Chapter 20 Grazing in Seagrass Ecosystems
485
This leads us to the broader issue of how grazer
populations—and their associated grazing impact—
are regulated in seagrass systems.
VII. Bottom-Up and Top-Down Control
in Seagrass Communities
Strong (1992) argued that community-wide trophic
cascades, in which total plant biomass is strongly reduced, are likely to be most common in low-diversity,
algal-based, systems, where single species or functional groups dominate trophic interactions at each
level. This entirely plausible suggestion has grown
into an article of faith among many ecologists, although it remains largely untested (Duffy 2002). In
the context of the present discussion, the important
point is that many seagrass systems may approximate the criteria Strong suggested should foster
trophic cascades, namely strong trophic interactions
among a relatively small number of keystone species
or homogeneous functional groups at each trophic
level. Thus, first principles might suggest that trophic
cascade dynamics may be common in seagrass beds.
Alternatively, it might be argued that dominance by
vascular plants, rather than by highly edible algae,
would foil trophic cascades in seagrass systems. We
find this argument unconvincing in light of the repeated observations from lab and mesocosm experiments that certain crustaceans and other animals
can greatly reduce seagrass biomass in the absence
of predator control. Thus, our specific questions become: (1) what controls grazer biomass and impact
under normal circumstances, and (2) what controls
seagrass biomass and relative dominance? We must
consider the first of these before proceeding to the
primary issue of what controls seagrass abundance.
A. Bottom-Up or Top-Down Control
of Grazer Populations?
Herbivores are central in food webs both as converters of primary production to a form available
to higher-order consumers, and in their important
structuring influence on plant communities. As such,
the factors regulating herbivore populations are central to understanding dynamics of seagrass (and
other) ecosystems more generally. However, determining what controls populations of mesograzers in
shallow benthic systems has proven as difficult as
the parallel question of what regulates seagrass populations (Heck and Orth 1980; Heck et al. 2000;
Williams and Heck 2001; Schanz et al. 2002). A primary reason for this ambiguity is the small sizes and
rapid population growth rates of mesograzers, which
seriously challenge our ability to conduct the standard field experimental manipulations (e.g. caging)
that have proven so successful in understanding the
community ecology of larger invertebrates and algae, particularly on hard substrata (e.g. Menge 1995;
Bertness et al. 2001).
Historically, the conventional wisdom has held
that mesograzer populations are usually controlled
by predators, primarily fishes (Nelson 1979, 1981;
Heck and Orth 1980; Orth et al. 1984; Duffy and
Hay 1991). Several lines of indirect evidence support this view. First, small mobile invertebrates, particularly crustaceans, in the mesograzer size range
dominate diets of many nearshore fishes and larger
invertebrates (Kikuchi 1974; Adams 1976). Indeed,
production of small crustaceans is one of the best
predictors of fish production in vegetated marine
ecosystems (Edgar and Shaw 1995; Taylor 1998).
Moreover, seasonal abundance patterns of fishes
and mobile epifauna often show negative covariance (e.g. Nelson 1979; Stoner 1980; Choate and
Kingett 1982; Duffy and Hay 1991; Valentine and
Heck 1993). These patterns, together with a perceived superabundance of attached microalgae in
most seagrass ecosystems, led many researchers to
conclude that fish predation regulates mesograzer
abundance and by extension grazing pressure in
most coastal ecosystems. Despite the circumstantial
evidence for predator regulation of epifaunal populations, experimental tests have largely failed to
provide definitive support for this hypothesis (references in Edgar 1993). There are several possible
methodological reasons for this, including enhancement of mesograzer food and habitat by cage structures, as well as increased predation on mesograzers in predator “exclusion” cages by small decapod
crustaceans that often “bloom” or immigrate into
cages designed to exclude fishes (Young et al. 1976;
Virnstein 1978; Nelson 1981; Olafsson et al. 1994).
Moreover, many smaller consumers in seagrass beds
are in fact omnivores that feed both on mesograzers
and their epiphytic forage (Heck et al. 2000), further
complicating the interpretation of the relationship
between mesograzer density and grazing intensity.
As a result, our understanding of the overall role of
top-down control in seagrass beds remains poorly
understood.
485
This leads us to the broader issue of how grazer
populations—and their associated grazing impact—
are regulated in seagrass systems.
VII. Bottom-Up and Top-Down Control
in Seagrass Communities
Strong (1992) argued that community-wide trophic
cascades, in which total plant biomass is strongly reduced, are likely to be most common in low-diversity,
algal-based, systems, where single species or functional groups dominate trophic interactions at each
level. This entirely plausible suggestion has grown
into an article of faith among many ecologists, although it remains largely untested (Duffy 2002). In
the context of the present discussion, the important
point is that many seagrass systems may approximate the criteria Strong suggested should foster
trophic cascades, namely strong trophic interactions
among a relatively small number of keystone species
or homogeneous functional groups at each trophic
level. Thus, first principles might suggest that trophic
cascade dynamics may be common in seagrass beds.
Alternatively, it might be argued that dominance by
vascular plants, rather than by highly edible algae,
would foil trophic cascades in seagrass systems. We
find this argument unconvincing in light of the repeated observations from lab and mesocosm experiments that certain crustaceans and other animals
can greatly reduce seagrass biomass in the absence
of predator control. Thus, our specific questions become: (1) what controls grazer biomass and impact
under normal circumstances, and (2) what controls
seagrass biomass and relative dominance? We must
consider the first of these before proceeding to the
primary issue of what controls seagrass abundance.
A. Bottom-Up or Top-Down Control
of Grazer Populations?
Herbivores are central in food webs both as converters of primary production to a form available
to higher-order consumers, and in their important
structuring influence on plant communities. As such,
the factors regulating herbivore populations are central to understanding dynamics of seagrass (and
other) ecosystems more generally. However, determining what controls populations of mesograzers in
shallow benthic systems has proven as difficult as
the parallel question of what regulates seagrass populations (Heck and Orth 1980; Heck et al. 2000;
Williams and Heck 2001; Schanz et al. 2002). A primary reason for this ambiguity is the small sizes and
rapid population growth rates of mesograzers, which
seriously challenge our ability to conduct the standard field experimental manipulations (e.g. caging)
that have proven so successful in understanding the
community ecology of larger invertebrates and algae, particularly on hard substrata (e.g. Menge 1995;
Bertness et al. 2001).
Historically, the conventional wisdom has held
that mesograzer populations are usually controlled
by predators, primarily fishes (Nelson 1979, 1981;
Heck and Orth 1980; Orth et al. 1984; Duffy and
Hay 1991). Several lines of indirect evidence support this view. First, small mobile invertebrates, particularly crustaceans, in the mesograzer size range
dominate diets of many nearshore fishes and larger
invertebrates (Kikuchi 1974; Adams 1976). Indeed,
production of small crustaceans is one of the best
predictors of fish production in vegetated marine
ecosystems (Edgar and Shaw 1995; Taylor 1998).
Moreover, seasonal abundance patterns of fishes
and mobile epifauna often show negative covariance (e.g. Nelson 1979; Stoner 1980; Choate and
Kingett 1982; Duffy and Hay 1991; Valentine and
Heck 1993). These patterns, together with a perceived superabundance of attached microalgae in
most seagrass ecosystems, led many researchers to
conclude that fish predation regulates mesograzer
abundance and by extension grazing pressure in
most coastal ecosystems. Despite the circumstantial
evidence for predator regulation of epifaunal populations, experimental tests have largely failed to
provide definitive support for this hypothesis (references in Edgar 1993). There are several possible
methodological reasons for this, including enhancement of mesograzer food and habitat by cage structures, as well as increased predation on mesograzers in predator “exclusion” cages by small decapod
crustaceans that often “bloom” or immigrate into
cages designed to exclude fishes (Young et al. 1976;
Virnstein 1978; Nelson 1981; Olafsson et al. 1994).
Moreover, many smaller consumers in seagrass beds
are in fact omnivores that feed both on mesograzers
and their epiphytic forage (Heck et al. 2000), further
complicating the interpretation of the relationship
between mesograzer density and grazing intensity.
As a result, our understanding of the overall role of
top-down control in seagrass beds remains poorly
understood.