The fucalean macroalga Hormosira banksii facilitates diverse rocky intertidal communities. Along the east coast of Australia, the alga can also persist in mangroves as a free-living form trapped amongst pneumatophores. We investigated (1) whether the alga has an effect on molluscan species richness and abundance in mangroves similar to that on rocky shores, and (2) whether, in mangroves, the source (phenotypically distinct estuarine or rocky shore populations) of H. banksii influences the outcome of its interspecific interactions. Sampling of 3 rocky shore and 3 mangrove sites along the east coast of Australia revealed that patches of H. banksii consistently supported a greater species richness of molluscs than adjacent substratum. Whereas the alga increased the abundance of molluscs in the mangrove forest, it had no effect or decreased molluscan abundance on the rocky shore. Transplant of H. banksii from rocky shores and estuarine tidal flats into the mangrove indicated that the source of the algae influenced the magnitude of effects. Although all algae enhanced molluscan abundance and species richness, estuarine H. banksii, which had larger vesicles and a longer thallus, supported more molluscs of more species than rocky shore H. banksii. These results support the growing consensus that the influence of foundation species at the community level is dependent on environmental conditions. As human activities place increasing pressure on coastal ecosystems, it will be important to understand the mechanisms and conditions that determine community-level effects of foundation species so that biodiversity may be conserved.