The geographic distribution of most species is expected to alter as a consequence of global climate change. Predictions for the extent of these range shifts are frequently based on anticipated changes in temperature using a 'climate envelope' approach, which oversimplifies predictions because it does not consider interactions with other physical and biological factors. The aim of this thesis was to investigate how biological interactions modulate species responses to climate change. On many rocky shores in the NE Atlantic the interaction between limpets, barnacles and canopy forming macroalgae have an important role in structuring rocky shore communities. In particular, limpets control the abundance of macroalgae on the shore through their grazing activities. Through descriptive studies and manipulative experiments the behaviour of a northern/boreal species of limpet, Patella vulgata and a southern/lusitanian species of limpet, P. depressa were compared in relation to canopy forming algae (Fucus patches). As a result of differences in the spatial distribution, behaviour and grazing activity of these two species, if as predicted, there are changes in their relative abundance it is likely there will be implications for rocky shore community dynamics. The second part of my thesis investigated intra- and interspecific competition between two coexisting barnacle taxa with northern and southern centres of distribution. It is predicted that Increased warming will result in a reduction in the abundance of Semibalanus balanoides either as a direct result of increased temperatures or due to an increase in the number of poor spawning years. My results suggest that as a consequence of the gregarious nature of settling S. balanoides cyprids, recruitment success may be reduced irrespective of the numbers of cyprids in the plankton. This will result in more space becoming available for the competing and later settling Chthamalus spp, resulting in a change in barnacle population structure. The likely impacts of population changes and species range shifts in response to increased warming are discussed, with particular emphasis on how the interaction between limpets, barnacles and Fucus may alter. The implications of altered species interactions are then discussed in terms of the effects on community dynamics and ecosystem functioning. Finally the role of biotic interactions in modulating species responses to climate change are discussed with reference to the use of the 'climate envelope' approach in making predictions of species range shifts.

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