Resistance of rocky intertidal communities to oceanic climate fluctuations

densities (100-400m-2) of limpets are not shown in plots to enable visualizations of fits. https://doi.org/10.1371/journal.pone.0297697.g007 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 17 / 34

Table 3. Analysis results for the focal functional groups with the highest average annual dissimilarity in the a) high, b) mid and c) low zone. Multiple regression models tested the effects of mean Pacific Decadal Oscillation index (mean PDO), mean North Pacific Gyre Oscillation index (mean NPGO), mean El Niño Southern Oscillation index (mean ENSO), mean and standard deviation of the Biologically Effective Upwelling Transport Index (mean and SD BEUTI, respectively), mean daily sea surface temperature (Mean SST), mean daily maximum air temperature (Max Air Temp), and years since the sea star wasting disease epidemic (Yr Since SSWD) Only those model terms that met the significance threshold of P < 0.00625 (Bonferroni correction) are listed, and these were subsequently tested for bivariate correlations between the climate term and the taxon abundance. Bold statistics indicate bivariate adjusted R2 of > 0.1. a) High zone Model Response Average Annual Dissimilarity Model Marginal R2 Sig. Terms (p < 0.0065) Bivariate Correlation Herbivorous snails 8.40 0.34 Mean PDO -0.057 Mean NPGO +0.157 ENSO -0.054 SD BEUTI -0.001 Max Air Temp -0.048 Mean SST < -0.001 Yr Since SSWD -0.239 Limpets 6.45 0.42 Mean PDO -0.038 Mean NPGO +0.122 ENSO -0.037 BEUTI +0.044 SD BEUTI +0.164 Max Air Temp +0.059 Mean SST -0.029 Yr Since SSWD -0.151 Balanus glandula 4.55 0.05 None sig. Fucoid algae 4.46 0.51 SD BEUTI -0.077 Chthamalus dalli 3.12 0.12 None sig. Mussels 3.10 0.32 PDO <+0.001 Bare Space 2.74 0.13 None sig. Mastocarpus papillatus 2.35 0.18 None sig.

b) Mid zone Model Response Average Annual Dissimilarity Model Marginal R2 Sig. Terms (p < 0.0065) Bivariate Correlation Limpets 7.98 0.40 Mean PDO -0.222 Mean NPGO +0.107 Mean ENSO -0.025 SD BEUTI +0.109 Max Air Temp -0.005 Mean SST -0.065 Yr Since SSWD -0.131 Predatory Snails 5.13 0.39 Mean NPGO +0.030 Mean ENSO <+0.001 SD BEUTI -0.034 Max Air Temp -0.117 Yr Since SSWD -0.012 Bare Space 4.74 0.32 None sig. Large barnacles 4.37 0.33 None sig. Gooseneck barnacles 3.63 0.27 Mean NPGO -0.010 Yr Since SSWD -0.001 Mussels 2.71 0.22 Yr Since SSWD +0.136 Endocladia muricata 2.30 0.63 Max Air Temp +0.255 Balanus glandula 2.18 0.60 None sig. (Continued) PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 18 / 34

since SSWD (R2 = 0.040, 0.029, & 0.006, respectively; Fig 11c–11e; Table 5b;). Low zone diver- sity increased with calmer NPGO conditions, increased upwelling, and warmer sea surface temperatures (R2 = 0.030, 0.001, & 0.065, respectively; Fig 11f–11h; Table 5c). The strongest trend was increased diversity with years since SSWD in the low zone (R2 = 0.229; Fig 11i, Table 5c). Discussion Marked resistance of intertidal communities to oceanic variation Our most striking finding was the consistency in rocky intertidal communities along the Ore- gon and Northern California coasts over the 14 years of study. We found little to no support for strong oceanic climate-driven fluctuations in community structure (Figs 7–9, Table 2), and only modest correlations between taxon abundance and climate fluctuations (Table 3). Analy- ses of rates of community change and change in diversity yielded similar results (Tables 3 and 4), suggesting that communities within the Oregon and northern California intertidal meta- ecosystem were resistance to environmental variation, at least during the 14 years of our study for the particular community parameters measured. Since several key ecological processes (e.g. growth, recruitment, resilience) have declined or become more variable with to climate change [11,12], this apparent resistance came as a surprise. We expected intertidal communities to be sensitive to oceanic climate for several reasons. First, several studies have found evidence of range shifts or shifting dominance of northern/

southern species in the intertidal over the past decades [67,68,98], and rocky intertidal Table 3. (Continued) c) Low zone Model Response Average Annual Dissimilarity Model Marginal R2 Sig. Terms (p < 0.0065) Bivariate Correlation Limpets 2.93 0.52 Mean PDO -0.088 Mean NPGO 0.059 Mean ENSO -0.097 BEUTI 0.001 SD BEUTI -0.004 Max Air Temp -0.003 Mean SST -0.123 Yr Since SSWD -0.050 Surf grass 2.61 0.09 Mean PDO 0.009 Mean NPGO 0.007 SD BEUTI -0.111 Hedophyllum sessilis 2.51 0.06 Mean SST -0.030 Crustose Coralline Algae 2.12 0.20 None sig. Cryptopleura and Hymenena 2.08 0.11 None sig. Odonthalia and Neorhodomela 1.94 0.05 None sig. Bare Space 1.77 0.08 Mean NPGO 0.009 Mean ENSO -0.018 Max Air Temp 0.029 Mean SST -0.085 Yr Since SSWD -0.047 Articulated Coralline Algae 1.73 0.07 BEUTI -0.027 Max Air Temp <0.001 https://doi.org/10.1371/journal.pone.0297697.t003 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 19 / 34

communities are widely expected to be particularly sensitive to a changing climate [99]. Sec- ond, other studies in this system have detected sensitivity of ecological processes to ocean cli- mate fluctuations like PDO, NPGO, ENSO upwelling, and temperature [20,54,62,64,100]. Third, there were multiple notable climate-related events during this time frame that we cap- tured in our analysis (Figs 2 and 3). Most notably, there were several concurrent and intercon- nected shifts in 2014–2016 that included a severe marine heat wave (“the blob”), a strong El Niño event, and a shift to less windy NPGO and warmer PDO conditions [46,47,101]. While Fig 8. Bivariate regressions between the percent cover of focal functional groups (significant functional groups in Table 2b with the 8 focal climate variables in the mid zone. Each data point is the average abundance of a functional group for a site and year. Environmental drivers included Pacific Decadal Oscillation (mean PDO), North Pacific Gyre Oscillation (mean NPGO), Multivariate El Niño Southern Oscillation Index (mean ENSO), Biologically Effective Upwelling Transport Index (mean BEUTI), the standard deviation in BEUTI (SD BEUTI), the maximum daily air temperature by site (Max Air Temp), the mean daily water temperature by site (Mean SST) and the years since the initial sea star wasting disease outbreak (Yrs since SSWD). Colors correspond to functional groups, with Endocladia muricata in red, mussels (Mytilus spp.) in blue, gooseneck barnacles (Pollicipes polymerus) in cyan, limpets in dark purple, and

predatory snails in light purple. Units are in percent cover except limpets and predatory snails are in density m-2. https://doi.org/10.1371/journal.pone.0297697.g008 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 20 / 34

other studies have found changes in intertidal communities following these particular climate events [102], we did not. Fourth, our recent experiments at many of these same sites showed that recovery rates from annual experimental disturbance are declining and becoming more variable, suggesting declining resilience [11]. Additionally, several key ecological metrics (eco- logical subsidies, mussel performance, and sea star performance) are either diminishing or increasing in variability [12]. Below we discuss the apparent disconnect between ecological processes and community patterns, ask why this apparent resistance might exist, explore the Fig 9. Bivariate regressions between the percent cover of focal functional groups (significant functional groups in Table 2c with the 8 focal climate variables in the low zone. Each data point is the average abundance of a functional group for a site and year. Environmental drivers included Pacific Decadal Oscillation (mean PDO), North Pacific Gyre Oscillation (mean NPGO), Multivariate El Niño Southern Oscillation Index (mean ENSO), Biologically Effective Upwelling Transport Index (mean BEUTI), the standard deviation in BEUTI (SD BEUTI), the maximum daily air temperature by site (Max Air Temp), the mean daily water temperature by site (Mean SST) and the years since the initial sea star wasting disease outbreak (Yrs since SSWD). Colors correspond to functional groups, with bare space in gray, surf grass (Phyllospadix spp.) in green,

Hedophyllum sessile in yellow, articulated coralline algae in pink, and limpets in dark purple. Units are in percent cover except limpets are in density m-2. https://doi.org/10.1371/journal.pone.0297697.g009 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 21 / 34

subtle changes that we were able to detect in our study, and contemplate whether we can expect this resistance to continue in the future. The disconnect between varying ecological processes and emergent community structure Our analysis suggests that as of 2020, the general structure of rocky intertidal communities on our shore appeared resistant to climate change. What is the underlying basis for the resistance of these communities to oceanic climate variation? We explore two ecological and possibilities: Table 4. Results of multiple regression analyses and bivariate regressions testing drivers of rate of community change in a) high, b) mid, and c) low intertidal zones in Oregon and northern California from 2006–2021. We tested the effects of peak seasonal climate indices for mean Pacific Decadal Oscillation index (mean PDO), mean North Pacific Gyre Oscillation index (mean NPGO), mean El Niño Southern Oscillation index (mean ENSO), mean and stan- dard deviation of the Biologically Effective Upwelling Transport Index index (mean and sd BEUTI, respectively), mean daily sea surface temperature (Mean SST), mean daily maximum air temperature (Max Air Temp), and years since the sea star wasting disease epidemic (Yr Since SSWD). Communities were analyzed at the quadrat level in the mid and high zones (0.5 x 0.5m) and the transect level in the low zone (the average of ~10 0.5 x 0.5m quadrats). Bold statistics indicate significant terms (Bonferroni cutoff P < 0.002). Factor Chisq Df Pr(>Chisq) Bivariate Adj R2 a) High Zone Intercept 0.76 1 0.3820 Mean PDO 0.12 1 0.7339

Mean NPGO 2.30 1 0.1298 Mean ENSO 1.92 1 0.1653 Mean BEUTI 2.25 1 0.1334 SD BEUTI 3.33 1 0.0681 Mean SST 0.14 1 0.7069 Max Air Temp 0.91 1 0.3398 Yr Since SSWD 3.33 1 0.0679 b) Mid Zone Intercept 5.55 1 0.0185 Mean PDO 0.03 1 0.8556 Mean NPGO 8.34 1 0.0039 Mean ENSO 9.71 1 0.0018 -0.003 Mean BEUTI 0.85 1 0.3557 SD BEUTI 0.53 1 0.4681 Mean SST 0.19 1 0.6612 Max Air Temp 6.05 1 0.0139 Yr Since SSWD 9.51 1 0.0020 -0.019 c) Low Zone Intercept 9.21 1 0.0024 Mean PDO 4.46 1 0.0347 Mean NPGO 1.77 1 0.1836 Mean ENSO 0.55 1 0.4587 Mean BEUTI 0.10 1 0.7575 SD BEUTI 1.62 1 0.2037 Mean SST 0.28 1 0.5953 Max Air Temp 13.39 1 0.0003 0.069 Yr Since SSWD 2.74 1 0.0980 https://doi.org/10.1371/journal.pone.0297697.t004 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 22 / 34

1) time lags in responsiveness deriving from longevity of component species and 2) impor- tance of top-down trophic forcing. First, many species in our system are long-lived and possess strong regenerative capabilities. For example, once they survive the extremely high mortality rates experienced as larvae and juveniles, many invertebrates in our system can live for years, decades, and even centuries. Some examples include sea anemones (Anthopleura xanthogram- mica and A. elegantissima) that may live >100 years [103], sea urchins (Strongylocentrotus pur- puratus) that likely live several decades [104], and mussels and large barnacles (Mytilus californianus and Semibalanus cariosus) that can live at least ~15 years [105,106]. Similarly, many of the dominant algae die back annually or when stressed, but can quickly regrow from their holdfasts [107–110]. So, while changes in climate mode or stress events can have sub- lethal physiological effects on these species [64,111–114], this may not translate into long-last- ing changes in abundance. In addition, long-life spans of many component species mean that climate-sensitive processes like recruitment and growth may take years to translate into com- munity change. Ultimately, long-life span and perennating capabilities suggest that sensitivity at the community level may lag behind the changes in oceanic climate and may not be detect- able for years to come. Second, climate-sensitive rates of recruitment and growth of individuals versus their sur-

vival in the face of top-down trophic forcing may influence the disconnect between ecological processes and community structure. For example, recruitment and/or growth rates of short- lived species of barnacles and mussels (B. glandula, C. dalli, and the mussel M. trossulus) at some of our sites (primarily sites on Cape Perpetua) can be extremely high [20,48,54,62,115]. However, while adult cover of these weedy species may spike as a result, these increases do not usually persist more than a few weeks to months due to high annual mortality from predation Fig 10. Correlations of the rate (vector lengths in Fig 4d–4f) of community change with significant environmental drivers (identified by models in Table 3) in Oregon and northern California from 2007–2020. Focal comparisons included the a) rate of change in the mid zone with Multivariate El Niño Southern Oscillation Index (mean ENSO) zone, b) rate of change in the mid zone with the years since the initial sea star wasting disease outbreak (Yrs since SSWD), and c) rate of change in the low zone with the maximum daily air temperature (Max Air Temp). https://doi.org/10.1371/journal.pone.0297697.g010 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 23 / 34

[116]. In fact, climate-driven high recruitment may simply transfer biomass to the top trophic levels, since these sites have high whelk and sea star densities [117,118]. Thus, predators may mute community sensitivity to climate fluctuations by transferring excess prey biomass to higher growth and reproductive output, as found for whelks by Navarrete et al. [116] and sea stars by Menge et al. [119]. Fig 11. Linear correlations of taxon diversity (Shannon-Weiner index ± SE) with significant environmental drivers (identified by models in Table 4) in Oregon and northern California from 2006–2020. Zones included the a-b) high, c-e) mid, and f-i) low zones (yellow, green and blue, respectively). Environmental drivers included a) Pacific Decadal Oscillation (mean PDO), b) the maximum daily air temperature by site (Max Air Temp), c & f) North Pacific Gyre Oscillation (mean NPGO), d) Multivariate El Niño Southern Oscillation Index (mean ENSO), e & i) the years since the initial sea star wasting disease outbreak (Yrs since SSWD), g) Biologically Effective Upwelling Transport Index (mean BEUTI), and h) the mean daily water temperature by site (Mean SST). https://doi.org/10.1371/journal.pone.0297697.g011 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 24 / 34

Shifts in intertidal communities with sea star wasting disease and climate fluctuations Our analysis did detect a shift in rocky intertidal communities in relation to the years since sea star wasting disease, which caused sea star biomass to decline by 60–90% [9]. We found that SSWD accounted for 15.9% of variation in community structure in the mid zone (Table 2b) and was correlated to an increase in mussels (Fig 8, Table 3b), and a decrease in diversity (Fig 11e, Table 5b). These findings are consistent with the keystone predation hypothesis [73,74] that suggests P. ochraceus increases diversity by consuming mussels, thereby freeing space for other taxa in the intertidal zone thus increasing diversity. In the high zone, diversity showed an unexpected opposite trend with sea star wasting disease (Fig 11i, Table 5c). Upon further inspection, however, this seemed to be driven by low diversity estimates in the early part of the study (2006–2008) while no increase was evident since the outbreak in 2014. This suggests this trend is not related to the disease but instead may be due to low taxonomic resolution in the Table 5. Linear model results testing the trends in taxon diversity (Shannon Index) with mean Pacific Decadal Oscillation index (mean PDO), mean North Pacific Gyre Oscillation index (mean NPGO), mean El Niño Southern Oscillation index (mean ENSO), mean and standard deviation of the Biologically Effective Upwelling Transport Index index (mean and sd BEUTI, respectively), mean daily sea surface temperature (Mean SST), mean daily max-

imum air temperature (Max Air Temp), and years since the sea star wasting disease epidemic (Yr Since SSWD). Model terms meeting the Bonferroni cutoff of P < 0.002 are in bold, and for these we subsequently tested the bivariate correlation of each significant term with diversity (bold indicates a bivariate correlation > 0.10). Term Chisq Df Pr(>Chisq) Bivariate R2 a) High zone Mean PDO 10.21 1 <0.001 0.042 Mean NPGO 0.19 1 0.660 ENSO 5.29 1 0.021 BEUTI 6.15 1 0.013 SD BEUTI 2.24 1 0.134 Max Air Temp 17.49 1 <0.001 0.029 Mean SST 2.39 1 0.122 Yr Since SSWD 2.71 1 0.099 b) Mid zone Mean PDO 1.18 1 0.277 Mean NPGO 87.94 1 <0.001 0.040 ENSO 38.28 1 <0.001 0.029 BEUTI 5.22 1 0.022 SD BEUTI 0.06 1 0.805 Max Air Temp 0.69 1 0.406 Mean SST 9.02 1 0.003 Yr Since SSWD 76.56 1 <0.001 0.006 c) Low zone Mean PDO 6.50 1 0.011 Mean NPGO 17.51 1 <0.001 0.030 ENSO 4.01 1 0.045 BEUTI 9.97 1 0.002 0.001 SD BEUTI 5.26 1 0.022 Max Air Temp 0.16 1 0.688 Mean SST 18.36 1 <0.001 0.065 Yr Since SSWD 68.03 1 <0.001 0.229 https://doi.org/10.1371/journal.pone.0297697.t005 PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 25 / 34

early years. The response to SSWD was not the primary focus of the present study but was included because it was a major event that was concurrent with the focal climate fluctuations. A more extensive investigation of this SSWD event will be reported elsewhere. Averaging across sites and capes, mobile gastropods including limpets (Lottia species), her- bivorous snails (primarily Littorina spp. and Tegula funebralis) and predatory snails (primarily Nucella ostrina and N. canaliculata) were quite variable over time (S4, S5 and S6 Figs in S3 Appendix). Some trends were notable (i.e. bivariate trends in Fig 8 showing an R2 of >0.10, in Table 3b), including increased limpets with windier NPGO in the high and mid zones, increased limpets with windier NPGO and higher upwelling variability in the high zone, and decreased predatory snails with warmer air temperatures in the mid zone. Limpets and whelks are relatively small and short-lived (e.g., 2–5 yr; [120–122]) so are likely susceptible to environ- mental stress. Thus, these species may be indicators of climate change, suggesting that the modest patterns noted above warrant further investigation. We were surprised by the lack of responses of algal taxa to climate fluctuations, since they are primary producers that should conceivably be sensitive to variation in nutrients, tempera- ture and environmental stress [123]. In particular, we expected kelps to be responsive to oce- anic climate since others have noted negative responses to El Niño and temperature [64,124].

Further, subtidal bull kelp (Nereocystis luetkeana) populations in Oregon and northern Cali- fornia collapsed just adjacent to several of our sites due to a combination of warm water and sea urchin overgrazing [75,125]. However, despite the clear effects of climate on nearby subti- dal kelps, the intertidal kelps (primarily Hedophyllum sessile) in our study were markedly con- sistent. This may be linked to their ability to to perennate and regrow from their holdfasts each year [108] unlike the nearby subtidal canopy-forming kelps [126]. The only notable trends in algal/plant taxa were an increase in the mid-to high zone species Endocladia muricata in the mid zone only during years of high air temperature (Fig 8, Table 3b), which could indicate a seaward shift in hot years. Surfgrass (Phyllospadix scouleri and P. serrulatus) abundance also increased with upwelling intermittency in the low zone (Fig 9, Table 3c), which may be linked to higher nutrients in the water during these years [16]. Limited inferences for long-term stability and the potential for imminent change It is possible that the 14-year duration of our study may have been too short to detect climate- induced change, especially in relation to the longer-period climate cycles like PDO and NPGO (20–30 year and 7–15 year cycles, respectively). For example, the declines in mussel cover and increases in algal cover referred to earlier occurred over 20 years [54], and a ~30 year study recently found that west coast intertidal communities were shifting northward at about 3.5–5

km/yr [98]. While it is certainly true that further decades of study are necessary before we can make concrete determinations, several other studies performed at similar decade-long time- scales have detected sensitivity of ecological processes to oceanic climate fluctuations in this system during this period [20,36,54,62]. This suggests our approach is robust, and if strong and immediate effects of these cycles were occurring, we should have been able to detect them. On the other hand, it is likely that there are responses to climate change occurring in this system that we were not able to detect with the indicators of annual community structure, functional group abundance, and diversity we measured here. For example, our plots were not fixed in space, so we were unable to detect small changes in cover of any given species over time. We also did not focus on rarer species that may be responding to climate change. Also, unlike latitudinally-focused studies that have detected range shifts over dozens or hundreds of intertidal sites [67,98,99] our 14-site study was well within the range edges of most of our PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 26 / 34

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

intertidal communities of Oregon and Northern California appear to have been broadly resis- tant to oceanic climate fluctuations over the past 14 years. Supporting information S1 Appendix. Supplemental tables. (DOCX) S2 Appendix. Background on ocean climate indices. (DOCX) S3 Appendix. Supplemental figures. (DOCX) Acknowledgments We thank the numerous technicians and students who performed the community surveys, including Roy Anderson, Silke Bachhuber, Allison Barner, Kris Bauer, Melissa Britsch, Jamie Canepa, Elizabeth Cerny-Chipman, Amanda Chiachi, John Dickens, Kamala Earl, Maya Fee- zell, Laurel Field, Emma Fraser, Heather Fulton-Bennett, Sean Garrity, Angela Johnson, Kailey Kraus, Chantelle MacAdams, Kinsey Matthews, Zech Meunier, Sydney Ngo, Skyler Peven, Megan Poole, Jerod Sapp, Haley Schmidt, Chenchen Shen, Seth Strumwasser, Barbara Spiecker, Jenna Sullivan-Stack, and Megan Urton. This research builds on studies begun by B. Menge, T. Gouhier, S. Hacker, F. Chan, and K. J. Nielsen. We thank Marti Anderson for a great PERMANOVA workshop where she helped us formulate our approach and Cristia´n Monaco for a helpful review. B. Menge conceived of the project and S. Gravem refined its scope. All authors collected the data and J. Robinson managed the data. S. Gravem and B. Poir- son performed the analyses. S. Gravem wrote the manuscript with editing by B. Menge. This is contribution number 536 by the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO, www.PISCOweb.org). Author Contributions Conceptualization: Bruce A. Menge.

Data curation: Sarah A. Gravem, Brittany N. Poirson, Jonathan W. Robinson, Bruce A. Menge. Formal analysis: Sarah A. Gravem, Brittany N. Poirson. Funding acquisition: Sarah A. Gravem, Bruce A. Menge. Investigation: Sarah A. Gravem, Brittany N. Poirson, Jonathan W. Robinson, Bruce A. Menge. Methodology: Sarah A. Gravem, Brittany N. Poirson, Jonathan W. Robinson, Bruce A. Menge. Project administration: Bruce A. Menge. Resources: Bruce A. Menge. Supervision: Bruce A. Menge. Visualization: Sarah A. Gravem. Writing – original draft: Sarah A. Gravem. PLOS ONE Resistance of rocky intertidal communities PLOS ONE | https://doi.org/10.1371/journal.pone.0297697 May 29, 2024 28 / 34

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