Roles of Sea Turtles in Marine Ecosystems: Reconstructing the Past
265
(McNaughton, 1985). A study now underway (Moran and Bjorndal, unpublished
data) on the effects of green turtle grazing on T. testudinum productivity should
greatly improve our estimates of K .
Under such heavy grazing regimes, seagrass ecosystems in the Caribbean would
have had very different structures and dynamics than they do today. The current
green turtle population in the Caribbean has been estimated to represent 3–7% of
preexploitation population levels (Jackson et al., 2001). Major changes in biodiversity, productivity, and structure of T. testudinum pastures would be expected between
grazed pastures with blade lengths of 2–4 cm and the essentially ungrazed pastures
of today with blade lengths of up to 30 cm or more (Zieman, 1982). Dampier (1729)
observed that blades of T. testudinum were only “six inches long” (15 cm) at a time
when green turtles were much more abundant in the Caribbean. Grazing by green
turtles significantly shortens nutrient cycling times in T. testudinum pastures (Thayer
et al., 1982). Reduced blade life in grazed stands and thus reduced time for epibiont
colonization would affect the epibionts that cover T. testudinum blades in some areas.
Shorter blade lengths in grazed stands would decrease the baffling effect and thus
the entrapment of particles and deposition of substrate and would substantially
change the physical structure of these ecosystems that are important nursery areas
for many species of fish and invertebrates. This change in structure may have
contributed to the mass mortality of Florida seagrasses in the 1980s (Jackson, 2001).
Seagrass mortality was positively density dependent and was correlated with high
temperatures and salinities, sulfide toxicity, self-shading, hypoxia, and infection by
the slime mold Labyrinthula spp. (Robblee et al., 1991; Harvell et al., 1999; Zieman
et al., 1999). All of these factors, except temperature and salinity, are greatly
increased in ungrazed seagrass pastures (Jackson, 2001). Again, the study now
underway (Moran and Bjorndal, unpublished data) on the effects of green turtle
grazing on T. testudinum productivity and structure should provide quantitative
estimates of some of these effects.
We can conclude that natural populations of green turtles consumed a tremendous amount of T. testudinum . A population of 100 million green turtles with an
average mass of 50 kg (a relatively modest population estimate from Jackson [1997]
and Bjorndal et al. [2000]) with an average annual intake of 1.23 kg T. testudinum
dry mass per kg turtle (Table 10.2) would consume 6.2 ¥ 10 9 kg T. testudinum dry
mass each year. That value is approximately half of the estimated total annual
production of 1.2 ¥ 10 10 kg T. testudinum dry mass in the Caribbean (6,600,000 ha
T. testudinum in the Caribbean [Jackson, 1997] ¥ 1750 kg T. testudinum dry mass
produced annually per ha [Table 10.2]).
10.4 CASE STUDY: CARIBBEAN HAWKSBILL
As stated above, Caribbean hawksbills have been extensively exploited for centuries
for tortoiseshell, the keratinized scutes that cover their shells (Parsons, 1972; Groombridge and Luxmoore, 1989; Meylan, 1999). The current number of adult female
hawksbills that nest each year in the Caribbean is estimated at 5000, on the basis
of a thorough review by Meylan (1999). Because each female nests at an average
interval of 2.7 years (Richardson et al., 1999), the estimate of adult female hawksbills
1123 book.book Page 265 Monday, November 11, 2002 11:11 AM
265
(McNaughton, 1985). A study now underway (Moran and Bjorndal, unpublished
data) on the effects of green turtle grazing on T. testudinum productivity should
greatly improve our estimates of K .
Under such heavy grazing regimes, seagrass ecosystems in the Caribbean would
have had very different structures and dynamics than they do today. The current
green turtle population in the Caribbean has been estimated to represent 3–7% of
preexploitation population levels (Jackson et al., 2001). Major changes in biodiversity, productivity, and structure of T. testudinum pastures would be expected between
grazed pastures with blade lengths of 2–4 cm and the essentially ungrazed pastures
of today with blade lengths of up to 30 cm or more (Zieman, 1982). Dampier (1729)
observed that blades of T. testudinum were only “six inches long” (15 cm) at a time
when green turtles were much more abundant in the Caribbean. Grazing by green
turtles significantly shortens nutrient cycling times in T. testudinum pastures (Thayer
et al., 1982). Reduced blade life in grazed stands and thus reduced time for epibiont
colonization would affect the epibionts that cover T. testudinum blades in some areas.
Shorter blade lengths in grazed stands would decrease the baffling effect and thus
the entrapment of particles and deposition of substrate and would substantially
change the physical structure of these ecosystems that are important nursery areas
for many species of fish and invertebrates. This change in structure may have
contributed to the mass mortality of Florida seagrasses in the 1980s (Jackson, 2001).
Seagrass mortality was positively density dependent and was correlated with high
temperatures and salinities, sulfide toxicity, self-shading, hypoxia, and infection by
the slime mold Labyrinthula spp. (Robblee et al., 1991; Harvell et al., 1999; Zieman
et al., 1999). All of these factors, except temperature and salinity, are greatly
increased in ungrazed seagrass pastures (Jackson, 2001). Again, the study now
underway (Moran and Bjorndal, unpublished data) on the effects of green turtle
grazing on T. testudinum productivity and structure should provide quantitative
estimates of some of these effects.
We can conclude that natural populations of green turtles consumed a tremendous amount of T. testudinum . A population of 100 million green turtles with an
average mass of 50 kg (a relatively modest population estimate from Jackson [1997]
and Bjorndal et al. [2000]) with an average annual intake of 1.23 kg T. testudinum
dry mass per kg turtle (Table 10.2) would consume 6.2 ¥ 10 9 kg T. testudinum dry
mass each year. That value is approximately half of the estimated total annual
production of 1.2 ¥ 10 10 kg T. testudinum dry mass in the Caribbean (6,600,000 ha
T. testudinum in the Caribbean [Jackson, 1997] ¥ 1750 kg T. testudinum dry mass
produced annually per ha [Table 10.2]).
10.4 CASE STUDY: CARIBBEAN HAWKSBILL
As stated above, Caribbean hawksbills have been extensively exploited for centuries
for tortoiseshell, the keratinized scutes that cover their shells (Parsons, 1972; Groombridge and Luxmoore, 1989; Meylan, 1999). The current number of adult female
hawksbills that nest each year in the Caribbean is estimated at 5000, on the basis
of a thorough review by Meylan (1999). Because each female nests at an average
interval of 2.7 years (Richardson et al., 1999), the estimate of adult female hawksbills
1123 book.book Page 265 Monday, November 11, 2002 11:11 AM