The effects of sea otter (Enhydra lutris) foraging on shallow rocky communities off northwestern Vancouver Island, British Columbia
Title: The effects of sea otter (Enhydra lutris) foraging on shallow rocky communities off northwestern Vancouver Island, British Columbia, by Watson, Jane Catherine, Ph.D, report of 1993
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THE EFFECTS OF SEA OTTER (ENHYDRA LUTRlS) FORAGING ON
SHALLOW ROCKY COMMUNITIES OFF NORTHWESTERN VANCOUVER ISLAND
Jane Catherine Watson
The effects of sea otter foraging on rocky subtidal communities was examined
off northwestern Vancouver Island, British Columbia. The Vancouver Island otter
population originates from 89 animals re-introduced from 1969-1972. Since 1977 the
population has increased 19.2% yr-1 and expanded its range 15.3% yr-1.
The effects of otter foraging were inferred by comparing the species composition
of 60 randomly selected sites, 40 in two areas without sea otters, and 20 in an area with
otters. Areas without sea otters w!;}re dominated by urchins and brown algae were rare,
whereas areas with otters were dominated by brown algae and urchins were rare. Patchy
mosaics of urchins and algae observed at sites recently occupied by otters may represent
a transitional stage between the otter-free and otter-dominated configurations.
The effects of otters were documented by monitoring species abundance at four
sites before and after sea otters arrived. Two sites with sea otters and two without were
monitored concurrently. Despite small changes in species abundance, community
configuration did not change at sites with or without sea otters. With the arrival of sea
otters, urchin abundance declined and algal abundance increased. The rate and pattern of
change varied among sites, apparently affected by the frequency, intensity and
seasonality of otter foraging.
The response of red urchins (Strongylocentrotus jranciscanus) to damaged
conspecifics was examined as a mechanism for the urchin/algal mosaics observed at sites
recently invaded by otters. Red urchins avoided eviscerated conspecifics, creating
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urchin-free patches which lasted at least 16 days. Urchin tests dropped by foraging otters
may elicit an escape response in nearby urchins, creating urchin-free patches that allow
algal recruitment and explain how mosaics of urchins and algae form.
Long-term succession was inferred by comparing algal assemblages at 12 sites
where the modal age of Prerygophora cali/arnica was used to estimate when sea otters
had arrived. Algal assemblages at sites occupied by otters for 3-4 yrs were unpredictable
in composition or density, whereas assemblages at sites> 7 yrs old were dominated by
stipitate kelp and were highly predictable. Chance events appear to have little effect on
the eventual composition of brown-algal assemblages in otter-dominated areas off
western Vancouver Island.