Competition, a Major Factor Structuring Seaweed Communities - Seaweed Biology - page 135

at the surface by gas-filled pneumatocysts or are supported above the substrate by
thick woody stipes (e.g., Abbott and Hollenberg 1976 ). These canopies place their
photosynthetic blades above those of their competitors and enabling them to more
effectively capture light before it reaches their competitors (Horn 1971 ; Hay 1986 ).
This structure results in a rich mosaic of benthic light regimes below the canopies
(Gerard 1984 ) to which macroalgae can “photoadapt” by altering the amount and/or
arrangement of their photosynthetic pigments (Ramus 1981 ; Rosenberg and Ramus
1982 ). Combined with other factors such as substrate topography, hydrodynamic
forces, and grazing, this can result in benthic algal communities being highly
variable at a number of spatial scales (Dayton et al. 1984 , 1992 , 1999 ; Foster and
VanBlaricom 2001 ; Edwards 2004 ).
Much in the way competitive dominance by canopy-forming plants plays a
structuring role in terrestrial forest communities (e.g., Brokaw and Scheiner
1989 ; Connell 1989 ; Spies and Franklin 1989 ; Hubbell et al. 1999 ), shading from
kelp canopies can be the determining factor in regulating benthic macroalgal
populations (Reed and Foster 1984 ; Kennelly 1989 ; Edwards 1998 ; Connell
2003b ). For example, it is well understood that in the terrestrial forests, the removal
of only a few canopy dominants can prevent the competitive exclusion of many
understory species and thereby promote greater diversity (Whitmore 1989 ; Brokaw
and Scheiner 1989 ). These understory species can be grouped into three general
categories based on how they respond to canopy shading. This “ecological response
group” approach has been widely used by terrestrial plant ecologists to classify
individual species according to how they respond to changes in their light environ-
ment and has been very useful in studies on canopy shading (e.g., Collins et al.
1985 ; Whitmore 1989 ; Kursar and Coley 1999 ). The logic behind the response-
group approach is straightforward; canopy removal should elicit positive responses
in recruitment and/or growth for species that require high irradiances, negative
responses for species that require low irradiances, and little-to-no responses for
species adapted to a variety of irradiances (Brokaw and Scheiner 1989 ; Spies and
Franklin 1989 ; Whitmore 1989 ; Kursar and Coley 1999 ). Those species, then, that
remain rare under dense canopies but rapidly recruit into canopy gaps or following
widespread canopy removal have typically been referred to as “light-adapted,”
“gap-requiring,” or “shade-intolerant” and are often considered to represent fugi-
tive species, while those species that occur under forest canopies but do not
markedly respond to canopy loss have typically been referred to as “light-flexible”
or “shade-tolerant” species and are often considered to represent climax species
(Whitmore 1989 ; Hay 1994 ; Clark et al. 2004 ). Whereas light-flexible species are
generally numerically more abundant than light-adapted species both under
canopies and in canopy gaps, their ability to withstand low light environments
may result in a decreased ability to rapidly respond to sudden increases in light
(Canham 1989 ). As a consequence, light-adapted species, through greater recruit-
ment and growth, typically dominate areas following canopy loss.
In temperate marine communities, where shading by thick canopies of kelps and
rockweeds (Orders Laminariales and Fucales, respectively) can regulate understory
algal abundance in shallow (

30 m) water (Pearse and Hines 1979 ; Foster 1982 ;

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