Additive and site-specific effects of two foundation species on invertebrate community structure

Many aquatic and terrestrial ecosystems are comprised of assemblages of multiple foundation species. Despite the common occurrence of multiple foundation species, relatively few studies have examined the independent and combined effects of multiple co-occurring foundation species or investigated whether they facilitate similar or distinct associated communities. At 2 sites in a temperate Australian mangrove forest, we examined the independent and interactive effects of co-occurring intermediate foundation species (the macroalga Hormosira banksii and the oyster Saccostrea glomerata-each of which is facilitated by mangrove pneumatophores) on associated community structure. Because the identity of the associated species facilitated by these 2 foundation species can differ, we hypothesized that their combined effects on species richness would be independent and additive. We found that despite their mutual facilitation by mangrove pneumatophores, the 2 intermediate foundation species exhibited independent and positive effects on associated species abundance, richness, and composition. Associated species abundance and richness increased consistently with S. glomerata biomass. In contrast, associated species abundance and richness only responded to H. banksii presence (not biomass), and this response differed across sites. Our finding that functionally different species produce additive effects on biodiversity is consistent with predictions from other recent studies of facilitation cascades. Furthermore, the site-specific effects of foundation species in this study add to growing evidence that while foundation species can set the potential abundance and richness of associated communities, realized community structure is determined by processes operating at larger spatial and temporal scales.