468
John F. Valentine and J. Emmett Duffy
Herbivores occupy a central role in determining
the structure and productivity of most marine food
webs. Not only can grazers influence the biomass,
productivity, and community structure of primary
producers (Lubchenco and Gaines 1981; Cyr and
Pace 1993; Menge 1995; Shurin et al. 2002), they
are also an important conduit for the transfer of
energy from primary producers to higher trophic
levels. Thus, grazers exert an important bottom-up
control over the nutritional status, abundance, and
composition of higher-order consumers. This is especially true of the small invertebrate mesograzers,
the key link between primary producers and higher
trophic levels in shallow marine systems (Edgar and
Shaw 1995; Taylor 1998). As such, the influence of
grazers cascades both downwards and upwards in
food webs. Accordingly, the roles of grazers can be
approached from two distinct perspectives, focusing
on their influence on community structure, versus
on ecosystem processes. Paine (1980) emphasized
that these two types of influence, which he referred
to as functional webs (also called interaction webs)
versus energy-flow webs, respectively, are often
not necessarily concordant. That is, a trophic link
with negligible influence on energy and materials
flows under normal circumstances, can nonetheless
have a critical influence on the relative abundance
of species in the community (Paine 1980), and on
the system’s stability (McCann et al. 1998; Berlow
1999). Similarly, key conduits for the transfer of
energy among adjacent trophic levels need not be
strong interactors from a community perspective
(e.g. Kirsch et al. 2002).
Below we consider both the community-level and
ecosystem-level influences of grazing. We consider
separately the mostly tropical systems in which direct grazing of seagrasses can still be substantial, and
the invertebrate-dominated, mostly temperate systems in which grazers feed primarily on attached
epiphytic algae, and impacts on seagrasses are primarily indirect.
V. Direct Grazing on Seagrasses
A. 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 seagrass 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; Cebri´ an 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 seagrass 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
John F. Valentine and J. Emmett Duffy
Herbivores occupy a central role in determining
the structure and productivity of most marine food
webs. Not only can grazers influence the biomass,
productivity, and community structure of primary
producers (Lubchenco and Gaines 1981; Cyr and
Pace 1993; Menge 1995; Shurin et al. 2002), they
are also an important conduit for the transfer of
energy from primary producers to higher trophic
levels. Thus, grazers exert an important bottom-up
control over the nutritional status, abundance, and
composition of higher-order consumers. This is especially true of the small invertebrate mesograzers,
the key link between primary producers and higher
trophic levels in shallow marine systems (Edgar and
Shaw 1995; Taylor 1998). As such, the influence of
grazers cascades both downwards and upwards in
food webs. Accordingly, the roles of grazers can be
approached from two distinct perspectives, focusing
on their influence on community structure, versus
on ecosystem processes. Paine (1980) emphasized
that these two types of influence, which he referred
to as functional webs (also called interaction webs)
versus energy-flow webs, respectively, are often
not necessarily concordant. That is, a trophic link
with negligible influence on energy and materials
flows under normal circumstances, can nonetheless
have a critical influence on the relative abundance
of species in the community (Paine 1980), and on
the system’s stability (McCann et al. 1998; Berlow
1999). Similarly, key conduits for the transfer of
energy among adjacent trophic levels need not be
strong interactors from a community perspective
(e.g. Kirsch et al. 2002).
Below we consider both the community-level and
ecosystem-level influences of grazing. We consider
separately the mostly tropical systems in which direct grazing of seagrasses can still be substantial, and
the invertebrate-dominated, mostly temperate systems in which grazers feed primarily on attached
epiphytic algae, and impacts on seagrasses are primarily indirect.
V. Direct Grazing on Seagrasses
A. 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 seagrass 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; Cebri´ an 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 seagrass 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