268
The Biology of Sea Turtles, Vol. II
Because the dry mass of C. nucula is about 15% of wet mass (León and Bjorndal,
in press), these values are equivalent to 360–753 kg wet mass. The population of
27,000 adult hawksbills would consume from 1.5 to 3.1 million kg of sponge dry
mass or 10–21 million kg of sponge wet mass each year.
On first consideration, 10–21 million kg of sponge wet mass seems a large
quantity. We must consider that number, however, from the perspective of the
quantity of sponges that hawksbill populations once consumed in the Caribbean. As
noted above, hawksbills have been harvested in the Caribbean since prehistoric times
primarily for their scutes, but also for their meat and eggs (Meylan, 1999). On the
basis of a thorough review of available data, Meylan and Donnelly (1999) documented declines in hawksbill populations in the Caribbean ranging from 75 to 98%
over the last 100 years or less. Given the historic records of annual harvests of
thousands of hawksbills in the Caribbean during the eighteenth and nineteenth
centuries (summarized in Meylan and Donnelly, 1999), an estimate of an overall
decline of 95% in hawksbills from preexploitation to the present is conservative. If
adult hawksbills consumed only sponges when population densities were at preexploitation levels, then we estimate that 540,000 adult hawksbills (27,000/0.05) consumed from 200 to 420 million kg of sponge wet mass each year. We consider the
estimate of 540,000 adult hawksbills in preexploitation populations to be very
conservative — perhaps underestimating the true value by an order of magnitude.
This estimate does not include the amount of sponge consumed by the large number
of immature hawksbills in the population.
The effect of this massive increase in the consumption of Caribbean sponges in
the past would go beyond the direct effect of decreasing sponge populations. Hawksbills can also affect reef diversity and succession by influencing space competition.
Scleractinian corals and sponges commonly compete for space on reefs with up to
12 interactions per square meter, and sponges are more often the superior competitor
(references in León and Bjorndal, in press). Competition for space also exists among
sponge species, and predation by hawksbills is believed to have a major role in
maintaining sponge species diversity (van Dam and Diez, 1997).
The diet preference for C. nucula emphasizes the past role of hawksbills in space
competition on coral reefs because C. nucula is a very aggressive competitor for
space with reef corals. C. nucula is now a very common Caribbean demosponge.
As summarized in León and Bjorndal (in press), C. nucula was the dominant sponge
at 13% of shallow reef sites off Cuba (Alcolado, 1994), occupied up to 12% of the
area on some Puerto Rican reefs (Corredor et al., 1988), and was one of the dominant
sponges in the Exuma Cays, Bahamas (Sluka et al., 1996). C. nucula was involved
in nearly half of all scleractinian coral competitive interactions on a reef in Puerto
Rico (Vicente, 1990), caused >70% of all coral overgrowths in a study in the Florida
Keys (Hill, 1998), and was considered one of the major threats to corals in a reef
in Belize (Antonius and Ballesteros, 1998). Hill (1998) excluded sponge predators
from coral–sponge interactions and found that C. nucula would rapidly overgrow
the majority of corals with which it interacted. Hill (1998) concluded that spongivory
might have substantial community-level effects in coral reefs.
Acroporid coral cover in the Caribbean during the first half of the twentieth
century had declined dramatically from the Pleistocene (Jackson et al., 2001).
1123 book.book Page 268 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
Because the dry mass of C. nucula is about 15% of wet mass (León and Bjorndal,
in press), these values are equivalent to 360–753 kg wet mass. The population of
27,000 adult hawksbills would consume from 1.5 to 3.1 million kg of sponge dry
mass or 10–21 million kg of sponge wet mass each year.
On first consideration, 10–21 million kg of sponge wet mass seems a large
quantity. We must consider that number, however, from the perspective of the
quantity of sponges that hawksbill populations once consumed in the Caribbean. As
noted above, hawksbills have been harvested in the Caribbean since prehistoric times
primarily for their scutes, but also for their meat and eggs (Meylan, 1999). On the
basis of a thorough review of available data, Meylan and Donnelly (1999) documented declines in hawksbill populations in the Caribbean ranging from 75 to 98%
over the last 100 years or less. Given the historic records of annual harvests of
thousands of hawksbills in the Caribbean during the eighteenth and nineteenth
centuries (summarized in Meylan and Donnelly, 1999), an estimate of an overall
decline of 95% in hawksbills from preexploitation to the present is conservative. If
adult hawksbills consumed only sponges when population densities were at preexploitation levels, then we estimate that 540,000 adult hawksbills (27,000/0.05) consumed from 200 to 420 million kg of sponge wet mass each year. We consider the
estimate of 540,000 adult hawksbills in preexploitation populations to be very
conservative — perhaps underestimating the true value by an order of magnitude.
This estimate does not include the amount of sponge consumed by the large number
of immature hawksbills in the population.
The effect of this massive increase in the consumption of Caribbean sponges in
the past would go beyond the direct effect of decreasing sponge populations. Hawksbills can also affect reef diversity and succession by influencing space competition.
Scleractinian corals and sponges commonly compete for space on reefs with up to
12 interactions per square meter, and sponges are more often the superior competitor
(references in León and Bjorndal, in press). Competition for space also exists among
sponge species, and predation by hawksbills is believed to have a major role in
maintaining sponge species diversity (van Dam and Diez, 1997).
The diet preference for C. nucula emphasizes the past role of hawksbills in space
competition on coral reefs because C. nucula is a very aggressive competitor for
space with reef corals. C. nucula is now a very common Caribbean demosponge.
As summarized in León and Bjorndal (in press), C. nucula was the dominant sponge
at 13% of shallow reef sites off Cuba (Alcolado, 1994), occupied up to 12% of the
area on some Puerto Rican reefs (Corredor et al., 1988), and was one of the dominant
sponges in the Exuma Cays, Bahamas (Sluka et al., 1996). C. nucula was involved
in nearly half of all scleractinian coral competitive interactions on a reef in Puerto
Rico (Vicente, 1990), caused >70% of all coral overgrowths in a study in the Florida
Keys (Hill, 1998), and was considered one of the major threats to corals in a reef
in Belize (Antonius and Ballesteros, 1998). Hill (1998) excluded sponge predators
from coral–sponge interactions and found that C. nucula would rapidly overgrow
the majority of corals with which it interacted. Hill (1998) concluded that spongivory
might have substantial community-level effects in coral reefs.
Acroporid coral cover in the Caribbean during the first half of the twentieth
century had declined dramatically from the Pleistocene (Jackson et al., 2001).
1123 book.book Page 268 Monday, November 11, 2002 11:11 AM