Chapter 20 Grazing in Seagrass Ecosystems
479
Lanyon et al. 1989; Nietschmann 1990; Provancha
and Hall 1991; Preen 1995). These large herbivores
can have even greater impacts on seagrass productivity and abundance than sea urchins or fishes (Zieman
et al. 1984). All of these examples suggest that seagrass herbivory, although reduced in an historical
context, can represent an important and underestimated trophic pathway in many areas today, rather
than a highly localized anomalous event.
E. Dynamic Seagrass Responses can Lead
to Underestimates of Grazing Losses
Ecologists have long known that multiple factors
can determine the rate at which plants recover following bouts of grazing. Recognizing this, aquatic
and terrestrial ecologists have employed increasingly
sophisticated field manipulations of nutrients, grazers, light and their interactions (see recent reviews
by Vanni 1987b; Athey and Connor 1989; Neckles
et al. 1993; Rosemond et al. 1993) to identify the
dynamic nature of interactions between grazers and
their forage. Based on such studies in marine and
other ecosystems, we know that plants can compensate for losses to grazers via either enhanced growth,
increasing nutrient uptake from the surrounding environment (Lehman and Scavia 1982; Cargill and
Jeffries 1984; Bianchi 1988; Williams and Carpenter 1988; Seagle et al. 1992; Sand-Jensen et al. 1994;
McNaughton et al. 1996) or translocating nutrients
among physiologically integrated ramets (J´ onsd´ ottir
and Callaghan 1990). Yet the mechanisms by which
seagrasses respond to herbivory remain inadequately
studied.
Unlike many species of marine macroalgae and
phytoplankton that are wholly exposed to marine
grazers and often totally consumed by them (cf.
Hay and Steinberg 1992; Steneck and Dethier 1994),
the stored reserves and sites of nutrient uptake for
many seagrasses are located in a belowground refuge
which is not readily accessible to most grazers. This
belowground refuge represents a stabilizing influence that allows seagrasses to persist where herbivory is intense and can, depending on the season,
allow seagrasses to recover rapidly to pre-grazing
levels (Valentine and Heck 1999).
Many of the early estimates of grazing on seagrasses were based on simple comparisons of aboveground biomass in grazed and ungrazed areas. From
these studies it has been indirectly estimated that,
depending on site, somewhere between ∼3% and
100% of seagrass net primary production enters food
webs via the grazing pathway. Most of such studies relied on short-term laboratory measurements or
anecdotal field observations to identify the levels of
seagrass production entering local food webs. Such
calculations do not account for the possibility that
grazing stimulates production (e.g. McNaughton
et al. 1996), such that rates of aboveground seagrass production (via either increased production
by surviving shoots or via the increased recruitment of new shoots) following grazing could exceed
those occurring in ungrazed areas. It is likely that
this resulted in large underestimates of the amount
of seagrass production reaching higher-level consumers via the grazing pathway (Kirsch et al. 2002).
This enhanced production following grazing has
been recognized as significant in many ecosystems
(Lehman and Scavia 1982; Cargill and Jeffries 1984;
Bianchi 1988; Williams and Carpenter 1988; Littler
et al. 1995; McNaughton et al. 1996), and more recently has been shown to be substantial in seagrass
ecosystems (Valentine et al. 1997, 2000; Valentine
and Heck 2001). Since new shoots are produced
predominantly at rhizome apices (Tomlinson and
Vargo 1966), it is reasonable to expect that grazing
should also lead to increased belowground production (Valentine et al. 1997). And while we need to develop a better understanding of the role of sediment
porewater nutrient concentrations and rhizome carbohydrate stores in determining seagrass responses
to grazing (cf. Zieman et al. 1984; Valentine et al.
1997; Valentine and Heck 1999), there have been no
such sustained experiments that have simultaneously
considered the roles of each of these factors, how
they could be influenced by herbivory, or what impact they may have on the transfer of energy within
nearshore food webs. Thus, the evidence suggests
that the grazing of seagrasses often has been underestimated due to methodological artifacts.
F. Factors that Influence Temporal and
Geographic Variability of Seagrass Herbivory
Spatial and temporal variation in herbivory among
individual plants is one of the most consistent yet
poorly understood aspects of plant-animal interactions (cf. Louda and Collinge 1992; Hacker and Bertness 1995, 1996). While many explanations have
been advanced to explain why herbivores exhibit
such a discriminating palate, differences in nutritional quality among individual plants appear to be
479
Lanyon et al. 1989; Nietschmann 1990; Provancha
and Hall 1991; Preen 1995). These large herbivores
can have even greater impacts on seagrass productivity and abundance than sea urchins or fishes (Zieman
et al. 1984). All of these examples suggest that seagrass herbivory, although reduced in an historical
context, can represent an important and underestimated trophic pathway in many areas today, rather
than a highly localized anomalous event.
E. Dynamic Seagrass Responses can Lead
to Underestimates of Grazing Losses
Ecologists have long known that multiple factors
can determine the rate at which plants recover following bouts of grazing. Recognizing this, aquatic
and terrestrial ecologists have employed increasingly
sophisticated field manipulations of nutrients, grazers, light and their interactions (see recent reviews
by Vanni 1987b; Athey and Connor 1989; Neckles
et al. 1993; Rosemond et al. 1993) to identify the
dynamic nature of interactions between grazers and
their forage. Based on such studies in marine and
other ecosystems, we know that plants can compensate for losses to grazers via either enhanced growth,
increasing nutrient uptake from the surrounding environment (Lehman and Scavia 1982; Cargill and
Jeffries 1984; Bianchi 1988; Williams and Carpenter 1988; Seagle et al. 1992; Sand-Jensen et al. 1994;
McNaughton et al. 1996) or translocating nutrients
among physiologically integrated ramets (J´ onsd´ ottir
and Callaghan 1990). Yet the mechanisms by which
seagrasses respond to herbivory remain inadequately
studied.
Unlike many species of marine macroalgae and
phytoplankton that are wholly exposed to marine
grazers and often totally consumed by them (cf.
Hay and Steinberg 1992; Steneck and Dethier 1994),
the stored reserves and sites of nutrient uptake for
many seagrasses are located in a belowground refuge
which is not readily accessible to most grazers. This
belowground refuge represents a stabilizing influence that allows seagrasses to persist where herbivory is intense and can, depending on the season,
allow seagrasses to recover rapidly to pre-grazing
levels (Valentine and Heck 1999).
Many of the early estimates of grazing on seagrasses were based on simple comparisons of aboveground biomass in grazed and ungrazed areas. From
these studies it has been indirectly estimated that,
depending on site, somewhere between ∼3% and
100% of seagrass net primary production enters food
webs via the grazing pathway. Most of such studies relied on short-term laboratory measurements or
anecdotal field observations to identify the levels of
seagrass production entering local food webs. Such
calculations do not account for the possibility that
grazing stimulates production (e.g. McNaughton
et al. 1996), such that rates of aboveground seagrass production (via either increased production
by surviving shoots or via the increased recruitment of new shoots) following grazing could exceed
those occurring in ungrazed areas. It is likely that
this resulted in large underestimates of the amount
of seagrass production reaching higher-level consumers via the grazing pathway (Kirsch et al. 2002).
This enhanced production following grazing has
been recognized as significant in many ecosystems
(Lehman and Scavia 1982; Cargill and Jeffries 1984;
Bianchi 1988; Williams and Carpenter 1988; Littler
et al. 1995; McNaughton et al. 1996), and more recently has been shown to be substantial in seagrass
ecosystems (Valentine et al. 1997, 2000; Valentine
and Heck 2001). Since new shoots are produced
predominantly at rhizome apices (Tomlinson and
Vargo 1966), it is reasonable to expect that grazing
should also lead to increased belowground production (Valentine et al. 1997). And while we need to develop a better understanding of the role of sediment
porewater nutrient concentrations and rhizome carbohydrate stores in determining seagrass responses
to grazing (cf. Zieman et al. 1984; Valentine et al.
1997; Valentine and Heck 1999), there have been no
such sustained experiments that have simultaneously
considered the roles of each of these factors, how
they could be influenced by herbivory, or what impact they may have on the transfer of energy within
nearshore food webs. Thus, the evidence suggests
that the grazing of seagrasses often has been underestimated due to methodological artifacts.
F. Factors that Influence Temporal and
Geographic Variability of Seagrass Herbivory
Spatial and temporal variation in herbivory among
individual plants is one of the most consistent yet
poorly understood aspects of plant-animal interactions (cf. Louda and Collinge 1992; Hacker and Bertness 1995, 1996). While many explanations have
been advanced to explain why herbivores exhibit
such a discriminating palate, differences in nutritional quality among individual plants appear to be