Population & community ecology

Sea urchins, by their large size, voracious appetites, and often dense numbers potentially have profound effects on intertidal and subtidal community structure.  World-wide, they have been popular candidates for removal-type experiments and, on the west coast, there has been keen interest in the interaction of sea urchins, kelps, and sea otters.


photograph of a green urchin Strongylocentrotus droebachiensis and a red urchin S. franciscanus
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Interactions with kelps, other macroalgae, & sea otters

  Topics relating to population & community ecology include interactions with kelps, other macroalgae, & sea otters, considered here, and REMOVAL-TYPE STUDIES, MASS MORTALITIES, and GENE FLOW, considered in other sections.
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Research study 1
  artist's drawing of a sea otter eating red urchins courtesy Sue ColemanDwindling giant kelp beds in the Point Loma area near San Diego, California during the mid-1900's prompted members of the Kelp Habitat Improvement Project to eliminate sea urchins, both by application of quicklime (CaO) and crushing with hammers. The chief aim of the project was to "enhance" the underwater environment by encouraging kelps to grow and thereby improve habitat for fishes. What caused the decline of kelps in California? Several authors attribute this to almost complete removal of sea otters Enhydra lutris from Californian waters during the 1800's. This allowed red sea-urchins Strongylocentrous spp. to flourish, which caused kelp beds to diminish. When sea otters re-appear in an area, change can come quickly. For example, prior to 1963, southern Monterey Bay had dense populations of sea urchins, but little kelp. Within a year of incursion of sea otters, the area became mostly free of sea urchins and beds of giant kelp Macrocystis and other seaweeds were abundant. Cause and effect, no, but suggestive...yes. McLean 1962 Biol Bull 122: 95; Leighton et al. 1966 p. 141, In Proc Int Seaweed Symp, 5th, Halifax (Young, ed.) Oxford: Pergamon Press; Kelp Habitat Improvement Project 1967, 1972, 1973 WM Keck Laboratory of Environmental Health Engineering, Cal Institut Tech; Lowry & Pearse 1973 Mar Biol 23: 213. Drawing of sea otter E. lutris eating red urchins S. franciscanus courtesy Sue Coleman.
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Research study 1.1

graph showing relationship of kelps Laminaria spp. and sea urchins Strongylocentrotus polyacanthus in Amchitka Island, AlaskaAlthough primarily dealing with competitive interactions between kelps, a study by a researcher from Scripps Institution of Oceanography, California at Amchitka Island, Alaska includes some points of interest relating to sea urchins Strongylocentrotus polyacanthus.  First, sea urchins are all but absent in depths shallower than about 20m and their presence in large numbers at that depth sets the lower limit of distribution of the deep-dwelling kelp species, mainly Laminaria spp. (see graph)  Second, kelp holdfasts in shallow water may provide shelter for small sea urchins against sea-otter Enhydra lutris predation.  This is suggested by disappearance of the urchins when the researcher removes the overlying kelp canopy as part of another experiment, presumably leaving the urchins potentially more open to discovery by otters.  Dayton 1975 Fishery Bull 73 (2): 230.

NOTE  the author notes that at the time of writing the actual species name was uncertain, either polyacanthus or droebachiensis

NOTE  sea otters are present in the area during the study period and are inferred by the author to be feeding intensively on sea urchins, but no otter-free control areas are surveyed

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Research study 2

histograms showing effect of sea otters on kelp diversity in areas of AlaskaWhat effect do sea otters Enhydra lutris, active consumers of sea urchins, have on seaweed biodiversity?  This is assessed in Alaska by comparing 3 different bays: 1) Torch Bay, where sea otters are absent, 2) Deer Harbor, where sea otters have been present for less than 2yr (Time 0yr = 1976), and 3) Surge Bay, an area in which sea otters have been present for about 10yr. Note that with increasing presence of sea otters, numbers of sea urchins correspondingly decrease, and densities of seaweeds increase.  For example, where sea otters are present and sea-urchin densities low (Deer Harbor), annual kelps predominate and the perennial kelp Laminaria groenlandica is in low density.  Where sea otters have had a long presence and sea urchins are absent (Surge Bay), the perennial kelp L. groenlandica dominates.  The study demonstrates the competitive superiority of L. groenlandica, the preference of Strongylocentrotus species for this seaweed as food, and the keystone-predator role played by Enhydra lutris in the dynamics of the nearshore marine community.  Duggins 1980 Ecology 61: 447.



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Research study 2.1

Seven years after sea otters Enhydra lutris are re-introduced into the Bunsby Islands, British Columbia researchers from the Pacific biological Station, Nanaimo compare “before and after” areas for differences in community composition. At 7 locations where sea otters are thought to have been feeding, sea urchins Strongylocentrotus franciscanus are scarce and restricted to crevice and under-rock habitats.  Kelps and other algae colonise the bottom to 10m depths, extending downwards from sublittoral fringe communities (such areas are apparently identifiable from the air).  Other sea-otter prey items, including abalone, top shells, rock scallops, and green and purple sea-urchin species are also scarce.  In contrast, at 5 sites where sea otters are thought not to have been feeding, sea urchins are abundant and large in size, and kelps are limited to shallow water.  Extensive grazing by sea-urchins at these sites creates “barrens” areas that  begin in shallow waters.  Although direct evidence photograph of red sea-urchins Strongylocentrotus franciscanus in an algal "barrens" areais lacking, the researchers infer that sea-otter depredation on sea urchins is the cause of these community differences.  Breen et al. 1982 Mar Ecol Progr Ser 7: 13.

NOTE  from the time of Cook’s explorations in 1778 to the early 1900s, sea otters were hunted to virtual extinction.  Introductions were made to the Bunsby Islans area of Vancouver Island in 1969-72

NOTE  this inferred hiding behaviour is a common finding in such studies, but no researcher has yet questioned how sea urchins are aware that predators are in the vicinity.  Is a “fright” pheromone released from damaged conspecifics that induces a hiding behaviour? or is it just that individuals with a propensity to hide are more likely to survive?


Large numbers of red sea-urchins Strongylocentrotus franciscanus
in an algal "barrens" area about 1m below the seaward edge of a kelp bed Nereocystis luetkeana in Barkley Sound, British Columbia. The
only plants visible in the area are coralline algae Lithothamnion spp.

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Research study 3

photograph of several red sea-urchins Strongylocentrotus franciscanus eating kelpA study conducted in southern California over a 3yr period provides insight into the way in which food, or the lack of it, changes the behaviour of large aggregations of red sea-urchins Strongylocentrotus franciscanus. The urchins live either in small stationary aggregations or in large motile ones, located within 100m of one another.  The stationary groups subsist mainly on drift kelp and play little or no role in population dynamics of the kelp.  The motile ones, however, advance in “fronts” at about 2m . mo-1 and consume almost all macroalgae in their paths.  Examination of relative gonad size in individuals of these moving groups shows that they are starved.  It appears that a scarcity of drift algae changes the behaviour of the stationary urchins, leading to the moving groups.  These motile urchins run into other stationary aggregations, quickly outstrip the food supply, and move on in a “snowballing” manner to form the large, motile aggregations, or fronts.  White urchins Lytechnus anamesus, also present in aggregations but at the offshore periphery of the kelp bed, remain relatively stationary over the study period.  This species rarely eats adult kelps, but feeds instead on the developmental stages and thus prevents seaward expansion of the bed.  In different ways, then, the 2 species of urchins profoundly affect the distribution and survival of large kelps.  Dean et al. 1984 Mar Biol 78: 301.

NOTE   the San Onofre kelp bed is located offshore from Oceanside, California and, at the time of the study, occupied about 10,000 hectares.  Principal kelps at a depth of 12-15m are giant Macrocystis pyrifera and smaller understory kelps Pterygophora californica

NOTE  while red urchins average about 11cm test diameter, white urchins are quite a bit smaller, averaging a little over 1cm test diameter

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Research study 3.1

A 5yr study on the effects of  2 severe winter storms on kelp-forest community structure at Naples Reef, California by researchers at the University of California, Santa Barbara provides an interesting timeline of how such effects can differ depending upon past history.  The first storm broke off large pieces of shale rock and removed all giant kelp Macrocystis pyrifera including large accumulations of drift kelp, thus depriving sea-urchin populations Strongylocentrotus franciscanus and S. purpuratus of their preferred foods.  The urchins were forced out of shelter and ate most of the remaining algae, including most of the understory kelps Pterygophora californica and Laminaria farlowii.  Disruption of the detritus-based food chain had important secondary negative effects on abundance of turf algae and, down the line, on surfperch fishes that were accustomed to eating turf-inhabiting invertebrates.  A second severe storm 3yr later killed off most of the sea urchins, thus allowing extensive kelp canopies to become re-established by the following summer (see drawings below).  The study illustrates the different effects that storms may have on community structure depending upon the state of the community prior to the disturbance.  Ebeling et al. 1985 Mar Biol 84: 287.

diagram of Naples Reef before the first severe winter storm in 1979 diagram of Naples Reef one year after the first severe winter storm in 1979
In summer 1979 before the first winter storm sea urchins shelter beneath the widespread kelp canopy and feed mainly on drift algae By autumn 1980 the urchins have left the kelp-forest area and feed near the last remaining Pterygophora beds.  Most of the reef is now bare rock (“barrens”)
diagram of Naples Reef 2 years after the first severe winter storm in 1979 diagram of Naples Reef after the second severe winter storm in 1983
By spring 1981 most of the reef is bare and urchins are constrained to a small area.  The second storm hits in winter 1983 and wipes out most of the remaining sea urchins By summer 1984 the reef is essentially restored.  “Barrens” areas are minimal and sea urchins are returning.  Lines on each figure indicate transecti lines surveyed by SCUBA
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red urchin Strongylocentrotus franciscanus with corallimorpharian colony Corynactis californicaCould the formation of such “fronts” by sea urchins be related to an inability or perhaps reluctance to cross physical barriers such as sand patchs, vertical rock walls, and the like, or perhaps discomfort to cross biological barriers such as large colonies of stinging corallimorpharians Corynactis californica? These ideas are tested by researchers at the University of California, Santa Barbara for short-spined purple urchins Strongylocentrotus purpuratus and long-spined red urchins S. franciscanus in both laboratory and field situations. Results show that both species can negotiate sand using their oral spines, but with purple urchins being much less adept than red urchins. Sloping ramps installed in the field are readily climbed by both species to reach kelp growing on higher ledges. Colonies of Corynactis californica represent a barrier to the urchins, mostly owing it seems to lack of suitable surface for attachment of tube feet. Laur et al. 1986 Mar Biol 93: 209.

NOTE the authors refer to this species as a "coral-like anemone", but this is not correct. Sea anemones are in Order Actiniaria, while Corynactis is in O. Corallimorpharia

NOTE spines are used for locomotion only on soft substrata; on hard surfaces both species locomote using their tube feet

A red urchin Strongylocentrotus franciscanus (trapped?)
in a field of corallimorpharians Corynactis californica.
Too soft? Too stinging? Too little food? 0.5X

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Research study 4

graph showing decline of kelp cover Macrocystis pyrifera in relation to densities of red and purple sea-urchinsphotograph of sea urchins Strongylocentrotus purpuratus and S. franciscanus in a kelp bedIn spring 1984 a large settlement of  red sea-urchins Strongylocentrotus franciscanus and purple urchins S. purpuratus occurred within forests of kelp Macrocystis pyrifera in Carmel Bay, California.  By end of summer 1986 these cohorts of urchins had removed most macro-algae from one large reef area (Outer Pinnacles), and a good portion of the sessile invertebrates as well (most notably compound ascidians and bryozoans).  By 1989 most purple urchins are gone, but the population of red urchins remains intact.  Study by researchers at the Monterey Bay Aquarium reveals that settlement of algal spores remains high during the grazing episode, but algal recruitment does not occur until the sea-urchin numbers have declined.  The authors discuss reasons for the deline, such as predation and disease, but discount the possibility that predation by sea otters could have had such a major impact on one, but not the other, sea-urchin species.  Watanabe & Harrold 1991 Mar Ecol Progr Ser 71: 125.


Mixed populations of purple and red sea-urchins
in a kelp bed, S. purpuratus and S. franciscanus

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Research study 5

Sea urchins attack kelps in several ways, including at the juncture of stipes with holdfast, often leading to entire loss of plant biomass, and also by crawling up and weighing down individual stipes, leading to consumption of entire fronds by individuals on the rocks below. graph showing effects over time of sea urchins on kelp holdfasts in the Point Loma, California area during the early 1990s The holdfasts are perennial, may last for 4-7yr in kelps such as Macrocystis pyrifera, and can be massive in size. Sea urchins around Point Loma, California, most notably Strongylocentrotus franciscanus and S. purpuratus, may burrow deeply into these holdfasts and the “cavitation” produced can lead to wholesale structural failure of the plant in storm photograph of several kelp plants along with anchoring rocks cast up on a beach after a stormsurge.  Studies of M. pyrifera holdfasts at 18m depth off Point Loma reveal high incidence of lethal cavitation, correlative with high densities of both species of sea urchins.  Tegner et al. 1995 J Exp Mar Biol Ecol 191: 83.

Holdfasts and stipes of bull kelps Nereocystis luetkeana, an annual species and much smaller than Macrocystis spp.

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Research study 5.1

addition to their own study on the interrelationships between sea urchins, kelps, and sea otters in the Aleutian Islands and Southeast Alaska, researchers from University of Santa Cruz and University of Washington provide a summary of 17 studies to date on the subject done in different areas of the west coast.  Of these, 6 show that sea otters reduce sea-urchin population numbers but with no reference to secondary effects on kelps, 7 show that sea-otter predation on urchins increases algal biomass, and 4 show that sea urchins affect algal biomass but do not involve sea otters.  In their own study at several sites in the Aleutian Islands the authors report that where sea otters have beeneither continuously present or continuously absent, kelp and sea-urchin abundances remain generally unchanged over periods ranging from 3-15yr.  However, at sites where sea otters are newly colonising, sea-urchin numbers decline significantly (by 50-100%), with concomitant increases in photograph of sea urchins Strongylocentrotus polyacanthus eating kelp in Aleutian Islands, Alaskakelp biomass.  The value of the study is in the selection of 2 widely separated sites and in the long period of study at one of the sites (15yr).  The researchers are satisfied that they have answered most of the criticisms put forward by other authors that past studies have focused too much on the top-down role of sea otters, with not enough attention being paid to other factors, such as physical disturbances.  The authors provide an excellent review of the subject of sea urchins, sea otters, and kelp on the west coast.   Estes & Duggins 1995 Ecol Monogr 65 (1): 75. Photograph courtesy Museum of Aleutians, Unalaska, Alaska MOA.

NOTE  species comprise Strongylocentrotus polyacanthus in the Aleutian sites, and S. franciscanus, S. droebachiensis, and S. purpuratus in Southeast Alaska

NOTE  the authors actually list 23 papers, but 6 have little or no relevance to this section of the ODYSSEY. Synopses of all of the remaining 17 papers can be found in this or other sections of the ODYSSEY.  However, given that there have been at least 6 west-coast publications concluding that sea otters eat sea urchins with resultant increase in algal biomass, one wonders how long interest in, and originality of, such studies will be maintained

A large stand of kelps in Aleutian Islands,
Alaska showing grazing effects of many sea
urchins Strongylocentrotus polyacanthu

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Research study 6

maps of before and after sea-otter presence on the Olympic Peninsula coastExpansion of the sea-otter Enhydra lutris population along the Olympic coast of Washington after their introduction in 1969 allows an assessment of their impact on sea-urchin stocks.  An initial survey in 1987 sets the baseline for distribution and numbers of otters and sea urchins, with a follow-up survey in 1995 assessing changes due to the sea otter population moving northwards into previously unoccupied areas.  The map shows the sea-otter range in 1987 and 1995 with the benthic sites sampled on both dates indicated by black dots.  Note the northwards expansion in range of Enhydra during 1987-1995.  The data presented in the paper are complex but can be summarised as an order of magnitude decline photograph of sea otter Enhydra lutrisin numbers and biomass of urchins Strongylocentrotus franciscanus in the areas newly colonised by sea otters, as well as further decline in areas previously occupied.  The only exception to this is the wave-exposed and current-swept areas around Cape Flattery, including Tatoosh Island.  Here, urchin numbers and biomass are higher than anywhere else sampled in the 2 studies.  The otters appear to have skipped over this area, perhaps because of the typically heavy seas and strong tidal currents found there.  At sites monitored for seaweeds, cover of red foliose species increases greatly in correspondence with the decline in sea-urchin numbers.  As a result of being covered by new growths of foliose species, coralline-crust algae drop from virtually 100% cover to 42% cover.  The authors’ results support previous conclusions that sea otters eliminate sea-urchin grazing as a dominant community-structuring force in near-shore benthic regions.  Kvitek et al. 1998 Mar Mammal Sci 14: 895; see also Kvitek et al. 1989 Mar Mammal Sci 5 (3): 266. 

NOTE  the founding members of the population were introduced from Alaska in 1969-70, with numbers being augmented in 1987 and again in 1995.

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Research study 7

The Exxon-Valdez oil spill in 1989 caused an immediate 50% reduction in abundance of sea otters Enhydra lutris in northern Knight Island, Alaska.  Nine years after the spill numbers of sea otters were still about 2/3rds fewer than they had been in 1973.  A decade later, at the time of the present study, athough some areas with reduced sea-otter densities have proportionately more larger green sea urchins Strongylocentrotus droebachiensis, in other areas there appears to have been little or no effect of sea-otter absence on either sea-urchin abundance or kelp abundance.  This contrasts with data from the western Aleutian Islands that show greatly increased sea-urchin biomass and greatly reduced kelp density after a 90% reduction in sea-otter abundance.  The authors discuss possible reasons for the differences.  Dean et al. 2000 Mar Ecol Progr Ser 199: 281.

NOTE  overall, the spill killed an estimated 1000-2800 sea otters, most in Prince William Sound

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Research study 8

One doesn’t think that kelps could defend themselves from sea-urchin grazing, but in Shemya Island in the Alaskan archipelago sea urchins Strongylocentrotus polyacanthus having found themselves deprived of food in barren areas, may actually be prevented from crossing into the food-rich kelp stands by the sweeping motion of the kelp fronds over the sea floor.  When researchers from the University of California, Santa Cruz experimentally translocated sea urchins into the kelp forest, the urchins prospered there until winter storms swept them away or caused them to seek shelter in crevices.  Gonad sizes of these kelp-inhabiting individuals are significantly larger than those in the barren areas, indicating that a kelp diet will improve fitness.  The finding that kelp plants are able to defend themselves from sea-urchin grazing in this way may explain why kelp-forest ecostystems often exist as stable patchwork mosaics of kelps and barren areas.  In Alaska, only in areas where sea otters are present  is the stability of this system upset, for  otters preferentially consume the sea urchins which prevents the formation of barren areas.  Konar & Estes 2003 Ecology 84 (1): 174.

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Research study 9

Does the close coupling of sea urchins and kelp forests as witnessed in the northern parts of their ranges extend also to their southern range limits?  This is examined for purple sea-urchins Strongylocentrotus purpuratus and a favoured kelp food Macrocystis pyrifera at 4 sites along the Pacific Coast of Baja California, Mexico.  Two of the sites are just south of the California border and 2 are in the region of Bahia Tortugas at latitude 27.7oN (see map below).  These last sites are in a biogeographic transition zone between the temperate waters of the California Current and the subtropical waters further south.  The authors establish transect lines (30 x 2m) at each site and sample 11 one-m2 quadrats along each line for density of kelp plants and their fronds, and numbers of sea urchins at 2 times of year (winter: Dec-Jan and spring: Mar-Apr).  Sea urchins are more dense in the southern populations than in the northern ones.  Overall, the authors conclude that at all 4 sites densities of kelps and sea urchins are positively correlated, but some significant seasonal and latitudinal variations also exist.  Where the community disappears south of the transition zone is not mentioned by the authors. Beas-Luna & Ladah Bot Mar 2014 57 (2): 73.

NOTE  many species of shallow-water algae and invertebrates encounter their southern limits of distribution in this area.  These include several large kelps, abalones, and red and purple sea urchins


schematic showing quadrat array for study of kelp and purple sea-urchin Strongylocentrotus purpuratus distributions photograph of purple sea-urchins Strongylocentrotus purpuratus in barrens area near kelp bed

The barrens area on the seaward
side of the kelp bed hosts many
sea urchins, mainly the purple
Strongylocentrotus purpuratus

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Research study 10

graph showing correlation between extent of sedentary behaviour in sea urchins Strongylocentrotus franciscanus and their drift-algae catching effectiveness Red sea-urchins Strongylocentrotus franciscanus often pack together like tinned sardines, and have little or no ability to move. In one assessment, researchers at Friday Harbor Laboratories, Washington note that 87% of individuals in study populations below the macroalgal zone do not move at all over a 3wk study period. Sedentary behaviour is especially noted in areas where availability of drift algae for capture is relatively high (see graph). At depths deeper than about 20m in the San Juan Archipelago region, macroalgae are scarce owing to diminished light for growth, and the sea urchins rely almost entirely on drift algae for their sustenance. At depths deeper than this, urchin density decreases as individuals spread out to search for food. One hypothesis in the present study is that red sea-urchins will migrate vertically from deeper to shallower habitats in winter when abundance of drift algae is low. The researchers also predict that differences in sea-urchin density will be accompanied by significant differences in composition of the benthic communities beneath them. In fact, seasonal monitoring of experimental and control quadrats at depths of 10, 20, and 30m using SCUBA at 3 sites reveals no significant seasonal change in density. However, the researchers do find that photograph of red sea-urchins Stronglyocentrotus franciscanus dispersed below the kelp zonethe benthos under the urchins is significantly more bare and has greater densities of coralline algae, snails, crabs, and shrimps than nearby control areas lacking sea urchins. The authors conclude that even when sedentary, urchins can exert a strong influence on benthic community structure. Lowe et al. 2015 Ecology 36: 129.

NOTE the authors determine the significance of these data using linear-regression analysis but, as this is a correlation, wouldn’t testing of a correlation coefficient have been more appropriate?

Sea urchins Strongylocentrotus franciscanus
in an algal-barrens area below the kelp level

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