Biology Reference
In-Depth Information
Regardless, it is clear that self-thinning is important to many seaweed populations,
though this may be more important for unitary species such as
Macrocystis pyrifera
than for clonal species such as
Mastocarpus papillatus
where self-thinning during
the growing season is not expected even under high density (Flores-Moya et al.
1996
; Scrosati
2006
).
While studies along the west coast of North America have shown dramatic
responses by the turf-forming algae to kelp canopy clearings, results from other
areas of the world are more variable. For example, clearings to the surface and
subsurface kelp canopies within
Nereocystis leutkeana
forests in coastal Alaska
resulted in complete overgrowth by subsurface kelps within 1 year, while similar
clearings within the
Eualaria fistulosa
forest throughout the Aleutian Archipelago
resulted in little to no macroalgal recruitment over 1-year period (B. Konar et al.
unpublished data). While the reason for this is unclear, we believe it is primarily
due to variation in identities and life histories of the species involved (i.e., whether
they are annual or perennial), oceanographic conditions (i.e., temperature, nutrient,
and wave conditions), grazing pressure, and idiosyncrasies associated with the
experiments themselves (i.e., when the clearings were done, how large and thor-
ough they were). However, even with this variability, certain similarities exist
among geographic areas. For example, throughout much of Australia, the
conditions created by subtidal kelp canopies include reductions in irradiance,
sedimentation, and the abundance of turf-forming algae (Kennelly
1987a
). Conse-
quently, the kelp canopies maintain encrusting coralline algae from overgrowth by
turf-forming algae through shade (Kennelly
1987a
; Connell
2003b
) and possibly
through reduced accumulation of sediments (Melville and Connell
2001
). Thus, the
clearance of kelp canopies generally leads to the overgrowth of encrusting algae by
turf-forming algae, which then trap sediments (Kennelly and Underwood
1993
;
Connell
2005
). Conversely, recovery of the kelp canopies invariably leads to the
reverse effect (e.g., Kennelly
1987b
), demonstrating that the co-occurrence of
canopy and encrusting coralline algae is not just a spatial and temporal coincidence,
but rather one driven by the state of the dominant canopy-forming species. Given
the strength of association across regions and different taxa in Australia, there
appears to be considerable predictive capacity in understanding how the understory
macroalgae benefit from the physical light environment created by the canopies
(e.g., Bertness and Leonard
1997
). These patterns are consistent with the relative
abundance of encrusting and turf-forming algae, where the former are relatively
abundant in locations of low light and sedimentation (Steneck
1986
,
1997
), while
the latter monopolize locations of high light and high sedimentation (Airoldi
1998
).
7.2.3 Competition for Space
On rocky surfaces, access to adequate space is crucial for the attachment and
survival of organisms (e.g., Connell
1961
). Except for a few examples of free-
living algal (e.g., some species in genera
Ulva
(formerly
Enteromorpha
) and
