aim of the present study was to experimentally evaluate how the frequency of physical disturbances that generate gaps in beds of the mussel Brachidontes rodriguezii affects mussel recolonization and the succession process on two rocky shore sites of the Southwestern Atlantic. Experimental disturbances (removal of mussels and other forms of biotic cover) were applied to small plots (14 cm × 14 cm) at three different frequencies over two years: 1) only at the beginning of the experiment (i.e., 1× removal frequency), 2) at the beginning of the experiment and 12 months later (2× removal frequency) and, 3) at the beginning of the experiment and 6, 12, and 18 later (4× removal frequency). Unmanipulated areas were maintained as controls. Before applying the experimental disturbance, we measured the abundance of all organisms present in all plots, obtaining data every 6 months. The data were analyzed considering two factors associated with the disturbance frequency: the time since the last removal and the number of removals on the same plot. Our results indicate that increases in disturbance frequencies were sufficient to maintain mussel bed recovery below control levels and increased the cover of crustose algae at the end of the two-year experimental period in one of the rocky shores studied. The decreases observed in the mussel recovery rates under increased disturbance scenarios are expected to have cascading consequences on rocky shore biodiversity, such as reductions in the overall abundance of interstitial invertebrate species that depend on mussel bed habitats and increases in the abundance of sessile organisms that preferentially occur in gaps and are otherwise excluded by mussels, such as the non-native barnacle Balanus glandula.