Climate change is directly and indirectly impacting biota, A common prediction is that biogeographic ranges will extend poleward, with increases in abundance near the leading edge. Understanding how marine species will respond is hindered by a scarcity of long-term datasets. However, within the British Isles there is a long history of well-documented intertidal research. Historical data are stored in a variety of forms from grey-literature and national databases to published journals. Sabellaria alveolata is a conservationally important biogenic reef-forming species that reaches its northern range limit within the British Isles. The aim of this thesis was to establish if S. alveolata has responded predictably to climate change, and to investigate abiotic and biotic drivers of observed change. Through comparisons with historical and contemporary collated and collected data on different spatial scales, it was clear that S. alveolata has exhibited persistence in distribution, and stability in abundance and morphology on a long term, broad spatial scale with no significant difference in distribution, abundance or reef-forming morphology exhibited from the 1980s to the 2010s (>50 % stability in all paired data; Bush et al., Chapter 2). Within this, S. alveolata populations have demonstrated change on reduced spatial scales, increasing in response to increased temperature near the northern range edge, with some decreases in response to increased wave exposure well within the range (explaining ~ 50 % of the variance in both instances; Bush et al., Chapter 3). Through long-term monitoring studies, this study demonstrated high within-site variability. Additionally evidence was provided that, within Britain, the intertidal ecosystem engineers S. alveolata and Mytilus edulis are alternative stable state communities on pebble shores, with complete change of state from S. alveolata dominated to M. edulis dominated reef in 1 year (Bush & Davies, Chapter 4). State is partially maintained by settlement success in both instances (R2 ≥ 0.55). Finally, through a combination of traditional monitoring and laboratory techniques with state of the art modelling approaches, this study provided insights into reproductive strategy, larval dispersal and population connectivity patterns of S. alveolata and provided evidence of subpopulations of reef-forming species on British coastlines (e.g. Scotland to North Wales, Mid Wales, and South Wales to Southwest England; Bush et al., Chapter 5).