Biology Reference
In-Depth Information
consumption of adult seaweeds is negligible, strong community-structuring effects
may occur in the case of selective removal of algal propagules. Selective grazing
reduces seaweed interspecific competition, influencing overall community compo-
sition and algal distribution patterns. Mesograzing at high densities can damage
valuable parts of the algal thallus such as reproductive or meristematic regions,
facilitate access for other grazers or pathogens, or weaken the thallus and render the
seaweed prone to breakage. Seaweed biomass reduction may be greater with
increased herbivore diversity, in addition to high abundance, because the comple-
mentary feeding modes and preferences of different grazers are more effective in
removing algal biomass (Table
8.2
). Conversely, while there is little directed
research yet, meta-analysis indicates that higher algal diversity may lead to
decreased grazer effects (Hillebrand and Cardinale
2004
). It was argued that, if
more algal species are present, there is a higher likelihood that the community
contains unpalatable species, or that a more diverse system may have increased
indirect and facilitative effects that may benefit seaweeds overall (Hillebrand and
Cardinale
2004
). Similar to macrograzers, mesograzers themselves are often under
strong control of predators, which can efficiently decimate grazer densities and thus
indirectly enhance seaweed growth. For example, micro-carnivorous fishes effi-
ciently prey upon mesograzers in a Californian kelp bed and enhance kelp frond
density (Davenport and Anderson
2007
).
Among other environmental variables, mesograzers may be particularly suscep-
tible to hydrodynamic forces that can dislodge them from their host alga, thus
reducing the effects that grazing has on seaweed communities. Nutrient levels, e.g.,
due to eutrophication in coastal waters (“bottom-up” effects), may partially coun-
teract grazer-induced “top-down” effects on seaweed diversity and production. The
relative importance of grazer versus nutrient effects on seaweed communities is
highly context dependent; nutrient effects seem to be strongest in naturally high-
nutrient systems but less important in oligotrophic systems, and they may also vary
based on algal functional group (e.g., foliose, filamentous, crustose; Burkpile and
Hay
2006
).
Occasionally, herbivore feeding can also benefit seaweeds and communities
(Hay et al.
2004
). A common effect of mesograzers is the removal of filamentous
epiphytes from a host, possibly affording better light and nutrient acquisition to the
host plant by reducing algal epiphyte biomass (Duffy
1990
). Also, coralline algae
have higher accretion rates when grazed because the positive effects of grazing,
such as the removal of competitively superior fleshy algae, outweigh the grazing
damage to the coralline (Littler et al.
1995
). Grazing, in general, cannot only open
new space for algal recruits, but in the case of grazing gastropods, the pedal mucus
trail left on the rocky surface can effectively trap spores and other seaweed
propagules and support their germination (Santelices and Bobadilla
1996
). Consid-
ering the typically high abundance of gastropods in rocky intertidal systems, this
may enhance new seaweed recruitment (and new food resources for the grazer!)
into an area where seaweeds are under strong grazing pressure. Grazing may also
enhance algal productivity and algal recruitment even if algal biomass is reduced.
For example, it has been hypothesized that either the reduction of self-shading or
