Sea Turtle Population Ecology
277
of these articles emphasize the need for high survival rates in the large juvenile,
subadult, and adult stages to achieve positive or stable long-term population growth.
A general conclusion for sea turtles and species with similar life histories is that
they are unlikely to be able to sustain even moderate levels of harvest, especially if
the populations are already at reduced levels.
Variable remigration intervals (periods between nesting seasons), highly variable
survival of nests and eggs, and fluctuating ocean conditions contribute to short-term
variability in population abundance and population cycles (Limpus and Nicholls,
1988; Chaloupka and Limpus, 2001). These short-term changes are more difficult
to predict, but are actually easier to measure in the field and can have a major impact
on our assessment of population change.
Long-term population growth rates are also driven by density-dependent changes
in vital rates. We know very little about how sea turtle populations respond to density,
beyond a few examples of reduced egg survival on crowded nesting beaches (Bustard
and Tognetti, 1969; Carr, 1986; Ballestero et al., 2000; Chaloupka, 2001) and evidence of slower growth rates of juvenile green turtles on crowded feeding grounds
(Bjorndal et al., 2000a). Because most sea turtles are in low abundance relative to
historical conditions (e.g., Jackson, 1997), some modelers have ignored the potential
influences of density dependence on vital rates and population growth. However, as
some populations recover, carrying capacity to growth and reproduction will become
an important area of research. Likewise, the effects of species interactions on survival
and growth are density dependent and may change dramatically through time.
In this chapter, we review the life history characteristics that are most relevant
to sea turtle population dynamics, techniques used to assess critical vital rates and
population trends, and application of population models to population dynamics.
11.2 LIFE HISTORY
11.2.1 L IFE S TAGES AND O NTOGENETIC S HIFTS
Sea turtle species share a common life cycle composed of a series of stages. Ontogenetic shifts , or shifts in location and habitat that occur during the life cycle in
response to changes in vital rates (Werner and Gilliam, 1984), have a major impact
on where sea turtles of different sizes or stages occur and, subsequently, the humancaused hazards to which they are exposed. We know of at least one major ontogenetic
shift that occurs in the hard-shelled sea turtles: the shift from pelagic to benthic
feeding areas. There may be additional ontogenetic shifts in microscale habitat use
or resource utilization of which we are not yet aware, but may have important
ramifications for management.
The general life cycle and specifics for individual species are described by
Musick and Limpus (1997). Briefly, adult females dig nest cavities on sandy, oceanfacing beaches and, depending on species, deposit anywhere from 50 to 130 eggs
per nest (Van Buskirk and Crowder, 1994). After incubation, hatchlings emerge from
the nests, crawl down the beach to the water, and swim out to the open ocean. Young
juveniles remain pelagic for a length of time that varies by species and potentially
by geographic location within species (Musick and Limpus, 1997). Following the
1123 book.book Page 277 Tuesday, November 12, 2002 7:43 AM
277
of these articles emphasize the need for high survival rates in the large juvenile,
subadult, and adult stages to achieve positive or stable long-term population growth.
A general conclusion for sea turtles and species with similar life histories is that
they are unlikely to be able to sustain even moderate levels of harvest, especially if
the populations are already at reduced levels.
Variable remigration intervals (periods between nesting seasons), highly variable
survival of nests and eggs, and fluctuating ocean conditions contribute to short-term
variability in population abundance and population cycles (Limpus and Nicholls,
1988; Chaloupka and Limpus, 2001). These short-term changes are more difficult
to predict, but are actually easier to measure in the field and can have a major impact
on our assessment of population change.
Long-term population growth rates are also driven by density-dependent changes
in vital rates. We know very little about how sea turtle populations respond to density,
beyond a few examples of reduced egg survival on crowded nesting beaches (Bustard
and Tognetti, 1969; Carr, 1986; Ballestero et al., 2000; Chaloupka, 2001) and evidence of slower growth rates of juvenile green turtles on crowded feeding grounds
(Bjorndal et al., 2000a). Because most sea turtles are in low abundance relative to
historical conditions (e.g., Jackson, 1997), some modelers have ignored the potential
influences of density dependence on vital rates and population growth. However, as
some populations recover, carrying capacity to growth and reproduction will become
an important area of research. Likewise, the effects of species interactions on survival
and growth are density dependent and may change dramatically through time.
In this chapter, we review the life history characteristics that are most relevant
to sea turtle population dynamics, techniques used to assess critical vital rates and
population trends, and application of population models to population dynamics.
11.2 LIFE HISTORY
11.2.1 L IFE S TAGES AND O NTOGENETIC S HIFTS
Sea turtle species share a common life cycle composed of a series of stages. Ontogenetic shifts , or shifts in location and habitat that occur during the life cycle in
response to changes in vital rates (Werner and Gilliam, 1984), have a major impact
on where sea turtles of different sizes or stages occur and, subsequently, the humancaused hazards to which they are exposed. We know of at least one major ontogenetic
shift that occurs in the hard-shelled sea turtles: the shift from pelagic to benthic
feeding areas. There may be additional ontogenetic shifts in microscale habitat use
or resource utilization of which we are not yet aware, but may have important
ramifications for management.
The general life cycle and specifics for individual species are described by
Musick and Limpus (1997). Briefly, adult females dig nest cavities on sandy, oceanfacing beaches and, depending on species, deposit anywhere from 50 to 130 eggs
per nest (Van Buskirk and Crowder, 1994). After incubation, hatchlings emerge from
the nests, crawl down the beach to the water, and swim out to the open ocean. Young
juveniles remain pelagic for a length of time that varies by species and potentially
by geographic location within species (Musick and Limpus, 1997). Following the
1123 book.book Page 277 Tuesday, November 12, 2002 7:43 AM