The effects of single versus successive warm summers on an intertidal community

To accurately predict how organisms and ecological communities will respond to future conditions caused by climate change, we must consider the temporal dynamics of environmental stressors, including the effects of repeated exposures to stress. We performed a two‐year passive thermal manipulation in coastal British Columbia, Canada to determine how intertidal communities responded to single and successive warm summers. Warm temperatures had both negative contemporaneous effects within years and persistent negative effects across years. Warming reduced organism densities, altered population dynamics, and affected community structure and diversity, patterns which were likely mediated by differences in foundation species (barnacle) abundance between treatments. Unexpectedly, the effects of warming in the second year were rarely dependent on whether temperatures were warm during the first year (i.e. no interactive effect). Our study suggests that, while this intertidal community can recover from single warm summers, recurring warming has additive negative effects, resulting in a more depauperate, less diverse community over time, particularly if foundation species are negatively affected.