466
John F. Valentine and J. Emmett Duffy
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.
III. An Evolutionary Perspective
on Seagrasses and their Grazers
To put the role of grazing in seagrass systems in
perspective, we begin with a long-term evolutionary
perspective, inspired by comparisons with terrestrial grasses and the steppe-dominated ecosystems
whose food webs are dominated by large herds of
grazing ungulates. It is generally appreciated that
the plants of these terrestrial grassland ecosystems
share a long and close evolutionary history with large
vertebrate grazers, and that important traits of both
the grazers and the grasses represent, at least in part,
adaptations to their long and intense coevolutionary
interactions (Barnard and Frankel 1964; Mack and
Thompson 1982; McNaughton 1984; Coughenour
1985). Many of these ungulates are characterized by
dental adaptations resulting from grazing on gritty,
silica-rich grasses. Terrestrial grasses, in turn, developed belowground basal meristems, rhizomes, adventitious fibrous roots, clonal propagation through
tillering, rapid growth of deciduous leaves, high tolerance to partial defoliation, and other traits that minimize the impacts of sustained grazing by herds of
these large grazers. Grasses with these traits dominate most heavily grazed terrestrial ecosystems, although they are also present in many ungrazed areas.
Similarly, wherever large domestic grazing livestock
have been introduced, rhizomatous grasses with the
same suite of traits, whether native or alien, are dominant. These trends, along with the simultaneous rise
of extensive grasslands and diverse grazing ungulates, support the premise that rhizomatous grasses
have coevolved with chronic grazing by large vertebrates, even if some originally arose as adaptations
to drought tolerance (Barnard and Frankel 1964;
Coughenour 1985).
Seagrasses evolved independently of terrestrial
grasses (Larkum and Den Hartog 1989; Les et al.
1997), but these two groups are convergent in several conspicuous respects. Interestingly, seagrasses
share many of the same features with terrestrial
grasses discussed above, which have been suggested
as adaptations to heavy grazing pressure. These include clonal propagation and resultant physiological integration of ramets, in many cases the possession of largely inaccessible belowground basal
meristems and branching rhizomes, an abundance
of small deciduous shoots, and often rapid regeneration of defoliated tissues (Valentine and Heck
1999; Valentine et al. 2004). There are, of course,
many possible adaptive benefits of such growth patterns. Nevertheless, the convergent presence of these
morphological, growth, and life history characters
in many seagrasses, which are widely understood to
represent adaptations to chronic grazing in terrestrial
grasses, suggests the intriguing possibility that seagrasses also may have evolved under an historical
regime of intense and regular grazing. Who might
have been responsible for such grazing? In modern
ecosystems, certain fishes, sea turtles, sea urchins
and waterfowl feed directly on seagrasses and can
have important impacts on them, as we document
below. But the grazers that have had the strongest
ecological and evolutionary impacts on seagrasses
over time have probably been the green turtles, sirenians including dugongs and manatees, and waterfowl (Valentine and Heck 1999; Domning 2001).
Extant sirenians and some waterfowl feed preferentially or even obligately on seagrasses, and include the only animals known to feed on seagrass
rhizomes. Sirenians were both diverse and abundant in warm marine waters prior to the Pleistocene,
and appear to have been closely associated with
John F. Valentine and J. Emmett Duffy
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.
III. An Evolutionary Perspective
on Seagrasses and their Grazers
To put the role of grazing in seagrass systems in
perspective, we begin with a long-term evolutionary
perspective, inspired by comparisons with terrestrial grasses and the steppe-dominated ecosystems
whose food webs are dominated by large herds of
grazing ungulates. It is generally appreciated that
the plants of these terrestrial grassland ecosystems
share a long and close evolutionary history with large
vertebrate grazers, and that important traits of both
the grazers and the grasses represent, at least in part,
adaptations to their long and intense coevolutionary
interactions (Barnard and Frankel 1964; Mack and
Thompson 1982; McNaughton 1984; Coughenour
1985). Many of these ungulates are characterized by
dental adaptations resulting from grazing on gritty,
silica-rich grasses. Terrestrial grasses, in turn, developed belowground basal meristems, rhizomes, adventitious fibrous roots, clonal propagation through
tillering, rapid growth of deciduous leaves, high tolerance to partial defoliation, and other traits that minimize the impacts of sustained grazing by herds of
these large grazers. Grasses with these traits dominate most heavily grazed terrestrial ecosystems, although they are also present in many ungrazed areas.
Similarly, wherever large domestic grazing livestock
have been introduced, rhizomatous grasses with the
same suite of traits, whether native or alien, are dominant. These trends, along with the simultaneous rise
of extensive grasslands and diverse grazing ungulates, support the premise that rhizomatous grasses
have coevolved with chronic grazing by large vertebrates, even if some originally arose as adaptations
to drought tolerance (Barnard and Frankel 1964;
Coughenour 1985).
Seagrasses evolved independently of terrestrial
grasses (Larkum and Den Hartog 1989; Les et al.
1997), but these two groups are convergent in several conspicuous respects. Interestingly, seagrasses
share many of the same features with terrestrial
grasses discussed above, which have been suggested
as adaptations to heavy grazing pressure. These include clonal propagation and resultant physiological integration of ramets, in many cases the possession of largely inaccessible belowground basal
meristems and branching rhizomes, an abundance
of small deciduous shoots, and often rapid regeneration of defoliated tissues (Valentine and Heck
1999; Valentine et al. 2004). There are, of course,
many possible adaptive benefits of such growth patterns. Nevertheless, the convergent presence of these
morphological, growth, and life history characters
in many seagrasses, which are widely understood to
represent adaptations to chronic grazing in terrestrial
grasses, suggests the intriguing possibility that seagrasses also may have evolved under an historical
regime of intense and regular grazing. Who might
have been responsible for such grazing? In modern
ecosystems, certain fishes, sea turtles, sea urchins
and waterfowl feed directly on seagrasses and can
have important impacts on them, as we document
below. But the grazers that have had the strongest
ecological and evolutionary impacts on seagrasses
over time have probably been the green turtles, sirenians including dugongs and manatees, and waterfowl (Valentine and Heck 1999; Domning 2001).
Extant sirenians and some waterfowl feed preferentially or even obligately on seagrasses, and include the only animals known to feed on seagrass
rhizomes. Sirenians were both diverse and abundant in warm marine waters prior to the Pleistocene,
and appear to have been closely associated with