Population & community ecology
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Removal-type studies

  Topics relating to population & community ecology include removal-type studies, considered here, and INTERACTIONS WITH KELPS, OTHER MACROALGAE, & SEA OTTERS, GENE FLOW, and MASS MORTALITIES considered in other sections.
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Research study 1

photograph of a shore area in Barkley Soound featuring kelp Hedophyllum sessile, a favoured food of purple sea urchinsIf you want to know what role a species plays in the community, then one way to find out is to remove it and monitor what happens. A study in Mukkaw Bay, Washington, in which all purple urchins Strongylocentrotus purpuratus are removed from shallow intertidal tidepools, results in the appearance of numerous species of opportunistic algae.  The order of appearance of these algae is in accordance with the seasonal release of spores.  In time, the fleshy brown alga Hedophyllum sessile becomes the dominant of the larger canopy-forming species in the tidepools.  In nearby control tidepools, containing the original complement of sea urchins, the existing population of small calcareous and fleshy seaweeds is maintained. A longer-term photograph of purple sea-urchins Strongylocentrotus purpuratus in a tide-poolremoval experiment, this time with red urchins Strongylocentrotus franciscanus in subtidal regions in San Juan Island, Washington, leads during the first year to dominance in the habitat by bull kelps Nereocystis luetkeana, and in the second and third years to dominance by Laminaria groenlandica and other large kelps.   Paine & Vadas 1969 Limnol & Oceanogr 14: 710.

Purple urchins Strongylocentrotus purpuratus in a
tidepool with coralline algae and some fleshy forms

Brown alga Hedophyllum sessile (sea cabbage) growing
with the red alga Halosaccion glandiforme 0.1X

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Research study 2

Similar results are noted in a later sea urchin-removal experiment in Torch Bay, Alaska where, 1yr after removal of all sea-urchins Strongylocentrotus spp., an association of kelps develops of high biomass and diversity (5 species).  In the second year after removal of the photograph of kelp Lamniaria groenlandicasea urchins, however, a single kelp species Laminaria groenlandica becomes dominant.  Because of displacement of most annuals such as the kelp Nereocystis luetkeana by the competitively superior Laminaria, total biomass of algae actually decreases 2yr following removal of the sea urchins.  Duggins 1980 Ecology 61: 447.

NOTE this kelp is perennial and has broad fronds which not only shade smaller species, but likely sweep the area clean of microsopic gametophyte and young sporophyte stages during storms

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Research study 3

map showing study sites used in sea-urchin removal experiment, using Strongylocentrotus purpuratusphotograph of several purple sea-urchins Strongylocentrotus purpuratusA trend in the distribution of intertidal algae is for large brown species, either laminarians or fucoids, to predomonate at higher latitudes and turf-forming red algae to predominate at lower latitudes.  The role that sea-urchin grazing might be playing in preventing large brown algae from dominating in the low intertidal zone of southern California is tested by removing urchins Strongylocentrotus purpuratus at 2 locations.  Removal results in rapid algal recruitment.  Crustose corallines Lithothamnion spp. that are not eaten by urchins are rapidly overgrown by several species of short-lived red and brown algae, followed by recruitment of long-lived brown algae suth as Egregia, Cystoseira, and Halidrys.  Later on, however, both experimental sites become dominated by perennial red algae such as Gigartina, Laurencia, and Gastroclonium.  The extent of the red-algal ascendancy depends upon the timing of the sea-urchin removal.  If removed just prior to settlement season, then a dense cover of brown algae results.  However, if removed earlier, there is sufficient time for the perennial red algal species to preempt the space.  Sousa et al. 1981 Oecologia 48: 297.

NOTE  the study sites are Ellwood Beach and White's Point. The experiments are conducted over a period of 1-2yr

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Research study 4

Researchers at Moss Landing Marine Laboratories, California assess the effects of sea-urchin grazing on community structure in a forest of kelp Macrocystis pyrifera off the central California coast by removing all red urchins Strongylocentrotus franciscanus from shallow and deep red urchins Strongylocentrotus franciscanus in deep water on the seaward side of kelp bedsites, then monitoring algal and invertebrate populations for a 12mo period.  Analysis of the data is made more complex by a severe storm that hit the area midway through the study.  The storm removed most of the kelp canopy as well as portions of the substratum.  Those sea urchins not washed away by the storm initially clumped or sheltered under ledges, then dispersed, leading later on to a 6-fold increase in understory algal cover.  Although the storm complicated things somewhat, overall the urchin-removal treatments showed higher kelp cover later on than control plots. Cowen et al. 1982 J Exp Mar Biol Ecol 64: 189.

NOTE  a total of 3370 individuals is removed from 2 sites


Red sea-urchins Strongylocentrotus franciscanus
on the seaward side of a kelp bed

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Research study 5

A research study based at Friday Harbor Laboratories, Washington investigates effects of removal of red sea-urchins Strongylocentrotus franciscanus on kelp-forest density at several locations in San Juan Channel between the islands. There are 3 treatments over a 1.5yr period, each with 3 replicates: monthly removals, involving removal of all urchins from each of 3 sites each month; annual removals, involving removal of all legal-sized (102-114mm test diameter) urchins at Month 0 (March 1997) and again at Month 13 (March 1998) from each of 3 sites, and controls, involving no urchin removals at 3 sites. Results show no significant effects on densities of 8-or so species of kelps in any area for either removal treatment. The authors conclude that other factors, for example, grazing by non-urchin invertebrates, may be involved as major influences on community structure in the San Juan Channel. Carter et al. 2007 Hydrobiologia 579: 233.

NOTE the study is really a test of the so-called “sea-otter paradigm”, which is an aggrandised statement of the obvious, that by eating sea urchins, sea otters increase macroalgal biomass and vice versa. Supporters of the “paradigm” seem to be mostly researchers who favour Keystone Predator status for sea otters. Sea otters neither fit the definition for this nor has support for their elevation to this status been anything more than weak (see section in the ODYSSEY dealing with Keystone Predators; for early supporters/originators of the "paradigm see Estes & Palmisano 1974 Science 185 (4156): 1058). The present authors find no support for the idea in their study

NOTE this first treatment attempts to mimic sea-otter predation in an area where no sea otters exist, while the second attempts to mimic effects of annual commercial harvesting of sea urchins in an area where no harvesting is permitted. The study ostensibly deals with community effects of sea-otter predation but, as the otters are not actually featured, this synopsis is more appropriately placed here, in the section of the ODYSSEY dealing with sea-urchin removals

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Research study 6

graph showing relationship of consumption rate of purple sea urchins Strongylocentrotus purpuratus in relation to richness of bottom cover of macroalgae and sessile invertebratesAn interesting set of experiments by researchers from University of California and University of Michigan tests how richness of sessile species1 in kelp-forest understory communities affects consumption rates of sea urchins Strongylocentrotus purpuratus.  The design of the caged array2 at 8m depth is complex, involving replicated crosses of 5 sea-urchin densities with 3 levels of species richness (low, medium, and high), done at overall “high” and “low” sea-urchin densities.  Results are understandably complex and cannot be considered in detail here.  Overall, however, they indicate that consumption rates of sea urchins increase as a function of richness of macroalgal and sessile invertebrate species, but only under conditions of generally low sea-urchin densities (0-14 individuals per caged plot; see graph).  Note in the graph that as initial species richness goes up, significantly more of the bottom cover is removed by the urchins.  Where sea-urchin densities are generally high3 (0-50 individuals), however, the relationship breaks down, as nearly all algal and invertebrate biomass is consumed.  In “real-life” scenarios this leads to creation of “barrens areas” where the substratum is essentially denuded of all life.  Although the overall results are not unexpected, the research protocol, implementation difficulty, data analyses, and general originality of the study, are to be admired.  There is much more to the study than presented here.  Byrnes et al. 2013 Ecology 94 (7): 1636.

NOTE1 these include both algae and invertebrates.  When hungry, and oftentimes not even then, sea urchins will eat all manner of animal matter

NOTE2  the total number of fenced plots is 90 which, considering that assembly and monitoring over the 2wk experimental period are done using SCUBA, is a remarkable feat.  The cages, 0.5m2 area x 0.75cm height and constructed of weighted PVC tubing, are set up in summer in a kelp-forest area, but in one where all kelps Macrocystis pyrifera have been removed the preceding winter in a separate experiment

NOTE3 the highest densities correspond to the natural range in sea-urchin densities found in surrounding reef areas

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Research study 7

Red sea-urchins Mesocentrotus franciscanus and lined chitons Tonicella spp. commonly occupy the same alga- and invertebrate-encrusted subtidal rock habitats and, as they consume the same kinds of foods, are thought to be functionally redundant. In other words, if one of the grazers is absent its “job” of clearing and maintaining available space will be assumed by the other. Urchins eat macroalgae species of larger size and sessile invertebrates, while chitons graze on microalgae, sporlings of larger algae, and early settling stages of invertebrates. To test whether the 2 types of grazers are functionally redundant, researchers at Friday Harbor Laboratories, Washington remove one or the other, or both types to the best extent possible from horizontal subtidal (12-18m depth) transect plots in the San Juan Islands, Washington over a 9mo period (Apr-Jan). Changes in abunance of rock-dwelling flora and fauna are monitored for 1yr prior, during, and for 1yr following grazer removals. Results show no effects on the community for removal of each grazer in isolation, but removal of both leads to a significant increase in abundance of sessile invertebrates, mainly compound colonial tunicates (75% increase), and to a decrease in algal cover. Although the experimental protocol has an imbalanced design (see Note below) and the results highly variable, the researchers fairly conclude that the two grazers are redundant to a degree in the maintenance of available space (defined here as encrusting algae and bare rock). The researchers should be congratulated for undertaking such a challenging study. Elahi & Sebens 2013 PLoS ONE 8 (11): e78969.

NOTE owing to the different size and motility of the 2 species, removals are done at different scales: entire transects (2.5 x2m) for urchins, and quadrats (approx. 1m2 ) for chitons. Because the quadrats are nested within the transects, this design allows effects of urchin removal on chiton abundance to be tested, but not chiton removal on urchin abundance

photograph of red urchin Metacentrotus franciscanus clearing a patch in a corallimorphara colony "photograph of 2 lined chitons Tonicella spp. clearing an area in of encrusting red algae
The grazing activity of this red urchin Mesocentrotus franciscanus keeps a patch cleared of corallimorpharians Corynactis californica A patch cleared of encrusting red algae and other organisms by 2 lined chitons Tonicella spp.
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