Despite major advances in biodiversity-ecosystem functioning CBEF) research over the past two decades, there remains a critical need to evaluate the consequences of species loss from complex ecosystems against a background of global environmental change. The aim of this thesis was to determine the ecological effects of species loss and test their context-dependency with respect to current and predicted environmental change. Focusing on grazer-algal interactions and their role in regulating the functioning of rocky intertidal ecosystems, four empirical studies were conducted at different locations on the coast of Ireland. First, wave action and temperature were manipulated simultaneously in outdoor mesocosms containing synthetic rock pool assemblages. The results of this experiment suggest that predicted increases in wave disturbance may exacerbate the consequences of changes in grazer diversity. Second, the responses of macroalgal assemblages to grazer species loss were tested in mussel beds and rock pools on a wave-exposed shore. This demonstrated that the identity-driven effects of consumer species loss can transcend differences in environmental conditions between habitats. Third, algal communities in mussel beds on a moderately wave-exposed shore were subjected to differing temporal regimes of fluctuating grazer densities, combined with elevated sedimentation. As well as emphasising the importance of environmental variability in regulating ecosystem processes, this showed that increased grazing variability can alter the susceptibility of communities to perturbation. Finally, limpets and barnacles were removed on three sheltered and three wave-exposed shores. There were interactive effects of species loss that varied unpredictably with wave exposure, indicating that environmental context can determine the extent to which functional characteristics may predict the consequences of species loss. This research provides important empirical demonstrations of interactive effects of species diversity and environmental context on ecosystem processes in ecologically realistic systems. These findings are discussed and developed in relation to the modern BEF framework.