The effects of climate change in a dynamic competitive interaction between two or more species can be bought about either as direct responses of species to change or indirectly through effects on competing species. Intertidal barnacles are ideal model organisms to test these alternative causal mechanisms, being easily censussed and directly competing for space. Single- and multi- species models will be developed for barnacles in SW England to determine whether direct or indirect mechanisms better predict responses to change. The models will include functions for space-limitation, environmental influence and, in the latter models, functions for interspecific competition. Historical data from a network of sites collected over a 40-year period will be used to develop and test the models.