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ABSTRACT AimSurveying the demography of populations near species range edges may indicate their vulnerability to range contractions or local extinction as the climate changes. In the rocky intertidal, not only are latitudinal ranges constricted by thermal stress, but tides also create zonation or a ‘vertical range’ driven by sharp environmental gradients. By investigating demographics along the latitudinal and vertical ranges simultaneously, we can investigate whether populations may be vulnerable to a changing climate. LocationRocky intertidal habitats along west coast of the United States. TaxaOchre sea starPisaster ochraceus, six‐armed sea starLeptasteriasspp., emarginate whelks(Nucella ostrina and N. emarginata) and channeled whelkN. canaliculata. MethodsIn 2018, we surveyed the demographics of the taxa above at 33 sites spanning > 11° latitude from central Oregon to southern California, near the southern range limits of each taxon. We counted and sized individuals from the high to low intertidal zone. To understand how environmental stress changed with latitude, we evaluated intertidal temperaturesin situ, as well as tidal extremes, tidal amplitude and wave exposure using offshore buoys. ResultsFor all taxa, population density, the relative proportion of smaller individuals (except for emarginate whelks) and the upper vertical limits on the shore declined from north to south as temperatures increased and high tide height, tidal amplitude and wave heights decreased. In addition, smaller individualLeptasteriasspp. generally inhabited lower shore levels while smaller individual emarginate whelks inhabited higher shore levels coastwide. ForN. canaliculata, smaller animals were higher on shore northward, but lower on shore southward. Main ConclusionsWhile this study is a snapshot in time and cannot assess impacts of climate change, our surveys suggest environmentally‐related demographic limitation toward southern range limits and demographically vulnerable southern populations. Therefore, a warming climate may cause local extinctions or range contractions near southern limits. 

Abstract Keystone predation can be a determinant of community structure, including species diversity, but factors underlying “keystoneness” have been minimally explored. Using the system in which the original keystone, the sea starPisaster ochraceus, was discovered, we focused on two potential (but overlapping) determinants of keystoneness: intrinsic traits or state variables of the species (e.g., size, density), and extrinsic environmental parameters (e.g., prey productivity) that may provide conditions favorable for keystone predator evolution. Using a comparative‐experimental approach, with repeated field experiments at multiple sites across a variable coastal environment, we tested predation rates, or how quickly predators consumed prey, and predation effects, or community response to predator presence or absence. We tested five hypotheses: (H₁) predation rates and effects will vary in space but not time; (H₂) per population predation rates will vary primarily with individual traits and population variables; (HJHH₃) per capita predation rates will vary only with individual traits; (H₄) predation effects will vary with traits, variables, and external drivers; and (H₅) as predicted by the keystone predation hypothesis, diversity will vary unimodally with predation pressure. As hypothesized, predation rates differed among sites but not over time (H₁), and in caging exclusion experiments, predation effect varied with both intrinsic and extrinsic factors (H₄). Unexpectedly, predation rates varied with both intrinsic and extrinsic (H₂, per population), or only with extrinsic (H₃, per capita) factors. Further, in large‐plot exclusion experiments, predation effect was most closely associated with individual traits (contraH₄). Finally, taxon diversity varied unimodally with proxies of predation pressure (sessile prey abundance) and was sensitive to extrinsic factors (mussel growth, temperature, and upwelling,H₅). Hence, keystoneness depended on predator individual traits, predator population variables, and environmental parameters. However, temporal differences in caging experiments suggested that environmental characteristics underlying prey dynamics may be preeminent. Compared to prior experiments, predation was weaker with low prey input compared to periods with high prey input. Collectively, our results suggest that keystone predator evolution depends on the coalescence of species‐specific characteristics, and environmental parameters favoring high prey productivity. Our approach may be a model for future studies exploring the generality of keystoneness. 

A powerful way to predict how ecological communities will respond to future climate change is to test how they have responded to the climate of the past. We used climate oscillations including the Pacific Decadal Oscillation (PDO), North Pacific Gyre Oscillation, and El Niño Southern Oscillation (ENSO) and variation in upwelling, air temperature, and sea temperatures to test the sensitivity of nearshore rocky intertidal communities to climate variability. Prior research shows that multiple ecological processes of key taxa (growth, recruitment, and physiology) were sensitive to environmental variation during this time frame. We also investigated the effect of the concurrent sea star wasting disease outbreak in 2013–2014. We surveyed nearly 150 taxa from 11 rocky intertidal sites in Oregon and northern California annually for up to 14-years (2006–2020) to test if community structure (i.e., the abundance of functional groups) and diversity were sensitive to past environmental variation. We found little to no evidence that these communities were sensitive to annual variation in any of the environmental measures, and that each metric was associated with < 8.6% of yearly variation in community structure. Only the years elapsed since the outbreak of sea star wasting disease had a substantial effect on community structure, but in the mid-zone only where spatially dominant mussels are a main prey of the keystone predator sea star,Pisaster ochraceus. We conclude that the established sensitivity of multiple ecological processes to annual fluctuations in climate has not yet scaled up to influence community structure. Hence, the rocky intertidal system along this coastline appears resistant to the range of oceanic climate fluctuations that occurred during the study. However, given ongoing intensification of climate change and increasing frequencies of extreme events, future responses to climate change seem likely. 

Climate change threatens to destabilize ecological communities, potentially moving them from persistently occupied “basins of attraction” to different states. Increasing variation in key ecological processes can signal impending state shifts in ecosystems. In a rocky intertidal meta-ecosystem consisting of three distinct regions spread across 260 km of the Oregon coast, we show that annually cleared sites are characterized by communities that exhibit signs of increasing destabilization (loss of resilience) over the past decade despite persistent community states. In all cases, recovery rates slowed and became more variable over time. The conditions underlying these shifts appear to be external to the system, with thermal disruptions (e.g., marine heat waves, El Niño–Southern Oscillation) and shifts in ocean currents (e.g., upwelling) being the likely proximate drivers. Although this iconic ecosystem has long appeared resistant to stress, the evidence suggests that subtle destabilization has occurred over at least the last decade.