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Methods
S T U D Y
S Y S T E M
Our field research was based out of Friday Harbor Laboratories on
San Juan Island, Washington State, USA. The field experiment was
carried out at a site within the San Juan Islands Marine Preserve
network adjacent to Shaw Island, known locally as Point George
(48.5549
°
N, 122.9810
°
W). Field work was accomplished using
SCUBA in shallow subtidal communities.
The native algal community characteristic of sheltered, rocky
subtidal habitats in this region is species-rich and structurally
complex (see Britton-Simmons 2006 for a more detailed description).
In this ecosystem, space is an important limiting resource and in the
absence of disturbance there is little or no bare rock available for
newly arriving organisms to colonize. This habitat has a diverse
fauna of benthic herbivores, including molluscs and sea urchins,
that create disturbances by clearing algae from the rocky substrata.
The green sea urchin
Strongylocentrotus droebachiensis
is a generalist
herbivore that reduces the abundance of native algae and creates
relatively large disturbed patches (Vadas 1968; Duggins 1980). In
the shallow zone where
S. muticum
is found, the green urchin is
highly mobile and often occurs in aggregations (Paine & Vadas
1969; Foreman 1977; Duggins 1983; personal observation). Green
urchins avoid areas where
S. muticum
is present because it is not a
preferred food resource (Britton-Simmons 2004), but they can be
found feeding in uninvaded areas adjacent to existing
S. muticum
populations (personal observation). Green urchins therefore create
intermittent but relatively intense disturbances in areas where
S.
muticum
is absent and some proportion of these disturbances can
potentially be exploited by dispersing
S. muticum
propagules. In
contrast, herbivorous benthic molluscs (chitons, limpets and snails)
are ubiquitous in the shallow subtidal and unlike sea urchins they
are unaffected by the presence of
S. muticum
(Britton-Simmons
2004). Herbivory by individual molluscs creates relatively small-
scale disturbances, thereby providing a consistent supply of micro-
sites that can be colonized by newly arriving species, including
Sargassum muticum
(see Appendix S1 in Supplementary Material
for more information about mollusc diets).
T H E
I N V A D E R
Sargassum muticum
is a brown alga in the order Fucales that was
introduced to Washington State in the early 20th century, probably
with shipments of Japanese oysters that were imported for aqua-
culture beginning in 1902 (Scagel 1956). It is now common in shallow
subtidal habitats throughout Puget Sound and the San Juan Islands
(Nearshore Habitat Program 2001, personal observation). In the
San Juan Islands,
S. muticum
has a pseudoperennial life history.
Each holdfast produces as many as 18 laterals in the early spring,
each of which can grow as tall as three metres. In late summer to
early autumn the laterals senesce and are lost, leaving only the basal
holdfast portion of the thallus to overwinter.
Sargassum muticum
has a diplontic (uniphasic) life cycle, is mone-
cious, and is capable of selfing. Reproduction typically occurs between
late June and late August in our region. During reproduction the
eggs of
S. muticum
are released from and subsequently adhere to the
outside of small reproductive structures called receptacles. Once
fertilized, the resulting embryos remain attached while they develop
into tiny germlings (
<
200
μ
m in length) with adhesive rhizoids
(Deysher & Norton 1982). Germlings then detach from the receptacle
and sink relatively quickly, recruiting in close proximity to the
parent plant (Deysher & Norton 1982). Although most recruitment
occurs within 5 m of adult plants, recruits have been found as far as
30 m from the nearest adult (Deysher & Norton 1982). Longer
distance dispersal probably occurs when plants get detached from
the substratum and subsequently become fertile after drifting for
some period of time (Deysher & Norton 1982). One distinctive
feature of the
S. muticum
invasion is that it is extremely limited in
vertical extent. In the San Juan Islands,
S. muticum
is found from
the low intertidal to the shallow subtidal zone (Norton 1977;
personal observation), from approximately –0.5 m Mean Lower
Low Water (MLLW) to –7 m MLLW. However, it is most abundant
in the shallow subtidal, from approximately –2 m MLLW to – 4 m
MLLW. Thus, in areas where
S. muticum
has invaded it forms a
narrow band along the shore.
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