Chapter 20 Grazing in Seagrass Ecosystems
477
seagrass growth or biomass. Still fewer have directly
estimated seagrass consumption by herbivores in the
field. Of these, even fewer have included estimates
of grazer-induced changes in seagrass leaf turnover
rates, which, when unaccounted for, can lead to significant underestimates of the amount of seagrass
production consumed by herbivores and the degree
to which these consumers can impact seagrass density (cf. Sand Jensen et al. 1994; Zieman et al. 1984;
Valentine et al. 1997, 2000). In the next section we
discuss the limited data in detail.
D. Evidence for Direct Grazing Impacts
on Seagrasses
A combination of field observations and short-term
experiments show that the variegated sea urchin,
Lytechinus variegatus (Lamarck), can consume from
50 to 100% of aboveground seagrass biomass produced in some areas of the eastern Gulf of Mexico
and Caribbean Sea (Moore et al. 1963; Camp et al.
1973; Greenway 1976, 1995; Zimmerman and Livingston 1976; Bach 1979; Valentine and Heck 1991;
Heck and Valentine 1995; Rose et al. 1999). At densities of 10 inds/m
2 , L. variegatus can reduce turtlegrass habitats to barren unvegetated substrates from
fall through early spring (Valentine and Heck 1991).
If grazing is persistent throughout the winter and
spring, sea urchins can reduce these vegetated habitats to permanently barren unvegetated substrates
(Heck and Valentine 1995). In summer, turtlegrass
persists under severe grazing pressure and regrows
to levels that either equal or exceed the standing crop
of nearby ungrazed turtlegrass. The apparent mechanism by which turtlegrass overcomes the effects of
this grazing is to increase the production or recruitment of new shoots in the grazed area rather than to
increase the production of existing shoots (Valentine
et al. 1997). Similarly, the sea urchins Tripneustes
ventricosus and Diadema antillarum can, and the latter did until recently, consume large quantities of seagrass in some Caribbean settings (e.g. Ogden et al.
1973; Lilly 1975; Vicente and Rivera 1982; Keller
1983; Edmunds and Carpenter 2001). Although it
was once thought that grazing on seagrasses was
predominantly a Caribbean phenomenon (Ogden
and Zieman 1977; Ogden 1980; Ogden and Ogden
1982), observations elsewhere show that sea urchins
also consume significant amounts of seagrass in eastern temperate North Atlantic, and the tropical Pacific
and Indian Oceans (Bak and Nojima 1980; Kirkman
and Young 1981; Hulings and Kirkman 1982;
Verlaque and Nedelec 1983; Jafari and Mahasneh
1984; Larkum and West 1990; Klumpp et al. 1993;
Jernakoff et al. 1996; Alcoverra and Mariani 2002).
In tropical settings where fishing pressure is
low, herbivorous fishes, not sea urchins, are the
dominant herbivores (e.g. Ogden 1976, 1980; Hay
1981, 1984a; Carpenter 1986; but see Jackson 1997;
Lessios, et al. 2001). More than 30 species of
Caribbean fishes, predominantly parrotfishes and
surgeonfishes, have been found to have seagrasses in
their guts (Randall 1967; McAfee and Morgan 1996;
Lewis and Wainwright 1985; but see Hay1984a).
New evidence indicates that herbivorous fishes can
feed extensively on seagrass production in some areas of the Indo-Pacific Ocean as well (Blaber and
Blaber 1980; Klumpp and Nichols 1983a,b; Nichols
et al. 1986; Mariani and Alcoverra 2000). It is likely
that even more species draw nutrition from these
plants, as investigators have typically considered the
presence of seagrass leaves in the guts of fishes to
be incidental intake associated with the capture of
animal prey (e.g. Thompson 1959; Carr and Adams
1973; Bell et al. 1978). When seagrass leaves have
been isotopically labeled, or fishes have been presented with seagrass in laboratory studies, it has
been found that seagrass leaves can contribute to fish
growth (e.g. Conacher et al. 1979; Montgomery and
Targett 1992). Using stable isotopes, Fry et al. (1982)
found that seagrasses and benthic algae contributed
significantly to the diets of many fishes in the seagrass beds of St. Croix, USVI. Fry and Parker (1979)
also found that seagrasses and other benthic plants
contributed significantly to the diets of shrimps and
fishes in some areas of Texas. Similarly, Connolly
(2003), using stable isotope comparisons among
selected fishes and the potential basal sources of
food, found clear evidence of a seagrass signature
in the tissues of the snub nosed garfish (Arrhamphus
sclerolepis).
A conspicuous feature of many tropical reefs is
the bare-sand “halos” created around reefs; intense
fish grazing on sediment bottoms accessible from
the reef ’s shelter prevent seagrasses from growing in
these areas (e.g. Randall 1965; Ogden and Zieman
1977; Hay 1984a; McAfee and Morgan 1996). Not
all foraging in and on seagrasses is limited to the base
of coral reefs, however (Ogden and Zieman 1977).
While many herbivorous fishes seek shelter on coral
reefs at night, they commonly forage in nearby seagrass habitats throughout the day (Randall 1965;
477
seagrass growth or biomass. Still fewer have directly
estimated seagrass consumption by herbivores in the
field. Of these, even fewer have included estimates
of grazer-induced changes in seagrass leaf turnover
rates, which, when unaccounted for, can lead to significant underestimates of the amount of seagrass
production consumed by herbivores and the degree
to which these consumers can impact seagrass density (cf. Sand Jensen et al. 1994; Zieman et al. 1984;
Valentine et al. 1997, 2000). In the next section we
discuss the limited data in detail.
D. Evidence for Direct Grazing Impacts
on Seagrasses
A combination of field observations and short-term
experiments show that the variegated sea urchin,
Lytechinus variegatus (Lamarck), can consume from
50 to 100% of aboveground seagrass biomass produced in some areas of the eastern Gulf of Mexico
and Caribbean Sea (Moore et al. 1963; Camp et al.
1973; Greenway 1976, 1995; Zimmerman and Livingston 1976; Bach 1979; Valentine and Heck 1991;
Heck and Valentine 1995; Rose et al. 1999). At densities of 10 inds/m
2 , L. variegatus can reduce turtlegrass habitats to barren unvegetated substrates from
fall through early spring (Valentine and Heck 1991).
If grazing is persistent throughout the winter and
spring, sea urchins can reduce these vegetated habitats to permanently barren unvegetated substrates
(Heck and Valentine 1995). In summer, turtlegrass
persists under severe grazing pressure and regrows
to levels that either equal or exceed the standing crop
of nearby ungrazed turtlegrass. The apparent mechanism by which turtlegrass overcomes the effects of
this grazing is to increase the production or recruitment of new shoots in the grazed area rather than to
increase the production of existing shoots (Valentine
et al. 1997). Similarly, the sea urchins Tripneustes
ventricosus and Diadema antillarum can, and the latter did until recently, consume large quantities of seagrass in some Caribbean settings (e.g. Ogden et al.
1973; Lilly 1975; Vicente and Rivera 1982; Keller
1983; Edmunds and Carpenter 2001). Although it
was once thought that grazing on seagrasses was
predominantly a Caribbean phenomenon (Ogden
and Zieman 1977; Ogden 1980; Ogden and Ogden
1982), observations elsewhere show that sea urchins
also consume significant amounts of seagrass in eastern temperate North Atlantic, and the tropical Pacific
and Indian Oceans (Bak and Nojima 1980; Kirkman
and Young 1981; Hulings and Kirkman 1982;
Verlaque and Nedelec 1983; Jafari and Mahasneh
1984; Larkum and West 1990; Klumpp et al. 1993;
Jernakoff et al. 1996; Alcoverra and Mariani 2002).
In tropical settings where fishing pressure is
low, herbivorous fishes, not sea urchins, are the
dominant herbivores (e.g. Ogden 1976, 1980; Hay
1981, 1984a; Carpenter 1986; but see Jackson 1997;
Lessios, et al. 2001). More than 30 species of
Caribbean fishes, predominantly parrotfishes and
surgeonfishes, have been found to have seagrasses in
their guts (Randall 1967; McAfee and Morgan 1996;
Lewis and Wainwright 1985; but see Hay1984a).
New evidence indicates that herbivorous fishes can
feed extensively on seagrass production in some areas of the Indo-Pacific Ocean as well (Blaber and
Blaber 1980; Klumpp and Nichols 1983a,b; Nichols
et al. 1986; Mariani and Alcoverra 2000). It is likely
that even more species draw nutrition from these
plants, as investigators have typically considered the
presence of seagrass leaves in the guts of fishes to
be incidental intake associated with the capture of
animal prey (e.g. Thompson 1959; Carr and Adams
1973; Bell et al. 1978). When seagrass leaves have
been isotopically labeled, or fishes have been presented with seagrass in laboratory studies, it has
been found that seagrass leaves can contribute to fish
growth (e.g. Conacher et al. 1979; Montgomery and
Targett 1992). Using stable isotopes, Fry et al. (1982)
found that seagrasses and benthic algae contributed
significantly to the diets of many fishes in the seagrass beds of St. Croix, USVI. Fry and Parker (1979)
also found that seagrasses and other benthic plants
contributed significantly to the diets of shrimps and
fishes in some areas of Texas. Similarly, Connolly
(2003), using stable isotope comparisons among
selected fishes and the potential basal sources of
food, found clear evidence of a seagrass signature
in the tissues of the snub nosed garfish (Arrhamphus
sclerolepis).
A conspicuous feature of many tropical reefs is
the bare-sand “halos” created around reefs; intense
fish grazing on sediment bottoms accessible from
the reef ’s shelter prevent seagrasses from growing in
these areas (e.g. Randall 1965; Ogden and Zieman
1977; Hay 1984a; McAfee and Morgan 1996). Not
all foraging in and on seagrasses is limited to the base
of coral reefs, however (Ogden and Zieman 1977).
While many herbivorous fishes seek shelter on coral
reefs at night, they commonly forage in nearby seagrass habitats throughout the day (Randall 1965;