Co-occurring secondary foundation species have distinct effects on the recruitment and survival of associated organisms

There is growing realisation that foundation species often co-occur in nested or adjacent assemblages. Whether multiple co-occurring foundation species have additive or interactive effects on communities depends on the extent to which they are functionally redundant, and on the density-dependent functions within and across species. We compared how 2 secondary foundation species -the Sydney rock oyster Saccostrea glomerata and the free-floating fucalean algae Hormosira banksii, each facilitated by the grey mangrove Avicennia manna-influence the recruitment and survival of associated invertebrates. Field experiments revealed that effects of the 2 species on recruitment processes were generally distinct and additive. S. glomerata recruitment was enhanced in the presence of oysters but unaffected by algal biomass. Barnacle recruitment, however, decreased with oyster or algal habitat biomass. The efficacy of secondary foundation species in ameliorating predator-prey interactions was dependent on body size relative to the refuge space provided by the foundation species. The naticid gastropod Conuber sordidum was sufficiently small to penetrate habitats, such that neither foundation species influenced its predation on the gastropod Batillaria australis. By contrast, each foundation species reduced predation of the toadfish Tetractenos hamiltoni on small crabs, Paragrapsus laevis, which were able to seek refuge in the interstitial space provided by either habitat. Differential effects of co-occurring secondary foundation species on key ecological processes (recruitment and predation) will result in their facilitation of distinct ecological communities. Hence, models of community assembly should consider interactions among primary and secondary foundation species, and of co-occurring secondary foundation species, which may occur in complex networks.