Chapter 20 Grazing in Seagrass Ecosystems
469
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. Bjorndal 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 seagrass grazers (the green turtles, dugongs, manatees,
fishes, and waterfowl) anomalously low (Randall
1965; Heinsohn and Birch 1972; Lipkin 1975; Charman 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.
B. Nutritional Value of Seagrasses
Several investigators have shown that, in contrast
to the poor-nutrition hypothesis, nitrogen concentrations of seagrass leaves are not dramatically lower,
when compared on a standardized basis, than those
of algae that form the base of most marine food webs
(Lobel and Ogden 1981; Lowe and Lawrence 1976;
Thayer et al. 1984; Strong 1992; Pillans et al. 2004;
see also Section III.A in Mateo et al., Chapter 7 and
their Fig. 4). There is also evidence that detrital seagrass leaves can be an even poorer source of nutrition
(i.e. have higher C/N ratios) for consumers than are
living leaves (Klumpp and Van der Walk 1984), as
seagrass detritus resists decay (Zieman 1975; Harrison and Mann 1975; Fenchel 1977; Thayer et al.
1977; Rice 1982). The use of C/N ratios as criteria to
assess the nutritional value of marine macrophytes
can be traced to Russell-Hunter (1970) who concluded that food with a C/N ratio <17 was required
to satisfy human protein requirements. While the
C/N ratio has been correlated with growth of some
pelagic freshwater herbivores (Elser et al. 2003), it
has not been clearly linked to performance of marine
herbivores (cf. Thayer et al. 1977; Hatcher 1994).
Therefore, it seems unlikely that high C/N ratios
(>17), by themselves, are responsible for the relatively low grazing on seagrasses.
Many marine herbivores, in fact, have morphological adaptations or digestive capabilities that allow them to obtain nutrients and energy from marine
vascular plants. Some fishes have low gut pH, allowing them to digest cellulose (e.g. Lobel 1981; Lobel
and Ogden 1981; Montgomery and Targett 1992),
while other fishes, reptiles, and sirenians (i.e. manatees and dugongs) possess microbial symbionts capable of digesting cellulose in seagrass leaf tissues
(Bjorndal 1979; Thayer et al. 1984; Luczkovich and
Stellwag 1993). In addition, the presence of calcareous epiphytes on surfaces of some seagrass leaves
may play an important role, in enhancing assimilation of seagrass tissue in some fishes (T. Arnold,
per. comm.). These observations have led some to
conclude that, as has been suggested for many herbivorous insects and mammals, herbivore feeding on
seagrasses is determined largely by nitrogen availability, rather than by carbon content (e.g. Boyd and
Goodyear 1971; Mann 1972; Lilly 1975; Bjorndal
1980; Zieman et al. 1984; Williams 1988; McGlathery 1995; Preen 1995, this chapter). Thus, it is unclear whether low nutritional value of seagrass tissues is a major impediment to grazing, at least by
vertebrates.
C. The Ghost of Grazing Past
Mounting evidence indicates that human fishing and
other activities have had strong, far-reaching, and
sometimes catastrophic impacts on large marine animals throughout the world (Jackson et al. 2001;
Pauly and Maclean 2003; Myers and Worm 2003).
The ecosystem-level impacts of these declines in
consumer density can only be estimated indirectly,
but they are surely large (Jackson et al. 2001; Duffy
2003). In particular, severe reductions in abundance
of large vertebrate grazers (notably green turtles and
sirenians but also waterfowl) have almost certainly
reduced the importance of herbivory in seagrass food
webs (Madsen 1998; Domning 2001). Despite these
upheavals, some marine food webs have sufficient
functional redundancy that the remaining herbivores
continue to have significant effects on aboveground
seagrass biomass (next section, Table 1), even if it
is neither as intense, nor as consistent in space and
time, as it once was. The suggested impacts of these
historical reductions in grazing pressure are difficult to evaluate; however, because there have been
few controlled field experiments designed to determine the degree to which modern herbivores impact
469
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. Bjorndal 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 seagrass grazers (the green turtles, dugongs, manatees,
fishes, and waterfowl) anomalously low (Randall
1965; Heinsohn and Birch 1972; Lipkin 1975; Charman 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.
B. Nutritional Value of Seagrasses
Several investigators have shown that, in contrast
to the poor-nutrition hypothesis, nitrogen concentrations of seagrass leaves are not dramatically lower,
when compared on a standardized basis, than those
of algae that form the base of most marine food webs
(Lobel and Ogden 1981; Lowe and Lawrence 1976;
Thayer et al. 1984; Strong 1992; Pillans et al. 2004;
see also Section III.A in Mateo et al., Chapter 7 and
their Fig. 4). There is also evidence that detrital seagrass leaves can be an even poorer source of nutrition
(i.e. have higher C/N ratios) for consumers than are
living leaves (Klumpp and Van der Walk 1984), as
seagrass detritus resists decay (Zieman 1975; Harrison and Mann 1975; Fenchel 1977; Thayer et al.
1977; Rice 1982). The use of C/N ratios as criteria to
assess the nutritional value of marine macrophytes
can be traced to Russell-Hunter (1970) who concluded that food with a C/N ratio <17 was required
to satisfy human protein requirements. While the
C/N ratio has been correlated with growth of some
pelagic freshwater herbivores (Elser et al. 2003), it
has not been clearly linked to performance of marine
herbivores (cf. Thayer et al. 1977; Hatcher 1994).
Therefore, it seems unlikely that high C/N ratios
(>17), by themselves, are responsible for the relatively low grazing on seagrasses.
Many marine herbivores, in fact, have morphological adaptations or digestive capabilities that allow them to obtain nutrients and energy from marine
vascular plants. Some fishes have low gut pH, allowing them to digest cellulose (e.g. Lobel 1981; Lobel
and Ogden 1981; Montgomery and Targett 1992),
while other fishes, reptiles, and sirenians (i.e. manatees and dugongs) possess microbial symbionts capable of digesting cellulose in seagrass leaf tissues
(Bjorndal 1979; Thayer et al. 1984; Luczkovich and
Stellwag 1993). In addition, the presence of calcareous epiphytes on surfaces of some seagrass leaves
may play an important role, in enhancing assimilation of seagrass tissue in some fishes (T. Arnold,
per. comm.). These observations have led some to
conclude that, as has been suggested for many herbivorous insects and mammals, herbivore feeding on
seagrasses is determined largely by nitrogen availability, rather than by carbon content (e.g. Boyd and
Goodyear 1971; Mann 1972; Lilly 1975; Bjorndal
1980; Zieman et al. 1984; Williams 1988; McGlathery 1995; Preen 1995, this chapter). Thus, it is unclear whether low nutritional value of seagrass tissues is a major impediment to grazing, at least by
vertebrates.
C. The Ghost of Grazing Past
Mounting evidence indicates that human fishing and
other activities have had strong, far-reaching, and
sometimes catastrophic impacts on large marine animals throughout the world (Jackson et al. 2001;
Pauly and Maclean 2003; Myers and Worm 2003).
The ecosystem-level impacts of these declines in
consumer density can only be estimated indirectly,
but they are surely large (Jackson et al. 2001; Duffy
2003). In particular, severe reductions in abundance
of large vertebrate grazers (notably green turtles and
sirenians but also waterfowl) have almost certainly
reduced the importance of herbivory in seagrass food
webs (Madsen 1998; Domning 2001). Despite these
upheavals, some marine food webs have sufficient
functional redundancy that the remaining herbivores
continue to have significant effects on aboveground
seagrass biomass (next section, Table 1), even if it
is neither as intense, nor as consistent in space and
time, as it once was. The suggested impacts of these
historical reductions in grazing pressure are difficult to evaluate; however, because there have been
few controlled field experiments designed to determine the degree to which modern herbivores impact