Studies of the effects of biodiversity on ecosystem function (BDEF) have largely found positive, saturating relationships. However, these studies have been criticised for generating species loss randomly when real extinctions are strongly biased toward rare species. We investigated BDEF relationships in the mollusc fauna of an intertidal rock platform at Griffiths Point, San Remo, south-east Victoria, Australia. Field surveys found that areas with the lowest function (mollusc biomass) were associated with lowest diversity. Excluding individual species from experimental enclosures affected function differentially depending on species’ initial abundance. Rectangular enclosures were attached to the rock platform enabling molluscs to be enclosed while allowing sea water to flow through. Removal of the most abundant species had a positive effect on mollusc biomass, suggesting an inhibition of the other species in the community. In the absence of the most common species, the less abundant species were more productive in combination than when present singly. Taken collectively, these results provide evidence for a relationship between biodiversity and ecosystem function which is a product of both diversity per se and species identity.