Ecosystem stability has intrigued ecologists for decades, and the realization that the global climate was changing sharpened and focused this interest. Determination of climate change effects on community stability, however, requires long-term studies of structure and underlying dynamics, including bottom-up and top-down effects in natural ecosystems. Although relevant datasets were rare in the early years of community ecology, such information has increased in recent decades. In a rocky intertidal system, we investigated the changes in ecological subsidies (nutrients, phytoplankton, prey colonization), several performance metrics of the dominant space occupier (mussels) and its primary predator (sea stars), and rate of predation by sea stars on mussels in relation to climatic oscillations, temperature, and disease. We focused on spatio-temporal changes in the mean and variability of these metrics. The research protocol involved annually repeated multiyear (~1999–2018), multisite (13 sites nested within 5 regions along ~260 km of the Oregon coast) observations, measurements, and experiments. We analyzed associations between environmental variables and ecological performance of key elements of the sea-star-mussel-dominated mid-intertidal system. We found that upwelling declined in some regions, but became more variable across all study regions. Air and water temperatures oscillated, but their mean and variation increased through time, with peak values coinciding with the 2014–16 combined El Niño and Marine Heat Wave. Ecological subsidies generally declined during the study period, and bottom-up processes increased in variability. Excepting growth rate, mussel (Mytilus californianus) performance (condition index, reproductive output) generally decreased and became more variable. Primarily due to a sea star wasting epidemic, reproductive output of the top predator Pisaster ochraceus decreased and became more variable, and predation rate on mussels also dropped abruptly. Analyses indicated that the primary drivers of these changes were temperature-related environmental factors. Since declining means and increasing variability of ecological performances are thought to typify destabilizing ecosystems, and environmental trends are toward ever more stressful conditions, the outlook for this iconic ecosystem is discouraging. Immediate and rapid action to mitigate and ultimately reverse climate change likely is the only option available to prevent an irreversible shift in the future of this, and most other ecosystems.

Climate measures (NPGO, ENSO, PDO) and upwelling (CUTI) indices were downloaded from websites given in the paper. Temperature data were collected in situ with dataloggers. Mussel and barnacle colonization, mussel growth/reproduction/condition index, nutrients, chlorophyll-a, sea star reproduction rate and sea star predation rate data were collected as noted in the paper using observations and experiments. Processing of all samples is described in the paper, as is data analysis.

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