Quantitative biogeography: Decreasing and more variable dynamics of critical species in an iconic meta‐ecosystem

Abstract

Ecosystem stability has intrigued ecologists for decades, and the realization that the global climate was changing has sharpened and focused this interest. One possible early warning signal of decreasing stability is increasing variability in ecosystems over time with increasing climate variability. Determining 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. We investigated spatiotemporal changes in mean and variability of ecological subsidies (nutrients, phytoplankton, prey colonization), performance metrics of a dominant space occupier (mussels) and its primary predator (sea stars), and sea star predation rates on mussels in relation to climatic oscillations, temperature, and disease on rocky shores. The research involved annually repeated multiyear (~1999–2018), multisite (13 sites nested within five 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 2014–2016 combined El Niño and Marine Heat Wave. Ecological subsidies generally declined during the study period but 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 decreased. Analyses indicated that the primary drivers of these changes were temperature‐related environmental factors. As declining means and increasing variability of ecological performances can 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.

Resistance of rocky intertidal communities to oceanic climate fluctuations

“…0.12, 0.07, and 0.09, respectively). As reported earlier [ 12 ], a period of elevated water temperature occurred during the confluence of the 2014–16 marine heat wave and the 2015–16 El Niño ( Fig 3b ). As expected, warmer water temperature was correlated to stronger PDO (R 2 = 0.523, P = < 0.001), but no other correlations of temperatures with climate oscillations were significant.…”

“…Our recent work shows some worrisome ‘early warning signals’ of impending change in this rocky intertidal system [ 11 , 12 ]. For example, annually repeated disturbance experiments to track recovery rates in the low zone at several sites in Oregon revealed increasing community instability (more variance in disturbed and undisturbed plots over time) and diminishing resilience (slower recovery from disturbance over time [ 11 ]).…”

“…Further, overall declines and/or increasing variability in several key ecological metrics have occurred, including ecological subsidies to the nearshore from offshore (i.e. phytoplankton and invertebrate recruitment), mussel performance (body condition and reproductive output), and sea star performance (reproductive output and predation rates [ 12 ].…”

“…For example, the intensity and relative ecological dominance of NPGO versus PDO may be increasing [ 30 , 31 ], extreme El Niño and La Niña events are expected to increase [ 32 ]. On the other hand, expectations for upwelling are mixed [ 12 , 33 , 34 ]. Warming of both air and sea temperatures is ongoing and is expected to continue with climate change [ 12 , 35 ].…”

“…On the other hand, expectations for upwelling are mixed [ 12 , 33 , 34 ]. Warming of both air and sea temperatures is ongoing and is expected to continue with climate change [ 12 , 35 ].…”