262
The Biology of Sea Turtles, Vol. II
and Bjorndal, 2000). Loggerheads can modify the physical structure of their habitat
in a number of ways, including digging trenches through soft substrates in search
of infauna prey (Preen, 1996).
The roles of sea turtles as consumers are the best known, but information is
largely limited to lists of prey species. The diets of most species have been evaluated
(Table 10.1), although there are considerable gaps for early life stages and some
geographic areas (Bjorndal, 1997). Knowledge of selective feeding and rates of
consumption, which is critical for quantitatively evaluating the ecological function
of sea turtles as consumers, is generally lacking.
For the remainder of this chapter, we will present two case studies to illustrate
how the ecological role of sea turtles as consumers can be quantified, as indicated
by the amount of prey consumed. We selected the Caribbean green turtle ( Chelonia
mydas ) and the Caribbean hawksbill because of the availability of data and the
difference in diets: The green turtle is an herbivore that feeds primarily on seagrasses
in the Caribbean, and the hawksbill is a carnivore that feeds largely on sponges.
In the two case studies, we have had to assume that diet and intake (rate of
consumption) will not change with changes in population density. We realize that
these assumptions may not be true. As populations become denser, diet species may
change as preferred prey become less abundant and less-favored species must be
consumed. Intake may decrease as intraspecific competition for food increases or
may change with diet quality. Evidence for such density-dependent effects was
observed for a population of immature green turtles for which somatic growth rates
and condition index (mass/length 3 ) declined as population density increased, apparently in response to lower food resources (Bjorndal et al., 2000).
10.3 CASE STUDY: CARIBBEAN GREEN TURTLE
The decline of green turtles in the Caribbean during historic times is well recognized
(Parsons, 1962). The example of the extirpation of the Cayman Islands green turtle
nesting colony is relatively well recorded in historical documents. The Cayman
Islands were apparently never inhabited and their resources were never utilized by
Amerindians (Stokes and Keegan, 1996; Scudder and Quitmyer, 1998). Columbus
sighted the islands of Cayman Brac and Little Cayman during his last voyage in
1503, and named them Las Tortugas because of the great number of turtles on the
land and in the surrounding waters (Hirst, 1910). After that time, the Cayman Islands,
which were not permanently settled by humans until 1734 (Williams, 1970), were
visited by ships of many nations to take on green turtles and their eggs (Lewis, 1940).
Consistent exploitation of Cayman green turtles by ships from Jamaica was initiated
in 1655 when the English took Jamaica from Spain (Lewis, 1940). In 1684, when
French and Spanish corsairs chased English turtling vessels out of Cayman and Cuban
waters, Colonel Hender Molesworth reported to Britain that Jamaica would suffer
because green turtle “is what masters of ships chiefly feed their men in port, and I
believe that nearly 2000 people, black and white, feed on it daily at this point, to say
nothing of what is sent inland. Altogether it cannot be easily imagined how prejudiced
is this interruption of the turtle trade” (Smith, 2000). With safe access to the Caymans
restored, Jamaican ships carried 13,000 turtles each year from the Caymans between
1123 book.book Page 262 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
and Bjorndal, 2000). Loggerheads can modify the physical structure of their habitat
in a number of ways, including digging trenches through soft substrates in search
of infauna prey (Preen, 1996).
The roles of sea turtles as consumers are the best known, but information is
largely limited to lists of prey species. The diets of most species have been evaluated
(Table 10.1), although there are considerable gaps for early life stages and some
geographic areas (Bjorndal, 1997). Knowledge of selective feeding and rates of
consumption, which is critical for quantitatively evaluating the ecological function
of sea turtles as consumers, is generally lacking.
For the remainder of this chapter, we will present two case studies to illustrate
how the ecological role of sea turtles as consumers can be quantified, as indicated
by the amount of prey consumed. We selected the Caribbean green turtle ( Chelonia
mydas ) and the Caribbean hawksbill because of the availability of data and the
difference in diets: The green turtle is an herbivore that feeds primarily on seagrasses
in the Caribbean, and the hawksbill is a carnivore that feeds largely on sponges.
In the two case studies, we have had to assume that diet and intake (rate of
consumption) will not change with changes in population density. We realize that
these assumptions may not be true. As populations become denser, diet species may
change as preferred prey become less abundant and less-favored species must be
consumed. Intake may decrease as intraspecific competition for food increases or
may change with diet quality. Evidence for such density-dependent effects was
observed for a population of immature green turtles for which somatic growth rates
and condition index (mass/length 3 ) declined as population density increased, apparently in response to lower food resources (Bjorndal et al., 2000).
10.3 CASE STUDY: CARIBBEAN GREEN TURTLE
The decline of green turtles in the Caribbean during historic times is well recognized
(Parsons, 1962). The example of the extirpation of the Cayman Islands green turtle
nesting colony is relatively well recorded in historical documents. The Cayman
Islands were apparently never inhabited and their resources were never utilized by
Amerindians (Stokes and Keegan, 1996; Scudder and Quitmyer, 1998). Columbus
sighted the islands of Cayman Brac and Little Cayman during his last voyage in
1503, and named them Las Tortugas because of the great number of turtles on the
land and in the surrounding waters (Hirst, 1910). After that time, the Cayman Islands,
which were not permanently settled by humans until 1734 (Williams, 1970), were
visited by ships of many nations to take on green turtles and their eggs (Lewis, 1940).
Consistent exploitation of Cayman green turtles by ships from Jamaica was initiated
in 1655 when the English took Jamaica from Spain (Lewis, 1940). In 1684, when
French and Spanish corsairs chased English turtling vessels out of Cayman and Cuban
waters, Colonel Hender Molesworth reported to Britain that Jamaica would suffer
because green turtle “is what masters of ships chiefly feed their men in port, and I
believe that nearly 2000 people, black and white, feed on it daily at this point, to say
nothing of what is sent inland. Altogether it cannot be easily imagined how prejudiced
is this interruption of the turtle trade” (Smith, 2000). With safe access to the Caymans
restored, Jamaican ships carried 13,000 turtles each year from the Caymans between
1123 book.book Page 262 Monday, November 11, 2002 11:11 AM