The basic qualitative biology of herbivory in seagrass beds is reasonably well known. Despite considerable variance in feeding strategies among species, we can generalize that vertebrate herbivores (fishes, turtles, sirenians, and waterfowl) and sea urchins often graze seagrasses directly, whereas smaller meso-grazers (crustaceans, gastropods, and other invertebrates) usually feed on algae attached to seagrass leaves. These patterns have been thoroughly documented by previous reviews (Orth and van Mont-frans 1984; van Montfrans et al. 1984; Thayer et al. 1984; Klumpp et al. 1989; Jernakoff et al. 1996; Valentine and Heck 1999; Williams and Heck 2001).
Given the strength of these previously documented generalizations, which we update below, we focus here on placing grazing in the larger context of seagrass community and ecosystem dynamics. The time appears ripe to ask new questions such as the following: What environmental and biological factors influence grazing intensity? How frequently and strongly do grazers control seagrass biomass and density? How does grazing influence seagrass beds at the community level, particularly the relative dominance of seagrasses, macroalgae, microalgae, and sessile invertebrates? How does grazing influence ecosystem process rates (e.g. primary productivity, metabolism, and materials fluxes) in seagrass beds? Moving up a trophic level, how is grazing impact influenced by cascading predator control, and to what extent has this control been altered by the overfishing of larger marine piscivores? In short, what would seagrass beds look like and how would they work in the absence of grazers? Indeed, would they even exist in the absence of grazers? Although most of these questions cannot be answered at present, we hope this review will stimulate research focused on doing so.
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