Resistance of rocky intertidal communities to oceanic climate fluctuations

functional groups, so we cannot adequately investigate range shifts here. Finally, unlike others who have detected sensitivity to seasonal environmental changes in upwelling or to events like heat waves and storms [34,65,102,127], our annual surveys were unable to resolve any intra- annual response. Overall, the detected resistance of intertidal communities to climate change is only rele- vant within the environmental regime encompassed by the 14-year time frame of our study. Because intertidal communities are expected to be particularly prone to a changing climate and because several studies have already uncovered sensitivity of intertidal species and com- munities to climate change [reviewed by 98], readers should interpret our results with cau- tion. For example, as climate change intensifies these communities could reach tipping points and undergo sudden near-future phase shifts in response to extreme events, species invasions, sea level rise, or disease outbreaks [17,42,128,129]. Some evidence suggests that ecological effects of climate change may manifest as rapid ecological shifts after extreme events [130]. Examples include drought-driven fires in forests or marine heatwave driven bleaching events in coral reefs. A relevant case was the Pacific Northwest ‘heat dome’ event in late June 2021 that caused several days of record heat with 50˚C temperatures recorded on intertidal surfaces in the Salish Sea and killed around 70% mussels and barnacles [131]. In this case, however, Oregon’s communities were spared because in June the daily lower low

tides occur in early morning, i.e., prior to the hottest mid-day hours. But more incremental climate change may also eventually cause shifts. With the previously discussed climate- related changes in invertebrate recruitment, growth, and reproduction, it seems probable that as more species in these systems respond to shifting climate at population or sub-organ- ismal levels, the apparent inertia of entire communities to climate-related change may even- tually be overcome. Indeed, the very clear spatial differences we and others [50,53,132] observed geographically, and the shifts documented by Raimondi et al. [98] suggest that cumulative differences in oceanic climate will eventually cause community shifts. Further, it is widely expected that sea level rise will have profound effects on rocky intertidal communi- ties [133,134]. Ultimately, further research is needed to capture community responses to both within-year drivers like upwelling and heat waves and responses to among-decade driv- ers like ENSO, NPGO and PDO. Conclusion The intertidal communities in this meta-ecosystem appeared resistant to multiple oceanic cli- mate drivers, at least within the scope of environmental variation observed in our 14-year timescale. This apparent resistance to oceanic climate is especially notable because previous research indicates that multiple ecological processes are sensitive to one or more of these cli- mate drivers, and that this system has recently shown signs of diminishing resilience to distur-

bance [11,12]. Thus, there appears to be a disconnect between the sensitivity of ecological processes to climate variability and the emergent structure of these ecological communities. We suggest that the longevity of many taxa or the importance of top-down forcing might be responsible for this disconnect. While these results are promising for the fate of these commu- nities in the era of rapid global change, we are cautious with our optimism. It is possible that our study was too short for the effects of climate fluctuations to manifest, that our indicators were too coarse, or that response to changing climate is non-linear, and that we have not yet observed the conditions that would elicit a “tipping point”. Finally, catastrophic events like extreme weather, marine heat waves, and disease outbreaks are increasing with climate change, and these could have sudden and drastic effects on these communities. Regardless, the rocky PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 27 / 34