Environmental Engineering Reference
In-Depth Information
Such is the case also for the loggerhead sea turtles. Consideration of popu-
lation dynamics when different aged individuals can be grouped according
to a common life-cycle stage is called
stage-structure population dynamics
(Caswell 2001). Both age-structured and stage-structured approaches are
conceptually similar in the way they help to understand population dynam-
ics.The difference between them is that a stage-structured approach ac-
counts for the fact that some individuals remain within an age group for
longer than one breeding period.
Ecologists and conservation biologists devote much effort to quantify
age- or stage-specific survival and fecundity of species.This information is
used to develop mathematical models that use this information to under-
stand what the future population size and age or stage structure (number of
individuals in each age or stage class) might look like in the face of various
management regimes or environmental stressors.
Modeling Age-Structured Population Dynamics
Building a model of age-structured dynamics effectively is like building a
spreadsheet with specific information about each age class within rows of
a column. Each column then represents a subsequent year of life. Figure 5.2
shows how the different stage classes combine to influence population
growth from time 1 to time 2. For example, only two stage classes (novice
breeders and mature adults) contribute to the production of newborn off-
spring.They do so at the age-specific rate m. In natural populations it is usu-
ally the older, mature breeders that produce the lion's share of offspring
because younger individuals, while physiologically capable of reproduction,
may be too naive to breed or may have insufficient energy reserves to re-
produce.
In addition to breeding, individuals of the different stage classes also be-
come older.This is depicted in figure 5.2 by the downward-angled arrows.
Individuals within a stage class grow older at rate p (with appropriate sub-
script designation for a stage class). If all of the individuals in a stage class
survive to the next breeding season then p (the proportion surviving to the
next age class) would be one or 100 percent. However, there are few if any
populations (including humans) in which all members of one stage class sur-
vive to the next. Diseases and accidents all take their toll, independently of
age—albeit younger or older individuals often are more likely to succumb
to disease than are individuals in their prime.Thus, p for any age class is
likely less than one.The lower the p value the higher the mortality risk for
