OOS 37-1 - Increasing instability of rocky intertidal communities driven by climate change

Background/Question/Methods
Climate change threatens to destabilize ecological communities, potentially moving them from persistently-occupied “basins of attraction” to alternate states. Decreasing performance and increasing variation in communities can signal impending state shifts in ecosystems. Although species abundances can vary through time, Oregon coast rocky intertidal communities have appeared to persist around long-term “mean” states for decades. We asked if this perception was underpinned by resilience; i.e., if the system rebounded from repeated perturbations back to the mean state. We tested the resilience of low intertidal communities using annual disturbance-recovery experiments. We initiated the experiment in July 2011 by permanently marking five haphazardly-sited pairs of 0.5 x 0.5 m plots. Plots were photographed, then one plot of each pair was cleared of all macrobiota. Disturbed plots recovered for 12 months, then were photographed and re-cleared. Experiments were conducted at paired sites (separated by 0.5 to 10 km) nested within three regions (6 sites total) along ~260 km of the Oregon coast. Resilience was measured by comparing the recovery of plots at each site to the undisturbed plots, as measured by Euclidean distance, and stability was measured by plotting the mean and variability in community structure of both cleared and control plots over time.

Results/Conclusions
Compared to intact plots, the mean state of disturbed communities declined and became more variable during the 2010s. Further, Euclidean distance between intact and disturbed plots increased as did the variability in this metric. The conditions driving these shifts appeared external, with thermal disruptions (e.g., marine heat waves, El Niño-Southern Oscillation) and shifts in ocean currents (e.g., changing upwelling) being the likely proximate drivers. At the larger scale of the Oregon coast, our research suggests that, although these communities have persisted in similar cape-specific states for years, thus demonstrating resistance and resilience to major perturbations such as the 1982-83 and 1997-98 El Niño events, system resilience may be diminishing. That is, recovery of community structure has slowed and variability in community structure has increased, consistent with the ideas of “critical slowing down” and “early warning signals” as a system destabilizes. Although the future trajectories of these communities are uncertain, and in fact trends could reverse, with ever-worsening climate change the likelihood is long-term decline, and possibly crossing a tipping point into an alternate state. Only a drastic and immediate decline in greenhouse gas emissions is likely to prevent this catastrophe.