Biology Reference
In-Depth Information
Reed and Foster
1984
; Kennelly
1987a
; Harrold et al.
1988
; Dayton et al.
1992
), the
removal of the dominant canopies typically results in increased bottom light and a
corresponding increase in the abundance of opportunistic species (Dayton et al.
1984
,
1992
; Reed and Foster
1984
; Kennelly
1987b
; Cecchi and Cinelli
1992
;
Graham
1996
; Edwards
1998
). In fact, North et al. (
1986
) conclude that light is the
primary factor regulating species abundances within coastal forests, a claim that has
been experimentally tested in numerous studies via experimental canopy removal.
For example, Ambrose and Nelson (
1982
) observed that removal of the invasive
Sargassum muticum
at Santa Catalina Island, USA, resulted in reduced recruitment
of the giant kelp
Macrocystis pyrifera
by reducing benthic irradiance. Likewise,
Reed and Foster (
1984
) found that removal of the
Macrocystis pyrifera
surface
canopy and the subsurface
Pterygophora californica
canopy in a central California,
USA, kelp forest resulted in increased recruitment of understory algae, as well as
the kelps themselves. Similarly, Clark et al. (
2004
) replicated Reed and Foster's
canopy clearings at three areas in the same central California kelp forest and
followed changes to the understory algae for a period of 2 years. Their clearing
design (Fig.
7.1
) allowed for the simultaneous testing of the individual and com-
bined effects of shading from both surface and subsurface canopies on understory
algal assemblages, and from an opportunistic alga,
Desmarestia ligulata
, that
recruited into the clearings in very high abundances. Their results indicated that
while understory algae did respond to the canopy clearings, the low abundances of
individual species and the small magnitude of each species' response compared to
their natural temporal and spatial variability made detecting canopy effects diffi-
cult. However, when understory species were grouped together in ecological
response groups, they were able to detect otherwise cryptic increases in some
(i.e., light-adapted) species as much as 1 year earlier than when each species was
examined individually. The exception to this was the opportunistic brown alga
Desmarestia ligulata
which showed dramatic rapid increases within the canopy
clearings. Edwards (
1998
) examined this further and found that while
Desmarestia
ligulata
remained in low abundances under existing canopies, it recruited in high
abundance in the spring and ultimately reached high bottom cover in areas where
the canopies were removed (Fig.
7.2
). Similar patterns have been observed for
Desmarestia ligulata
in Point Loma, CA, following canopy removal by winter
storms (Dayton et al.
1984
) and in three central California kelp forests characterized
by different hydrodynamic conditions and canopy covers (Foster
1982
). Dayton
et al.'s work further described that disturbance to the dominant kelp canopies
resulted in variation in the benthic light regimes and a corresponding mosaic of
understory algal patches. Some of these patches were able to persist for extended
periods of time and competitively exclude or delay recovery of the otherwise
dominant kelps. The effects of this canopy shading, however, are not limited to
interspecific interactions but also impact individuals of their own species via
intraspecific interactions. For instance, shading from the dominant
Macrocystis
pyrifera
canopies also inhibits recruitment and growth of their own juvenile
sporophytes (Anderson and North
1969
; Reed and Foster
1984
; Dean et al.
1989
).
Similar negative effects of intraspecific competition were reported by Neushul and
