Functional redundancy buffers mobile invertebrates against the loss of foundation species on rocky shores

Abstract

Foundation species are vital to the maintenance of biodiversity and ecosystem functioning in many systems. On rocky shores, rockweeds (large brown algae in the Order Fucales) have the potential to provide habitat and ameliorate stress for mobile invertebrates. To determine the relative role of 2 rockweeds (Silvetia compressa and Pelvetiopsis spp.) as foundation species at sites along a latitudinal gradient, we conducted observational surveys and then initiated a 12 mo removal experiment. We found that richness and abundance of mobile invertebrates declined over time when rockweeds were removed, but only at the southernmost site. In contrast, at our other sites, there was no change in the richness and abundance of mobile invertebrates following rockweed removal. At the southern site, rockweeds played an important role in maintaining mobile invertebrate diversity. At our central and northern sites, rockweeds were less important in maintaining the diversity of mobile invertebrates. At these sites, alternative species, including bladed and branching taxa in the genera Mastocarpus, Mazzaella, Corallina, and Endocladia, co-occur with rockweeds and can buffer the system against their loss. However, these alternative foundation species are rare to absent at the southern site, potentially due to greater physical stress. The loss of rockweed foundation species, which are declining at our southern site, can have cascading effects by causing local co-extinctions of associated species. This study highlights the importance of foundation species, especially in areas where their functional redundancy is low, and how the loss of foundation species can alter diversity, leading to potential changes in ecosystem functioning.