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Mass mortalities in natural populations, particularly those that leave few survivors over large spatial areas, may cause long-term ecological perturbations. Yet mass mortalities may remain undocumented or poorly described due to challenges in responding rapidly to unforeseen events, scarcity of baseline data, and difficulties in quantifying rare or patchily distributed species, especially in remote or marine systems. Better chronicling the geographic pattern and intensity of mass mortalities is especially critical in the face of global changes predicted to alter regional disturbance regimes. Here, we couple replicated post-mortality surveys with preceding long-term surveys and historical data to describe a rapid and severe mass mortality of rocky shore invertebrates along the north-central California coast of the northeastern Pacific Ocean. In late August 2011, formerly abundant intertidal populations of the purple sea urchin (Strongylocentrotus purpuratus, a well-known ecosystem engineer), and the predatory six-armed sea star (Leptasterias sp.) were functionally extirpated from ~100 km of coastline. Other invertebrates, including the gumboot chiton (Cryptochiton stelleri) the ochre sea star (Pisaster ochraceus), and subtidal populations of purple sea urchins also exhibited elevated mortality. The pattern and extent of mortality suggest the potential for long-term population, community, and ecosystem consequences, recovery from which may depend on the different dispersal abilities of the affected species.
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26039349
???displayArticle.pmcLink???PMC4454560 ???displayArticle.link???PLoS One
Fig 1. Photograph of dead sea stars.Ochre sea star (Pisaster ochraceus) carcasses washed up en masse in late August 2011. Photo credit: Ashley Robart.
Fig 2. Map of the study region (north-central California, USA).The region between dashed lines marks the zone of invertebrate mortality, which spanned approximately 100 km of coastline (fractal unit 0.1 km). Sites are numbered north to south from Van Damme State Park (Site 1, 39.16° N) to Duxbury Reef (Site 24, 37.53° N). See S1 Table for GPS locations and details of surveys conducted at each location.
Fig 3. Purple urchin burrows in bedrock, with and without occupants.(A) A large tidepool outside the impact zone (site 4), with a dense population of live Strongylocentrotus purpuratus in November 2011. (B) Empty urchin burrows in a tidepool (site 16) after the mass mortality event. (C) Close-up of occupied purple urchin burrows, showing one urchin per burrow and 100% occupancy; note urchins are covered with debris. (D) Close-up of clean, empty urchin burrows such as those used to estimate pre-event densities.
Fig 4. Densities of purple urchins (A) before and (B) after the mass mortality event, by latitude.Bars depict standard deviation across sampled quadrats. Data are from the 11 sites surveyed between September and December 2011 (sites 1–4, 6, 10, 14, 16, 18, 23, 24). The shaded area denotes the impacted region, in which we found zero individuals at all sites surveyed after the event (n = 5).
Fig 5. Subtidal-zone densities of purple urchins.Data are shown by depth class, and represent counts (mean ± SE) in transects surveyed before (grey; n = 300) and after (black; n = 146) the mass mortality event. Data from four sites (11–13, 15) in the impact zone are pooled.
Fig 6. Six-armed sea star presence and absence before and after the mass mortality.Data are shown for surveys before the 2011 mass mortality (2001–2010) and after (2012). In the intervening period, Leptasterias sp. became absent from every site within the die-off region in which purple sea urchins experienced mortality (bordered by dashed lines; sites 7, 8, 10, 14, 18, 19). Six-armed sea stars were still found at sites outside the affected region in 2012, as in prior years (sites 3, 4, 24).
Fig 7. Post-event densities of P. ochraceus and C. stelleri.Data are from swath transects, shown by site latitude. Despite many carcasses of these species being observed on shores during the die-off, surveys did not show consistently depressed density within the impact zone (defined by the loss of S. purpuratus and Leptasterias sp. and indicated by dashed lines). See S3 Table for raw counts and area surveyed per site.
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