276
The Biology of Sea Turtles, Vol. II
ocean basins and feed in nearshore areas. These stressors affect the survival and
growth rates of each sea turtle life stage, which, in turn, influences population
growth rates and dynamics.
The first requirement for analysis of sea turtle population dynamics is a long
time series. Fortunately, several long-term studies on nesting beaches reveal both
variability and directional change through time. Long-term monitoring has revealed
dangerous declines in Pacific leatherbacks, encouraging trends in Kemp’s ridleys,
and low but steady nesting populations of hawksbills (Figure 11.1). Apparent cycles
of nesting in sea turtles may occur over short or long periods, although the exact
cause of such cycling in most species is unknown. Some species, such as green
turtles, show marked periodicity that is a function of environmental variance and
internesting (remigration) intervals (Hays, 2000; Chaloupka, 2001). In Australia,
analysis of long time series of nesting green turtles shows that this species responds
to environmental stochasticity and climate shifts such as the El Niño southern
oscillation (ENSO) (Limpus and Nicholls, 1988; Chaloupka, 2001). Ridley turtles,
at least, are capable of relatively rapid increase once critical mortality factors on
large juveniles and adults have been removed and populations are augmented by
egg protection programs (Peñaflores et al., 2000; Heppell et al., 2002a). Species that
mature later, such as loggerheads and greens, may take much longer to recover from
negative perturbations (Crowder et al., 1994; Chaloupka and Limpus, 2001). The
time lags in population response for species that take decades to reach maturity are
a daunting reality for conservation and management.
We can divide our discussion of sea turtle population dynamics into two components: factors that drive long-term population growth rates and factors that affect
short-term variability in populations. Our understanding of these factors and their
integration at the population level is limited temporally and spatially; most studies
examine a single life stage in one location over a limited time frame. We rely on
population models to put the pieces together and project how populations will
respond to perturbations. Such extrapolations, although necessary, must be interpreted cautiously and updated continuously with new information.
Specific life histories of individual sea turtle species vary, but the common
denominator in all of them is that sea turtles are long-lived, slow-growing species
that use multiple habitats over their course of development (Meylan and Ehrenfeld,
2000). General characterizations also include temperature-dependent sex determination, low and variable survival in the egg and hatchling stage, and high and
relatively constant annual survival in the subadult and adult life stages. Maximum
intrinsic growth rates of sea turtles are limited by the extremely long duration of
the juvenile stage in most species and fecundity that is limited by relatively large
eggs and infrequent nesting (Heppell, 1998; Gibbs and Amato, 2000). Annual survival, stage duration (growth rates), and reproduction are vital rates that are influenced by environmental change and human impacts. These vital rates are the foundation of long-term population trends, and we can use models to assess how longterm trends may be affected by perturbations.
Numerous authors have recently highlighted the management and conservation
issues that are critical to maintaining long-lived, slow-growing species (Congdon
et al., 1993; Heppell, 1998; Crouse, 1999; Heppell et al., 1999; Musick, 1999). All
1123 book.book Page 276 Tuesday, November 12, 2002 7:43 AM
The Biology of Sea Turtles, Vol. II
ocean basins and feed in nearshore areas. These stressors affect the survival and
growth rates of each sea turtle life stage, which, in turn, influences population
growth rates and dynamics.
The first requirement for analysis of sea turtle population dynamics is a long
time series. Fortunately, several long-term studies on nesting beaches reveal both
variability and directional change through time. Long-term monitoring has revealed
dangerous declines in Pacific leatherbacks, encouraging trends in Kemp’s ridleys,
and low but steady nesting populations of hawksbills (Figure 11.1). Apparent cycles
of nesting in sea turtles may occur over short or long periods, although the exact
cause of such cycling in most species is unknown. Some species, such as green
turtles, show marked periodicity that is a function of environmental variance and
internesting (remigration) intervals (Hays, 2000; Chaloupka, 2001). In Australia,
analysis of long time series of nesting green turtles shows that this species responds
to environmental stochasticity and climate shifts such as the El Niño southern
oscillation (ENSO) (Limpus and Nicholls, 1988; Chaloupka, 2001). Ridley turtles,
at least, are capable of relatively rapid increase once critical mortality factors on
large juveniles and adults have been removed and populations are augmented by
egg protection programs (Peñaflores et al., 2000; Heppell et al., 2002a). Species that
mature later, such as loggerheads and greens, may take much longer to recover from
negative perturbations (Crowder et al., 1994; Chaloupka and Limpus, 2001). The
time lags in population response for species that take decades to reach maturity are
a daunting reality for conservation and management.
We can divide our discussion of sea turtle population dynamics into two components: factors that drive long-term population growth rates and factors that affect
short-term variability in populations. Our understanding of these factors and their
integration at the population level is limited temporally and spatially; most studies
examine a single life stage in one location over a limited time frame. We rely on
population models to put the pieces together and project how populations will
respond to perturbations. Such extrapolations, although necessary, must be interpreted cautiously and updated continuously with new information.
Specific life histories of individual sea turtle species vary, but the common
denominator in all of them is that sea turtles are long-lived, slow-growing species
that use multiple habitats over their course of development (Meylan and Ehrenfeld,
2000). General characterizations also include temperature-dependent sex determination, low and variable survival in the egg and hatchling stage, and high and
relatively constant annual survival in the subadult and adult life stages. Maximum
intrinsic growth rates of sea turtles are limited by the extremely long duration of
the juvenile stage in most species and fecundity that is limited by relatively large
eggs and infrequent nesting (Heppell, 1998; Gibbs and Amato, 2000). Annual survival, stage duration (growth rates), and reproduction are vital rates that are influenced by environmental change and human impacts. These vital rates are the foundation of long-term population trends, and we can use models to assess how longterm trends may be affected by perturbations.
Numerous authors have recently highlighted the management and conservation
issues that are critical to maintaining long-lived, slow-growing species (Congdon
et al., 1993; Heppell, 1998; Crouse, 1999; Heppell et al., 1999; Musick, 1999). All
1123 book.book Page 276 Tuesday, November 12, 2002 7:43 AM