Chapter 20 Grazing in Seagrass Ecosystems
489
relatively resistant to perturbation. For example,
Heck et al. (2000) have shown that high seagrass
shoot density can maintain high mesograzer densities, which in turn can control algal biomass accumulation, even under strongly enhanced nutrient loading or enhanced abundance of invertebrate-feeding
fishes. Thus, a healthy seagrass bed has some resistance to both eutrophication and the increase in small
predator densities that might result from overfishing
of large predators.
Well-lit sediment bottoms can also show alternate
community state, being dominated by benthic microalgae or macroalgal beds resistant to seagrass
encroachment. Under high light and nutrient conditions, algae are inherently competitively superior
to seagrasses (Valiela et al. 1997). Thus, once in
place, algae are likely to resist the establishment of
dense seagrass populations. Moreover, during initial
phases of establishment, seagrasses will be patchy
and at relatively low shoot density. Many experiments have shown that predators are much more effective in capturing seagrass-associated prey at such
low shoot densities. Thus, during early (sparse) seagrass establishment, abundant light in the sparse
canopy should favor rapid epiphyte growth and
mesograzers should be maintained at low densities by predation (unless associated macroalgae can
support a reservoir of mesograzers protected from
predation). The low mesograzer densities may act
synergistically with the strong light, allowing algal
biomass to accumulate.
Thus, a degraded system, with low seagrass and
high algal biomass, will tend to resist restoration of
seagrasses. It is noteworthy that the several anthropogenic insults common to shallow nearshore systems all would appear to drive this system toward
the algal-dominated state, albeit via different points
in the cycle (Fig. 6).
IX. Questions and Recommendations
for Future Research
A quarter of a century ago, den Hartog (1977) suggested that quantifying effects of grazers on seagrasses should be a “priority desideratum” for future
research. Considerable progress has been made in
such quantification (Orth and van Montfrans 1984;
Jernakoff et al. 1996; Valentine and Heck 1999;
Tables 1 & 2), and, despite lingering uncertainty,
several converging lines of evidence suggest that
top-down forces are likely to play important roles in
structuring seagrass communities—in concert with
the always important influence of bottom-up processes. This evidence includes the typically strong
interactions among consumers and prey in marine
systems generally (Cyr and Pace 1993; Menge 1995;
Duffy and Hay 2001; Shurin et al. 2002; Fig. 1),
the strong grazing pressure, particularly on epiphytes, characteristic of seagrass systems specifically (Tables 1 & 2, Fig. 5), the high abundance of
grazers and predators in many seagrass ecosystems
(Heck and Orth 1980; Klumpp et al. 1989; Williams
and Heck 2001), and the relatively low species
diversity of many temperate seagrass ecosystems,
which should render trophic cascades more effective
(Strong 1992; Duffy 2002). Yet, despite progress,
consensus still eludes us regarding the importance
of direct grazing on seagrasses, specifically, and the
role of consumer control in seagrass communities
and ecosystems, more generally.
Uncertainty about the role of consumer control in
seagrass ecosystems will be resolved only by rigorous multifaceted field studies that include manipulations of grazing pressure, conducted under realistic conditions and of sufficient duration to detect
population- and community-level responses. Central challenges involve scaling up the lab, mesocosm, and small-plot field experiments conducted
so far to address processes occurring over larger
scales and involving highly mobile consumers. Laboratory and mesocosm experiments, such as those
that have dominated seagrass-grazer research to
date, have widely recognized limitations (Carpenter 1996; Schindler 1998; Drenner and Mazumder
1999; Huston 1999), and must be interpreted with
some caution. It is perhaps less widely recognized—
but worth emphasizing—that observational studies
and measurements of unmanipulated systems, no
matter how long-running or detailed, also have limited ability to identify the causal mechanisms that
structure communities (Hairston 1989; Paine 1994).
A cautionary example involves the most conspicuous pattern in marine ecology, the sharp zonation of rocky intertidal organisms, which was believed for decades if not centuries to reflect a simple bottom-up mechanism of differential physiological tolerances of species to physical factors. This
conventional wisdom proved erroneous when experiments showed that the patterns were strongly
controlled by competition and predation (Connell
1961a,b).
489
relatively resistant to perturbation. For example,
Heck et al. (2000) have shown that high seagrass
shoot density can maintain high mesograzer densities, which in turn can control algal biomass accumulation, even under strongly enhanced nutrient loading or enhanced abundance of invertebrate-feeding
fishes. Thus, a healthy seagrass bed has some resistance to both eutrophication and the increase in small
predator densities that might result from overfishing
of large predators.
Well-lit sediment bottoms can also show alternate
community state, being dominated by benthic microalgae or macroalgal beds resistant to seagrass
encroachment. Under high light and nutrient conditions, algae are inherently competitively superior
to seagrasses (Valiela et al. 1997). Thus, once in
place, algae are likely to resist the establishment of
dense seagrass populations. Moreover, during initial
phases of establishment, seagrasses will be patchy
and at relatively low shoot density. Many experiments have shown that predators are much more effective in capturing seagrass-associated prey at such
low shoot densities. Thus, during early (sparse) seagrass establishment, abundant light in the sparse
canopy should favor rapid epiphyte growth and
mesograzers should be maintained at low densities by predation (unless associated macroalgae can
support a reservoir of mesograzers protected from
predation). The low mesograzer densities may act
synergistically with the strong light, allowing algal
biomass to accumulate.
Thus, a degraded system, with low seagrass and
high algal biomass, will tend to resist restoration of
seagrasses. It is noteworthy that the several anthropogenic insults common to shallow nearshore systems all would appear to drive this system toward
the algal-dominated state, albeit via different points
in the cycle (Fig. 6).
IX. Questions and Recommendations
for Future Research
A quarter of a century ago, den Hartog (1977) suggested that quantifying effects of grazers on seagrasses should be a “priority desideratum” for future
research. Considerable progress has been made in
such quantification (Orth and van Montfrans 1984;
Jernakoff et al. 1996; Valentine and Heck 1999;
Tables 1 & 2), and, despite lingering uncertainty,
several converging lines of evidence suggest that
top-down forces are likely to play important roles in
structuring seagrass communities—in concert with
the always important influence of bottom-up processes. This evidence includes the typically strong
interactions among consumers and prey in marine
systems generally (Cyr and Pace 1993; Menge 1995;
Duffy and Hay 2001; Shurin et al. 2002; Fig. 1),
the strong grazing pressure, particularly on epiphytes, characteristic of seagrass systems specifically (Tables 1 & 2, Fig. 5), the high abundance of
grazers and predators in many seagrass ecosystems
(Heck and Orth 1980; Klumpp et al. 1989; Williams
and Heck 2001), and the relatively low species
diversity of many temperate seagrass ecosystems,
which should render trophic cascades more effective
(Strong 1992; Duffy 2002). Yet, despite progress,
consensus still eludes us regarding the importance
of direct grazing on seagrasses, specifically, and the
role of consumer control in seagrass communities
and ecosystems, more generally.
Uncertainty about the role of consumer control in
seagrass ecosystems will be resolved only by rigorous multifaceted field studies that include manipulations of grazing pressure, conducted under realistic conditions and of sufficient duration to detect
population- and community-level responses. Central challenges involve scaling up the lab, mesocosm, and small-plot field experiments conducted
so far to address processes occurring over larger
scales and involving highly mobile consumers. Laboratory and mesocosm experiments, such as those
that have dominated seagrass-grazer research to
date, have widely recognized limitations (Carpenter 1996; Schindler 1998; Drenner and Mazumder
1999; Huston 1999), and must be interpreted with
some caution. It is perhaps less widely recognized—
but worth emphasizing—that observational studies
and measurements of unmanipulated systems, no
matter how long-running or detailed, also have limited ability to identify the causal mechanisms that
structure communities (Hairston 1989; Paine 1994).
A cautionary example involves the most conspicuous pattern in marine ecology, the sharp zonation of rocky intertidal organisms, which was believed for decades if not centuries to reflect a simple bottom-up mechanism of differential physiological tolerances of species to physical factors. This
conventional wisdom proved erroneous when experiments showed that the patterns were strongly
controlled by competition and predation (Connell
1961a,b).