Environmental Engineering Reference
In-Depth Information
fore we can answer this question, we
must gain some insight into princi-
ples of life cycles and structured pop-
ulation dynamics.
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.
Life Cycles and
Population Dynamics
Individuals of most species develop
through several life-cycle stages be-
fore they become adults.There is considerable variety in the modes of life-
cycle development among different taxa. For example, most vertebrate
species start life as newborns then pass through juvenile, young prerepro-
ductive adult, prime adult, and old adult stages. In these cases, the young are
simply miniature copies of older members of a population. Usually, they re-
side in the same habitats as older individuals. Insects such as grasshoppers
can likewise have simple development where young individuals reside in
the same habitats as older individuals and they are also miniature copies of
their older counterparts. In these cases, individuals may pass through five
development stages (called
instars
) before reaching adulthood. Other insects
are born and pass through similar instar stages in the aquatic realm (e.g.,
mosquitoes, dragonflies) but they switch to terrestrial habitats when they
emerge as adults.
There is also complexity in life-cycle development.The most celebrated
examples, perhaps, are the amphibians that are born as tadpoles in the water.
Tailed and legless young tadpoles look very different than older tadpoles
that have developed legs and lost their tails. Even these older tadpoles look
different than adults who often leave the aquatic realm and spend a good
part of their life on land. Perhaps the most striking examples of complex life
cycles are in Lepidoptera (butterfly and moth) species. Here individuals de-
velop from eggs, become larvae (caterpillars), enclose themselves in a casing
(pupae), and remain dormant for a period of time after which they emerge
as winged adults.
The life cycles of species can be represented schematically (figure 5.1).
Consider the case of the loggerhead sea turtle with the six age classes listed
in Box 5.1: eggs/hatchlings, small juveniles, large juveniles, subadults, novice
breeders, and mature adults. Figure 5.1 illustrates with arrows the transi-
tion of individuals from one age class to another, or the aging process. It also
illustrates which age classes contribute toward reproduction.The letters as-
