Roles of Sea Turtles in Marine Ecosystems: Reconstructing the Past
261
be the effect of developing a commercial harvest of jellyfish in the Gulf
of Mexico — a major food resource for several sea turtle species? What
is the effect of the depletion of shark populations — major predators on
sea turtles around the world?
3. More meaningful goals for management and conservation of sea turtles:
To define goals for sea turtle recovery programs that allow sea turtles to
be ecologically functional in marine ecosystems. The mission of the
Marine Turtle Specialist Group of the World Conservation Union (IUCN)
is to “promote the restoration and survival of healthy marine turtle populations that fulfill their ecological roles” (Marine Turtle Specialist Group,
1995). Such goals coincide with the current emphasis on ecosystem management rather than single-species management. Sea turtles cannot be
conserved without restoring and competently managing the marine systems they inhabit. The recovery plans for sea turtle species developed by
the U.S. Fish and Wildlife Service and the National Marine Fisheries
Service contain specific recovery goals, as required under the U.S. Endangered Species Act. None of these plans has set a recovery goal to restore
sea turtle populations to their ecological roles (e.g., National Marine
Fisheries Service and U.S. Fish and Wildlife Service, 1991a, b). Our lack
of knowledge hinders setting such goals: How many sea turtles would be
required for a population to be ecologically functional?
10.2 ECOLOGICAL ROLES OF SEA TURTLES
Sea turtles range widely over the Earth. They occur in oceanic and neritic habitats
from the tropics to subarctic waters and venture onto terrestrial habitats to nest or
bask in tropical and temperate latitudes (Table 10.1). Before sea turtle populations
were depleted by humans, sea turtles occurred in massive numbers that are now
difficult to imagine (King, 1982; Ross, 1982; Jackson, 1997; Jackson et al., 2001).
At those high population levels, sea turtles had substantial effects on the marine
systems they inhabited as consumers, prey, and competitors; as hosts for parasites
and pathogens; as substrates for epibionts; as nutrient transporters; and as modifiers
of the landscape.
Bjorndal (in press) summarized the current state of our knowledge of the
ecological roles of loggerheads ( Caretta caretta ). Although our understanding of
the ecological role of the loggerhead is extremely limited, it is the best-studied
sea turtle species in this regard. Loggerheads prey upon a large number of species
and, particularly at small sizes, are preyed upon by a wide range of predators
(Bjorndal, in press). Sea turtles serve as substrate and transport for a diverse array
of epibionts. Loggerheads nesting in Georgia had 100 species of epibionts from
13 phyla (Frick et al., 1998), and loggerheads nesting at Xcacel, Mexico, carried
37 taxa of algae in total, with up to 12 species on an individual turtle (Senties
et al., 1999). Sea turtles can transfer substantial quantities of nutrients and energy
from nutrient-rich foraging grounds to nutrient-poor nesting beaches. Less than
one third of the energy and nitrogen contained in eggs deposited by loggerheads
in Melbourne Beach, FL, returned to the ocean in the form of hatchlings (Bouchard
1123 book.book Page 261 Monday, November 11, 2002 11:11 AM
261
be the effect of developing a commercial harvest of jellyfish in the Gulf
of Mexico — a major food resource for several sea turtle species? What
is the effect of the depletion of shark populations — major predators on
sea turtles around the world?
3. More meaningful goals for management and conservation of sea turtles:
To define goals for sea turtle recovery programs that allow sea turtles to
be ecologically functional in marine ecosystems. The mission of the
Marine Turtle Specialist Group of the World Conservation Union (IUCN)
is to “promote the restoration and survival of healthy marine turtle populations that fulfill their ecological roles” (Marine Turtle Specialist Group,
1995). Such goals coincide with the current emphasis on ecosystem management rather than single-species management. Sea turtles cannot be
conserved without restoring and competently managing the marine systems they inhabit. The recovery plans for sea turtle species developed by
the U.S. Fish and Wildlife Service and the National Marine Fisheries
Service contain specific recovery goals, as required under the U.S. Endangered Species Act. None of these plans has set a recovery goal to restore
sea turtle populations to their ecological roles (e.g., National Marine
Fisheries Service and U.S. Fish and Wildlife Service, 1991a, b). Our lack
of knowledge hinders setting such goals: How many sea turtles would be
required for a population to be ecologically functional?
10.2 ECOLOGICAL ROLES OF SEA TURTLES
Sea turtles range widely over the Earth. They occur in oceanic and neritic habitats
from the tropics to subarctic waters and venture onto terrestrial habitats to nest or
bask in tropical and temperate latitudes (Table 10.1). Before sea turtle populations
were depleted by humans, sea turtles occurred in massive numbers that are now
difficult to imagine (King, 1982; Ross, 1982; Jackson, 1997; Jackson et al., 2001).
At those high population levels, sea turtles had substantial effects on the marine
systems they inhabited as consumers, prey, and competitors; as hosts for parasites
and pathogens; as substrates for epibionts; as nutrient transporters; and as modifiers
of the landscape.
Bjorndal (in press) summarized the current state of our knowledge of the
ecological roles of loggerheads ( Caretta caretta ). Although our understanding of
the ecological role of the loggerhead is extremely limited, it is the best-studied
sea turtle species in this regard. Loggerheads prey upon a large number of species
and, particularly at small sizes, are preyed upon by a wide range of predators
(Bjorndal, in press). Sea turtles serve as substrate and transport for a diverse array
of epibionts. Loggerheads nesting in Georgia had 100 species of epibionts from
13 phyla (Frick et al., 1998), and loggerheads nesting at Xcacel, Mexico, carried
37 taxa of algae in total, with up to 12 species on an individual turtle (Senties
et al., 1999). Sea turtles can transfer substantial quantities of nutrients and energy
from nutrient-rich foraging grounds to nutrient-poor nesting beaches. Less than
one third of the energy and nitrogen contained in eggs deposited by loggerheads
in Melbourne Beach, FL, returned to the ocean in the form of hatchlings (Bouchard
1123 book.book Page 261 Monday, November 11, 2002 11:11 AM