482
John F. Valentine and J. Emmett Duffy
and van Montfrans 1984; Klumpp et al. 1989; Brawley 1992; Jernakoff et al. 1996; Heck et al. 2000).
Beds of seagrasses and benthic algae throughout the
world typically harbor dense populations of these
mesograzers. Estimates based on their consumption
and secondary production rates, as well as a limited
number of experimental studies (see below), suggest
that mesograzers play pivotal roles in marine benthic
communities, both in transfer of primary production
to higher trophic levels, including fisheries (Kikuchi
1974; Klumpp et al. 1989; Edgar and Shaw 1995;
Taylor 1998), and in structuring the assemblages of
plants on which they feed (Orth and van Montfrans
1984; Hootsmans and Vermaat 1985; Short et al.
1995; Williams and Ruckleshaus 1993; Jernakoff
et al. 1996; Duffy and Hay 2000; Duffy and Harvilicz
2001; Duffy et al. 2001, 2003).
Of particular interest in the context of seagrass
ecosystems is the potentially central role of mesograzers as ecological engineers (sensu Jones et al.
1994), specifically in indirectly facilitating seagrass
dominance by cropping epiphytic algae that would
otherwise outcompete the seagrasses for light and
water-column nutrients. Numerous lab and microcosm studies have documented that—with some
important exceptions—the majority of small grazing invertebrates associated with marine plants feed
preferentially or exclusively on benthic algae and detritus, and little if at all on seagrasses (reviewed by
Orth and van Montfrans 1984; van Montfrans et al.
1984; Klumpp et al. 1989; Brawley 1992; Jernakoff
et al. 1996). Because algae generally grow faster than
seagrasses, and can grow on the seagrasses themselves, algae are competitively superior to seagrasses
under many conditions and can potentially exclude
them from well-lit shallow waters when grazing is ineffective (Valiela et al. 1997). Thus, mesograzers potentially form one member of a critical three-way interaction with seagrasses and algae; their preferential
grazing on epiphytic algae potentially releases seagrasses from competition, allowing seagrasses and
the ecologically and economically important communities that depend on them to flourish. While the
general feeding preference of mesograzers for algae is well established, however, direct experimental confirmation that this grazing actually translates
into enhanced seagrass performance in the field has
proven more difficult (but see Heck et al. 2000).
Table 2 summarizes existing evidence from laboratory, mesocosm, and field experiments for mesograzer mediation of plant community structure in
seagrass beds. We restricted our consideration to
studies of living seagrasses rooted in sediment, and
that presented some quantitative measure of the
influence of mesograzers relative to treatments in
which mesograzer abundance was experimentally
reduced. The collective weight of these few experiments demonstrates convincingly that natural densities of small crustaceans and gastropods greatly reduce the accumulation of epiphytic algae on macrophytes, and suggests strongly, although indirectly,
that exclusion of grazers often has serious negative consequences for seagrass growth, biomass, and
cover.
Despite the clear potential for mesograzers to
facilitate seagrass dominance, however, the existing data have several significant limitations. First,
empirical evidence for such facilitation, particularly by crustaceans, comes largely from short-term,
small-scale laboratory and mesocosm experiments.
Many experimental studies of seagrass-mesograzer
interactions, particularly the early pioneering ones,
were unreplicated or pseudoreplicated, conducted
under artificial conditions, and/or of short duration (Table 2). For example, many early experiments were conducted in aquaria in the laboratory,
or with seagrasses planted in submerged flower pots.
These designs are understandable, given the logistical challenges of working in the field with clonal
plants rooted in sediments and manipulating abundances of very small animals with rapid population
growth rates. However, such experiments are probably unable to detect the potentially important effects
of physiological integration among seagrass ramets
growing in large beds (Valentine et al. 2004), or the
interactions between grazing and the often widely
fluctuating environmental conditions characteristic
of estuaries and coastal environments. Nonetheless, replicated experiments conducted in the field
(Robertson and Mann 1982; Phillipart 1995; Fong
et al. 2000; Schanz et al. 2002) and in outdoor mesocosms (Howard and Short 1986; Duffy et al. 2001,
2003) support earlier results showing that gastropod
and crustacean mesograzers are essential to healthy
seagrass stands. Exclusion of these grazers often increases biomass of epiphytes and other fouling material (including attached or sessile invertebrates) by
an order of magnitude, and reduced seagrass biomass
or cover by 50% or more (Table 2). Thus, the general
picture emerging from experimental studies is that
John F. Valentine and J. Emmett Duffy
and van Montfrans 1984; Klumpp et al. 1989; Brawley 1992; Jernakoff et al. 1996; Heck et al. 2000).
Beds of seagrasses and benthic algae throughout the
world typically harbor dense populations of these
mesograzers. Estimates based on their consumption
and secondary production rates, as well as a limited
number of experimental studies (see below), suggest
that mesograzers play pivotal roles in marine benthic
communities, both in transfer of primary production
to higher trophic levels, including fisheries (Kikuchi
1974; Klumpp et al. 1989; Edgar and Shaw 1995;
Taylor 1998), and in structuring the assemblages of
plants on which they feed (Orth and van Montfrans
1984; Hootsmans and Vermaat 1985; Short et al.
1995; Williams and Ruckleshaus 1993; Jernakoff
et al. 1996; Duffy and Hay 2000; Duffy and Harvilicz
2001; Duffy et al. 2001, 2003).
Of particular interest in the context of seagrass
ecosystems is the potentially central role of mesograzers as ecological engineers (sensu Jones et al.
1994), specifically in indirectly facilitating seagrass
dominance by cropping epiphytic algae that would
otherwise outcompete the seagrasses for light and
water-column nutrients. Numerous lab and microcosm studies have documented that—with some
important exceptions—the majority of small grazing invertebrates associated with marine plants feed
preferentially or exclusively on benthic algae and detritus, and little if at all on seagrasses (reviewed by
Orth and van Montfrans 1984; van Montfrans et al.
1984; Klumpp et al. 1989; Brawley 1992; Jernakoff
et al. 1996). Because algae generally grow faster than
seagrasses, and can grow on the seagrasses themselves, algae are competitively superior to seagrasses
under many conditions and can potentially exclude
them from well-lit shallow waters when grazing is ineffective (Valiela et al. 1997). Thus, mesograzers potentially form one member of a critical three-way interaction with seagrasses and algae; their preferential
grazing on epiphytic algae potentially releases seagrasses from competition, allowing seagrasses and
the ecologically and economically important communities that depend on them to flourish. While the
general feeding preference of mesograzers for algae is well established, however, direct experimental confirmation that this grazing actually translates
into enhanced seagrass performance in the field has
proven more difficult (but see Heck et al. 2000).
Table 2 summarizes existing evidence from laboratory, mesocosm, and field experiments for mesograzer mediation of plant community structure in
seagrass beds. We restricted our consideration to
studies of living seagrasses rooted in sediment, and
that presented some quantitative measure of the
influence of mesograzers relative to treatments in
which mesograzer abundance was experimentally
reduced. The collective weight of these few experiments demonstrates convincingly that natural densities of small crustaceans and gastropods greatly reduce the accumulation of epiphytic algae on macrophytes, and suggests strongly, although indirectly,
that exclusion of grazers often has serious negative consequences for seagrass growth, biomass, and
cover.
Despite the clear potential for mesograzers to
facilitate seagrass dominance, however, the existing data have several significant limitations. First,
empirical evidence for such facilitation, particularly by crustaceans, comes largely from short-term,
small-scale laboratory and mesocosm experiments.
Many experimental studies of seagrass-mesograzer
interactions, particularly the early pioneering ones,
were unreplicated or pseudoreplicated, conducted
under artificial conditions, and/or of short duration (Table 2). For example, many early experiments were conducted in aquaria in the laboratory,
or with seagrasses planted in submerged flower pots.
These designs are understandable, given the logistical challenges of working in the field with clonal
plants rooted in sediments and manipulating abundances of very small animals with rapid population
growth rates. However, such experiments are probably unable to detect the potentially important effects
of physiological integration among seagrass ramets
growing in large beds (Valentine et al. 2004), or the
interactions between grazing and the often widely
fluctuating environmental conditions characteristic
of estuaries and coastal environments. Nonetheless, replicated experiments conducted in the field
(Robertson and Mann 1982; Phillipart 1995; Fong
et al. 2000; Schanz et al. 2002) and in outdoor mesocosms (Howard and Short 1986; Duffy et al. 2001,
2003) support earlier results showing that gastropod
and crustacean mesograzers are essential to healthy
seagrass stands. Exclusion of these grazers often increases biomass of epiphytes and other fouling material (including attached or sessile invertebrates) by
an order of magnitude, and reduced seagrass biomass
or cover by 50% or more (Table 2). Thus, the general
picture emerging from experimental studies is that