Biology Reference
In-Depth Information
Since this debate began, a few studies have demonstrated how both top-down
and bottom-up controls can be important. Littler et al. (
2006
) developed a concep-
tual model, the Relative Dominance Model, to describe the process of bloom
formation on reefs under independent and combined factors of nutrients and
grazing pressure. Results of manipulative experiments, bioassays, and surveys of
communities showed that: (1) under reduced nutrients alone fleshy algae grow
when herbivory is low, (2) under high herbivory alone fleshy algae grow when
nutrients are high, (3) reduced nutrients and high herbivory prevent blooms of
macroalgae (Littler et al.
2006
). It has also been found that different controls are
important for different groups of algae; for example, McClanahan et al. (
2003
)
found that turf algae were positively affected by nutrients and negatively affected
by herbivory, while frondose brown algae grew better under low nutrients and low
herbivory. Vermeij et al. (
2010
) also found that turf algae can overgrow corals
under high nutrients, but that herbivores are not able to control their growth under
these conditions.
Despite the evidence that bottom-up control does play a role in triggering
macroalgal bloom formations on reefs, the majority of studies support that top-
down control is the primary driving factor (Thacker et al.
2001
; Burkepile and Hay
2006
; Sotka and Hay
2009
). A recent study in a pristine fisheries management area
along a Hawaiian reef revealed that in the absence of herbivores, fleshy macroalgal
abundance was higher than in any other treatment, while in the presence of
herbivores corals and crustose coralline algae were more abundant (Smith et al.
2010
). The effects of nutrient enrichment supported slightly higher macroalgal
percent cover than in nonenriched treatments, but the species composition
depended on the herbivore treatment. After reexposure to natural conditions, the
effects of enrichment and herbivore absence disappeared over a relatively short
time frame, supporting the notion that increasing herbivore populations is critical to
reduce macroalgal blooms on reefs (Smith et al.
2010
).
Based on the variable results of studies examining top-down and bottom-up
controls of macroalgal blooms on coral reefs, the relative role of these controls is
still not fully resolved. Only through larger scale changes in fishing regulations and
better management of nutrient inputs to coastal reefs will we be able to better
distinguish which of these or the combination of both is the causal factor.
21.5.2.3 Rocky Intertidal and Subtidal Ecosystems
Much of the knowledge defining the role of top-down controls on macroalgal
community dynamics began with the well-known classical ecological studies in
rocky intertidal and subtidal communities (Paine and Vadas
1969
; Lubchenco
1978
,
1983
; Estes et al.
1978
; Lubchenco and Menge
1978
). These studies showed how
removal of the dominant grazers or predators influenced macroalgal diversity and
community structure directly or indirectly through cascading effects from the top-
down. In the experimental study by Paine and Vadas (
1969
), the removal of the sea
urchin
Strongylocentrotus purpuratus
resulted in an increase in the biomass of
