Foundation species loss alters algal community structure and dynamics at a trailing range edge

Abstract Foundation species are the basis of entire ecosystems and are increasingly threatened by climate change. At trailing edges, population loss can be rapid as extreme warming events can exceed physiological thresholds, causing local extinctions where recruitment cannot sustain recovery. Whilst such losses can be projected, the impacts on remaining communities and potential recovery are difficult to predict, which limits our anticipatory power. Here, we simulated the decline and loss of a cold‐adapted marine foundation species (the kelp Laminaria digitata ) at two trailing‐edge sites, characterised by varying levels of wave exposure and L. digitata dominance. We first exposed macroalgal communities to a pulse disturbance (complete canopy removal) and then tracked trajectories of secondary succession for 2.5 years, under manipulated levels of recruitment (0%, 50% and 100% removal). Background recruitment (0% removal) resulted in the development of assemblages similar to pre‐disturbance levels, indicating a high degree of stability/resilience within the system. However, with increasing L. digitata removal, this resilience was lost, which led to decreased standing stock biomass of canopy‐formers/understorey algae and clear shifts in assemblage structure. Wave‐exposed conditions caused a complete shift to a short‐lived opportunistic species, which provided less stable and favourable habitat and supported distinct communities. Sheltered conditions caused more complex shifts towards multiple alternative canopy‐formers. Synthesis . These findings show the indirect effects of climate‐driven population loss, through changes in ecological interactions such as competition, can propagate through entire communities. Given kelp underpin the wider temperate reef food web, this will likely have serious knock‐on consequences at the ecosystem level.