Biology Reference
In-Depth Information
Sargassum,
and several species of coralline algae called rhodoliths), most marine
macroalgae are attached to rocky reef, other algae, invertebrates, or sediments. This
results in their interacting for attachment sites in two dimensions rather than three
and thus greatly increases the chances that they will compete for access to the
substrate. This will also occur between algae and sessile invertebrates such as
is commonly observed on degraded coral reefs where algae are considered compet-
itively dominant to coral larvae and new recruits, but competitively subordinate
to coral adults (reviewed in McCook et al.
2001
; see also Chap.
16
by Mejia et al.
and Chap.
18
by Bartsch et al.). As a consequence, algal proliferation often
occurs in areas where coral adults have died, which in turn prevents coral recruit-
ment and slows community recovery (e.g., Kuffner et al.
2006
). However, while
algal-invertebrate competition is common, we do not address it further here but
rather focus our attention to competition between and within algal species.
Although algae may compete strongly for attachment space on the substrate,
when examined closely, the actual area of attachment to the substrate is often very
small compared to the overall size of the algal thallus. For example, on coral reefs,
the actual attachment site of the dominant algae can be as small as 10-100
m with
lots of bare space present between the holdfasts (Hackney et al.
1989
). Further,
Dayton (
1975
) found that less than 20% of rocky reef in a subtidal temperate kelp
forest was occupied by the kelp holdfasts even though the kelp canopies covered the
majority of the reef. Consequently, competition among macroalgae for space, per
se, may not be a limiting factor in recruitment in many locations (reviewed in
Carpenter
1990
). However, given that the thalli of many algae tend to lie prostrate
on the substrate or physically abrade the substrate as they move in the waves
(Velimirov and Griffiths
1979
; Witman
1987
; Kennelly
1989
), they can prevent
settlement and/or growth of other algae on the substrate via interference (abrasion)
and exploitative (shading) competition (Duggins and Dethier
1985
). Removal of the
dominant species, then, generally results in an increase in other macroalgae as
observed for intertidal
Hedophyllum sessile
communities in the San Juan Islands,
off the coast of Washington State, USA (Duggins and Dethier
1985
). Further, some
of the strongest competitive interactions in the southern hemisphere tend to be
created by large canopy-forming algae (e.g., intertidal
Durvillaea antarctica
;
subtidal
Ecklonia radiata
). These individuals exclude many understory species
via their fronds which shade or whiplash the substratum (Kennelly
1987b
). How-
ever, these effects may vary geographically at a number of spatial scales (Connell
and Irving
2009
) and among different benthic species (Kennelly
1989
). Likewise, in
the Aleutian Archipelago, physical abrasion from the brown alga
Desmarestia
viridis
is an important factor keeping other algae from establishing patches on the
tops of pinnacles (Konar and Estes
2003
). Physical abrasion by kelp fronds can also
prevent recruitment of sessile invertebrates (Connell
2003a
) which otherwise
monopolize space under lower light and sedimentation such as observed on South
Australian coasts (Irving and Connell
2006
) or of motile invertebrates that would
otherwise graze on the algae (Konar and Estes
2003
). Hence, body size often serves
as the best measure of the potential for algae and other types of sessile species to
exert strong community-wide effects.
m
