Biology Reference
In-Depth Information
7.2 Competition Among Marine Macroalgae
7.2.1 Overview
The distribution of plants is regulated by both physical and biological factors, though
the relative importance of these factors is notoriously variable through time and space,
and is particularly dependent on the scale at which they are examined (Dayton and
Tegner
1984
; Edwards
2004
). While plant populations may persist within a given
ecosystem due to predation on herbivores as postulated by Hairston et al. (
1960
), in the
absence of herbivory, plants would increase in number and ultimately compete among
themselves for light, space, and nutrients. In fact, competition has been predicted to
be the most important factor regulating the distribution and abundance of plant species
in the absence of environmental stressors (Grime
1974
,
1977
,
1979
). It is considered
crucial in setting the latitudinal range limits (e.g., Edwards and Hern
ยด
ndez-Carmona
(e.g., Hawkings and Hartnoll
1985
) of some macroalgae, and regulating patterns of
their succession following disturbances in others (e.g., Sousa
1979
). However, the
mere presence of a competitormay not be sufficient to cause strong competitive effects.
Variation in morphology may have variable effects on the availability of and/or access
to resources. For example, the exclusion of erect species from the understory within
Australian
Ecklonia radiata
kelp forests depends on the morphology of the algae.
Shorter and more flexible forms exclude erect taxa by shading and whiplashing the
substratum (Connell
2003a
; Irving and Connell
2006
), whereas longer and more rigid
forms do not exclude erect taxa from the understorywhich is strongly affected by shade
(Kennelly
1987a
). These differences have profound effects on the understory
communities across Australia as 5,000 km of coastline across the Leeuwin Current
(west and south coast) are largely structured by the short-flexible forms whereas the
east coast (East Australian Current) understory is structured by the long-rigid forms
(Connell and Irving
2009
). These mechanisms match closely with variation in under-
story communities across temperate Australia (Irving et al.
2004
). Furthermore,
interactions amongmacroalgal holdfastsmay result in strong interspecific competition,
as seen in some intertidal red algae in Chile, but little-to-no intraspecific competition,
as the holdfasts of the same species tend to coalesce (Santelices et al.
2003
). What this
means for their populations, however, is uncertain. One thing that is clear is that marine
macroalgae are susceptible to competition as space, light, and nutrients are often in
short supply in coastal ecosystems (reviewed in Carpenter
1990
).
7.2.2 Competition for Light
Light is the primary resource requirement for algal growth, reproduction, and
survival. Both the quantity (irradiance) and spectral quality (wavelength) are
important in determining the distribution of many algal species, especially along
