Is Direct Grazing on Seagrasses Negligible

One of the few existing paradigms of marine ecology holds that, in most systems, little seagrass primary production is directly grazed (Fig. 2). Most marine texts report that, on average, <30% of sea grass production reaches higher order consumers via the grazing pathway (e.g. Lalli and Parson 1993; Valiela 1995; Levinton 2000; Nybakken 2000; see also Mateo et al., Chapter 7). As a result, the primary conduit for seagrass production to reach higher order consumers is consistently reported to be the detrital pathway (e.g. Fenchel 1970, 1977; Kikuchi and Peres 1977; Nienhaus and Van Ierland 1978; Kikuchi 1980; Thayer et al. 1984; Nienhaus and Groenendijk 1986; Zieman and Zieman 1989; Ce-brian 2002; Mateo et al., Chapter 7). One contributory factor to this view is the observation that in seagrass systems, as in most systems on land, "the world is green" (Hairston et al. 1960). That is, since seagrasses form lush vegetated habitats along the coasts of every continent except Antarctica, herbivores would seem unlikely to play a significant role in controlling their biomass. Despite this plausible qualitative picture, there have been relatively few critical assessments of the importance of grazing in seagrass beds, and fewer direct estimates of the proportion of seagrass production consumed in the field. Consequently, investigations of the factors controlling seagrass growth and biomass emphasize the primacy of the sediment nutrient supply (e.g. Patriquin 1972; Short 1987; Powell et al. 1989; Fourqurean et al. 1992; Short et al. 1993), light availability, and/or physical factors (e.g. Patriquin 1975; Backman and Barilotti 1976; Dennison and Alberte 1985; Thom and Albright 1990).

The history of this "bottom-up" paradigm for sea-grass ecosystems is intriguingly similar to the analogous worldview of early terrestrial ecologists, who also perceived grazing on terrestrial plants to be low. Just as early assessments of terrestrial plant-animal interactions are now known to have been overly simplistic (e.g. Karban and Baldwin 1997; Lowman 1984, 1985; McNaughton 1985; McNaughton et al. 1996), it is increasingly clear that seagrass-grazer interactions are more important and dynamic than previously recognized. In this section we provide an updated overview of recent evidence of direct grazing on seagrasses. Perhaps surprisingly, there is ample evidence that grazing on seagrasses is significant in many areas of the world's oceans. This evidence suggests that it is premature to conclude in general terms how important direct grazing of seagrasses is in determining the structure and productivity of modern coastal food webs.

Two distinct, but not mutually exclusive, explanations have been offered for the apparently low levels of herbivory on seagrasses. These can be classified roughly as bottom-up and top-down hypotheses. The first suggests that seagrass physiology results in low nutritional value that poses an intrinsic barrier to grazers. Specifically, the suggestion has been made that seagrass leaves are of little nutritional value owing to their high C/N ratios (e.g. Bjorn-dal 1980; Duarte 1990; Mateo et al., Chapter 7), and the inability of most grazers to digest cellulose (Lawrence 1975). The second hypothesis recognizes that several herbivores are capable of grazing and assimilating seagrass tissues, but posits that human overharvesting and other activities have left densities of these ecologically important large vertebrate sea-grass grazers (the green turtles, dugongs, manatees, fishes, and waterfowl) anomalously low (Randall 1965; Heinsohn and Birch 1972; Lipkin 1975; Char-man 1979; Bjorndal 1980; Kiorboe 1980; Jacobs et al. 1981; Thayer et al. 1984). These hypotheses are not mutually exclusive, as poor nutritional quality could deter many generalist herbivores, whereas human hunting may have reduced the densities of those specialists capable of grazing seagrasses. We consider each of these hypotheses in turn.

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