Population & community ecology
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Mass mortalities

  Topics relating to population & community ecology include mass mortalities, considered here, and INTERACTIONS WITH KELPS, OTHER MACROALGAE, & SEA OTTERS, REMOVAL-TYPE STUDIES, and GENE FLOW considered in other sections.
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Research study 1

photograph of a diseased red sea-urchin Strongylocentrotus franciscanusMass mortality of sea urchins is not uncommon, as witness several occurrences in California.  The latest report of these concerns red sea-urchins Strongylocentrotus franciscanus in 2 regions near Santa Cruz (Año Nuevo Island and Point Santa Cruz).  At the time of publication the disease agent is unknown but manifests itself in spine loss and test plates layered with a thin greenish surface layer and red friable middle layer, overlying a normal white inner layer.  The authors characterise the mortalities as localised.  Pearse et al. 1977 Fish Bull 75: 645.





Diseased Strongylocentrotus franciscanus.
The cause is not known, but possibly fungal 0.6X

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Research study 2
  photograph of kelp Macrocystis integrifoliagraph showing changes in density of red urchins Strongylocentrotus franciscanus off Point Santa Cruz, California over a 4-year periodA mass mortality of red sea-urchins Strongylocentrotus franciscanus in Monterey Bay, California in early 1976 is documented, along with its apparent effect on the extent of the kelp canopy. The graph on the Right shows changes in density of red sea-urchins between 1974-1977 at 4 locations in a kelp bed off Point Santa Cruz, California. The demise is unexplained.

This is followed by a large increase in canopy cover of brown kelps over the following year. The area of major increase is shown in brown colour on the maps below. Initially, there are 4 species of kelps but, within a year, the competitively dominant giant kelp Macrocystis pyrifera, a favoured food of red sea urchins, becomes a virtual monoculture. Pearse & Hines 1979 Mar Biol 51: 83.

NOTE  these are M. pyrifera, Laminaria dentigera, Pterygophora californica and, to a lesser extent, Nereocystis luetkeana

NOTE the large blades (fronds) of the kelp have gas-filled bladders that float them at the surface.  The area of fronds floating in this way represents the canopy cover of the kelp. The photo shows a northern species of kelp Macrocystis integrifolia with the floats visible.

schematic maps showing increase in size of kelp beds at Point Santa Cruz, California after demise of sea urchins
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Research study 3

graph showing water temperature in kelp forests off Point Loma, California during el Nino events in the mid-1980a In the Point Loma, San Diego area of California storms and warm water associated with El Niño events during 1982-84 are thought to have been the cause of major losses in giant kelps Macrocystis pyrifera.  Intensive fisheries for red sea-urchins Strongylocentrotus franciscanus in the area during the late 1970s and into the early 1980s, however, may have contributed to the eventual recovery of the kelp-beds.  Tegner & Dayton 1991 Mar Ecol Progr Ser 77: 49.

NOTE the graph shows temperature records from 3 sites in the kelp forest, the first 2 peaks representing the causative El Niño eventsphotograph of offshore kelps in Barkley Sound, British Columbia






An embayment in Barkley Sound, B.C. full of kelp. Nearshore
types are Laminaria spp. and Pterygophora sp, extending to
Macrocystis integrifolia
and Nereocystis luetkeana offshore

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Research study 4

graph showing prevalences of diseases in purple sea-urchins Strongylocentrotus purpuratus in southern Californa and Baja California in late summer/autumn correlated with mean summer seawater temperatureStudies of disease in purple sea-urchins Strongylocentrotus purpuratus at several locations in southern California and northern Baja California, Mexico disclose the presence of 2 distinct pathologies, rather than just a single one expected from previous studies.  Both pathologies are common in warmer southern waters, but are rare or absent in the cooler waters north of Point Conception (see graph).  Laboratory experiments show that elevated water temperature leads to larger lesions and significantly lower gonadal indices, suggesting that thermal stress may exacerbate the urchins’ susceptibility. Although a density dependence in disease prevalence is expected, this only occurs with the black-ring pathology, and then in negative relationship; that is, lower disease prevalence correlates with higher densities. Of the 2 diseases, black-ring appears to be most prevalent, with 2 of the Baja California populations exhibiting up to 15% infection in autumn (see graph).  In comparison, prevalence of red-spot disease is generally less than 5%, save for one site in California (South Laguna Beach) that reaches 15% prevalence also in autumn.  The authors note that if thermal stress does increase the urchins’ susceptibility to disease, then warming associated with climate change may have significant effects on the economies of kelp forests and sea-urchin fisheries.  Lester et al. 2007 Can J Fish Aquat Sci 64: 314.

NOTE  the 2 pathologies are “red-spot”, characterised by shortened spines and large areas of the test lacking spines, and “black-ring”, characterised by an outer black ring, and inner lighter area lacking spines and epithelium

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Research study 5

photograph of stranded urchins after mass-mortality incident with purple urchins Strongylocentrotus purpuratusA researcher from the Natural History Museum, Los Angeles describes 2 mass mortalities of purple urchins Strongylocentrotus purpuratus at Malibu Lagoon (2010 and 2011) in great detail. Both incidences appear to be related to exposure to low-salinity lagoon water possibly combined with stress of aerial exposure during especially low tides, and with a degree of sand burial. Hendler 2013 Bull South Cal Acad Sci 112 (1): 19. Photograph courtesy the author.


Tidally stranded Strongylocentrotus purpuratus at Malibu
Lagoon State Park in October 2010. The author remarks
that most individuals have intact spines after death

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Research study 6

before and after photographs documenting a mass-mortality event for purple urchins in central California during August 2015In a later paper, a consortium of mainly University of California scientists describes the August 2011 mass mortality of purple urchins Strongylocentrotus purpuratus in California in more detail and additionally lists incidences of mortalities of other species along the same 100km stretch of California coastline at the same time. During late summer 2011 urchin densities at 11 sites in central California went from 100% occupancy of burrows to essentially zero (only 11 survivors noted out of populations estimated in their millions; see photos). Populations of red sea-urchins Mesocentrotus franciscanus at the same sites but living subtidally suffered 50-90% mortalities. Analyses of climatic and oceanographic records during the main mortality period of August-September 2011 reveal no obvious physical stressors such as temperature, rainfall, or excessive storm surge. Chlorophyll-a concentrations, however, were unusually high during 2 blooms in August 2011, suggesting to the researchers that some type of toxic phytoplankton may have been involved, although how this might work to affect so many diverse species is unclear. Jurgens et al. 2015 PLoS ONE 10 (6): e0126280. Photographs courtesy the authors.

NOTE the authors include a listing of mass mortalities throughout the world from the year 2000, including zoanthids, corals, sponges, sea stars, sea urchins, and other marine species, so the paper provides good historical data, albeit of fairly recent occurrence

NOTE other echinoderms, including sea stars Leptasterias sp., were also decimated at the same time. Additionally, ochre stars Pisaster ochraceus, red abalone Haliotis rufescens, and gumboot chitons Cryptochiton stelleri suffered some mortality but not to the same extent as the other 2 species

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