Sea Turtle Population Ecology
291
ranges of temperatures determine whether a nest will produce males, females, or a
combination of both. Many sea turtle populations appear to produce strongly femalebiased offspring (Hanson et al., 1998; Godley et al., 2002; others). Freedberg and Wade
(2001) theorize that female-biased sex ratios in reptiles with environmentally controlled sex determination are reinforced by natal nest site fidelity of females. Nest sites
on beaches that produce predominantly female offspring will be perpetuated as the
female offspring mature and return to the same sites. The authors do not provide an
explanation for the adaptive significance of this in terms of population maintenance.
Although long-term sex ratio skews in offspring are not predicted by life history theory
(Fisher, 1930), they can be favored in spatially structured populations (Charnov, 1982).
We are gradually learning about the mating systems of sea turtles through genetic
studies (e.g., FitzSimmons, 1998), but it is difficult to determine the point at which a
population could become male-limited. Potential effects of a shortage of adult males
could include reduced eggs per nest, reduced clutches, or increased remigration interval. From a management perspective, temperature-dependent sex determination supports the need to protect and maintain populations in higher latitudes, such as the
northern nesting subpopulation of loggerheads (NMFS, 2001).
In addition to reduced reproduction through sex ratio bias, small populations
may suffer effects of density depensation , such as a reduction in nesting frequency
caused by an inability of females to find potential mates. The problem could be
severe for Pacific leatherbacks, which have wide geographic ranges and have suffered
a decrease of as much as 99% over the last two decades (Chan and Liew, 1996;
Spotila et al., 2000). Two centuries after the end of commercial exploitation, green
turtles have failed to recover on the Cayman Islands; this population may have been
driven to such a small size that recovery is no longer possible (Aiken et al., 2001).
11.3 POPULATION STRUCTURE
Understanding population structure is critical to the conservation and management of
a species. Tagging studies initially demonstrated that adult female sea turtles exhibit
a high degree of nest site fidelity in that they return to the same nesting region (e.g.,
Bjorndal et al., 1983). Studies of mitochondrial DNA (mtDNA) further indicate that
female sea turtles are actually exhibiting natal nest site fidelity and returning to the
area of beach at which they hatched (Bowen et al., 1992; 1993; Allard et al., 1994;
Encalada et al., 1998). These studies demonstrate that there is female-mediated genetic
differentiation between nesting areas that is maintained by natal nest site fidelity.
We have the best understanding of population structure for loggerheads in the
Atlantic Ocean and Mediterranean Sea (Turtle Expert Working Group, 2000). Studies
of mtDNA from rookeries in these regions have demonstrated at least eight genetically distinct nesting areas: (1) Greece/Cyprus; (2) Turkey; (3) Brazil; (4) Yucatán,
Mexico; (5) Dry Tortugas, FL; (6) south Florida; (7) Florida panhandle; and (8)
northeast Florida to North Carolina (Bowen et al., 1992; Encalada et al., 1998;
Laurent et al., 1998; Francisco et al., in press).
The ramification of the genetic structuring is that if one of these nesting aggregations
becomes extinct, it will not be recolonized on conservation-level time scales (Bowen
et al., 1992). We do not yet fully understand the level of interaction between the nesting
1123 book.book Page 291 Tuesday, November 12, 2002 7:43 AM
291
ranges of temperatures determine whether a nest will produce males, females, or a
combination of both. Many sea turtle populations appear to produce strongly femalebiased offspring (Hanson et al., 1998; Godley et al., 2002; others). Freedberg and Wade
(2001) theorize that female-biased sex ratios in reptiles with environmentally controlled sex determination are reinforced by natal nest site fidelity of females. Nest sites
on beaches that produce predominantly female offspring will be perpetuated as the
female offspring mature and return to the same sites. The authors do not provide an
explanation for the adaptive significance of this in terms of population maintenance.
Although long-term sex ratio skews in offspring are not predicted by life history theory
(Fisher, 1930), they can be favored in spatially structured populations (Charnov, 1982).
We are gradually learning about the mating systems of sea turtles through genetic
studies (e.g., FitzSimmons, 1998), but it is difficult to determine the point at which a
population could become male-limited. Potential effects of a shortage of adult males
could include reduced eggs per nest, reduced clutches, or increased remigration interval. From a management perspective, temperature-dependent sex determination supports the need to protect and maintain populations in higher latitudes, such as the
northern nesting subpopulation of loggerheads (NMFS, 2001).
In addition to reduced reproduction through sex ratio bias, small populations
may suffer effects of density depensation , such as a reduction in nesting frequency
caused by an inability of females to find potential mates. The problem could be
severe for Pacific leatherbacks, which have wide geographic ranges and have suffered
a decrease of as much as 99% over the last two decades (Chan and Liew, 1996;
Spotila et al., 2000). Two centuries after the end of commercial exploitation, green
turtles have failed to recover on the Cayman Islands; this population may have been
driven to such a small size that recovery is no longer possible (Aiken et al., 2001).
11.3 POPULATION STRUCTURE
Understanding population structure is critical to the conservation and management of
a species. Tagging studies initially demonstrated that adult female sea turtles exhibit
a high degree of nest site fidelity in that they return to the same nesting region (e.g.,
Bjorndal et al., 1983). Studies of mitochondrial DNA (mtDNA) further indicate that
female sea turtles are actually exhibiting natal nest site fidelity and returning to the
area of beach at which they hatched (Bowen et al., 1992; 1993; Allard et al., 1994;
Encalada et al., 1998). These studies demonstrate that there is female-mediated genetic
differentiation between nesting areas that is maintained by natal nest site fidelity.
We have the best understanding of population structure for loggerheads in the
Atlantic Ocean and Mediterranean Sea (Turtle Expert Working Group, 2000). Studies
of mtDNA from rookeries in these regions have demonstrated at least eight genetically distinct nesting areas: (1) Greece/Cyprus; (2) Turkey; (3) Brazil; (4) Yucatán,
Mexico; (5) Dry Tortugas, FL; (6) south Florida; (7) Florida panhandle; and (8)
northeast Florida to North Carolina (Bowen et al., 1992; Encalada et al., 1998;
Laurent et al., 1998; Francisco et al., in press).
The ramification of the genetic structuring is that if one of these nesting aggregations
becomes extinct, it will not be recolonized on conservation-level time scales (Bowen
et al., 1992). We do not yet fully understand the level of interaction between the nesting
1123 book.book Page 291 Tuesday, November 12, 2002 7:43 AM