Environmental Engineering Reference
In-Depth Information
standpoint, it stands to reason that we should protect the eggs of these won-
derful, defenseless species from their predators.And so, wildlife management
agencies and volunteers often hold vigil on beaches during the egg-laying
season and place protective wire baskets over turtle egg nests.
At face value, such management derives logically from natural history
observation and the retroductive reasoning (see chapter 2) that predators
must be the cause of population decline for sea turtles. Here, however, is
an example of management that may have acted on an untested hypothe-
sis.We must ask the critical question:Why do this kind of management in
the first place? To answer this question it will be insufficient to use the ma-
chinery of classic population ecology presented in chapter 4.This is because
classic population ecology assumes that all individuals in a population—
young and old—live in the same habitat all the time. But, we know that this
assumption does not apply to sea turtles where adults who spend most of
their time in the ocean live apart from eggs and young on the beach.To an-
swer the management question we need to consider each life-cycle stage
specifically—that is, we need to consider the
population age structure
—when
attempting to understand the dynamics of the turtle population.
Sea turtles are long-lived creatures. Some can reach eighty years of age
or more.They first begin to breed around age twenty; a female can produce
many hundreds of eggs in a single breeding season. Suppose we followed
the fate of individuals from the one hundred eggs of a single nest using sur-
vival estimates provided by Crouse et al. (1987).The data show that most
eggs are lost to predation, fungal infections, accidents, and so on before they
even hatch. Upon hatching, individuals must rush out to the ocean. Most
make it, but some succumb to accidents or predation (Box. 5.1). Once in
the ocean, individuals are highly likely to live; and to live to an old age.This
pattern holds for many other species of reptile and amphibian species (Pi-
anka 1988). Natural selection places a large toll on the egg and hatchling
stages of the turtles.
For a population to be stable or sustainable over the long term, a parent
only needs to be replaced by one surviving offspring. Of the thousands of
eggs that are laid by a single female sea turtle over the course of forty to
sixty years, only two turtle babies need to survive to replace their mother
and father.This then begs the question:Why do sea turtles employ a game-
of-life strategy of laying hundreds of eggs in a clutch and laying so many
clutches? They do this because the need to breed on beaches and to spend
