Sarah A. Gravem

Research on intertidal community structure and recovery in the California Current System has largely focused on macrophytes and invertebrates occupying two‐dimensional, readily studied “open” rock surfaces. However, most rocky shores have a “third” dimension that includes channels, cracks, crevices, and overhangs whose organismal assemblages, termed “cryptic communities,” are poorly studied. Cryptic communities not only share many species with those on more accessible surfaces but also include high abundances of colonial invertebrates such as tunicates, sponges, bryozoans, and hydrozoans. We investigated species abundance and diversity of cryptic communities and tested their recovery from disturbance by comparing removal plots to undisturbed controls for ~1.5 years. Additionally, we tested whether community structure and recovery varied with contrasting large‐scale levels of ecological subsidies (invertebrate recruitment, nutrients, and phytoplankton) and local‐scale microhabitat differences (emersion and solar irradiation) on the Oregon Coast. We compared cryptic recovery rates to recovery rates on open‐surface communities. In cryptic communities, site explained most (92%) variance in community structure of undisturbed plots, while microhabitat metrics had little (1.2%) effect. Further, recovery rates were faster at a site with higher subsidy inputs than one with lower subsidies in both cryptic and noncryptic communities. Hence, larger scale environmental drivers appeared more important than local‐scale drivers within cryptic communities. Our research provides novel insight into intertidal cryptic surge channel community structure and dynamics.

Determining the relative influence of biotic and abiotic processes in structuring communities at local to large spatial scales is best understood using a biogeographic comparative‐experimental approach. Using this approach, previous work suggests that intertidal community dynamics (top‐down and bottom‐up effects) vary unimodally along an upwelling‐based productivity gradient, termed the Intermittent Upwelling Hypothesis (IUH). Evidence consistent with the IUH comes from the sessile invertebrate/predator (SIP) subweb in certain rocky intertidal communities, but whether this pattern extends to macrophyte/herbivore (MH) subwebs is unknown. Here we ask: Are MH subwebs also structured as predicted by the IUH? What is the relative importance of herbivory and predation in structuring these communities? Under what conditions do ecological subsidies like nutrients or propagule production drive community dynamics? And are omnivorous interactions important? We hypothesize that MH subwebs are driven by a new construct, the Grazing‐Weakening Hypothesis (GWH), which states that MH interactions weaken monotonically with increasing nutrients, with strong (weak) herbivory and low (high) macrophyte productivity at low (high) nutrients. We explored local‐to‐large spatial scale dynamics of both subwebs using a biogeographic comparative‐experimental factorial field experiment testing joint and separate effects of herbivores and predators between two continents. Experiments at 10 sites ranging from persistent upwelling to persistent downwelling regimes ran for 26–29 months in Oregon and California, and New Zealand (NZ) South Island. For the MH subweb, results were consistent with the GWH: herbivory declined and macrophytes increased with increasing nutrients. As expected, results for the SIP subweb were consistent with the IUH: predator effect size was unimodally related to upwelling. Overall, herbivory explained more variation in community structure than did predation, especially in NZ. Omnivory was weak, sessile invertebrates outcompeted macrophytes, and ocean‐driven subsidies provided the basic template driving ecosystem dynamics. We propose a unified meta‐ecosystem dynamics model combining MH and SIP results: with increased upwelling, sessile invertebrates and underlying dynamics vary unimodally (as in the IUH), while herbivory decreases and macrophytes generally increase. While this model was based on research in temperate ecosystems varying in upwelling regime, its wider applicability remains to be tested.

The recent collapse of predatory sunflower sea stars ( Pycnopodia helianthoides ) owing to sea star wasting disease (SSWD) is hypothesized to have contributed to proliferation of sea urchin barrens and losses of kelp forests on the North American west coast. We used experiments and a model to test whether restored Pycnopodia populations may help recover kelp forests through their consumption of nutritionally poor purple sea urchins ( Strongylocentrotus purpuratus ) typical of barrens. Pycnopodia consumed 0.68 S. purpuratus d −1 , and our model and sensitivity analysis shows that the magnitude of recent Pycnopodia declines is consistent with urchin proliferation after modest sea urchin recruitment, and even small Pycnopodia recoveries could generally lead to lower densities of sea urchins that are consistent with kelp-urchin coexistence. Pycnopodia seem unable to chemically distinguish starved from fed urchins and indeed have higher predation rates on starved urchins owing to shorter handling times. These results highlight the importance of Pycnopodia in regulating purple sea urchin populations and maintaining healthy kelp forests through top-down control. The recovery of this important predator to densities commonly found prior to SSWD, whether through natural means or human-assisted reintroductions, may therefore be a key step in kelp forest restoration at ecologically significant scales.

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.