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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  Interaction with kelps & sea otters
  Topics relating to population & community ecology include interactions with kelps & 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 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 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 & Dixon 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 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 4-7 years 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 6

Expansion 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. 

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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